Endocrinology

Question 1

Describe the clinical presentation and findings in kittens with congenital hyposomatotrophism (pituitary dwarfism)

Answer 1

• Failure to grow after 1-2 months of age* If only GH is deficient, then the dwarphism will be proportionate* Dull hair coat* Retention of deciduous teeth* Hypoglycaemia may be seen due to the fragile nature of a small kitten, or the lack of gluconeogenic stimulation from GH

Question 2

What are the important differentials for failure to thrive / proportionate dwarfism in kittens?

Answer 2

• Hepatic disease * Porto-systemic vascular anomaly* Malnutrition* Gatrointestinal disease* Renal disease* Cardiac disease

Question 3

Discuss the testing options for investigation of growth hormone deficiency or excess in cats.What are the benefits of IGF-1 testing over serum GH concentration?

Answer 3

• There is no validated GH assay for cats* GH also has pulsatile secretion and there are significant random variations in the GH concentration in nnormal animals* GH stimulates the release of IGF-1 from the liver * IGF-1 release is non-pulsatile and reflects the GH concentration over the preceding ~ 24 hours* Provocative testing of GH or IGF-1 have not been validated in cats.* IGF-1 can be reduced with other disease states: * Newly diagnosed DM * Hepatic insufficiency * Lymphoma * Renal disease

Question 4

How does growth hormone interfere with insulin and glucose regulation?

Answer 4

• Growth hormone exerts both direct and indirtect effects * Direct antagonism of insulin and the effects of insulin* Indirect: * Growth hormone stimulates the release of IGF-1 from the liver* Both growth hormone and moreso IGF-1 antagonise the effect of insulin* IGF-1 binds to the insulin receptor and the IGF-1 receptor * IGF-1 competitively inhibits insulin binding at the insulin receptor * Binding to the insulin receptor essentially blocks intracellular signalling * IGF-1 stimulates the insulin receptor at ~ 0.1 times efficiency as insulin* The net effect is increased glucose / hyperglycaemia and insulin resistance.

Question 5

What is the primary pathological process in feline hypersomatotropism?Describe the potential environmental and genetic causes for the disease

Answer 5

• Most cases of feline hypersomatotrophism are caused by a pituitary adenoma. A tumour of the growth hormone producing somatotrophs_Environmental:_* Organohalogenated contaminants have been implicated * Polychlorinated biphenyls * Brominated flame retardants (BFRs)* One study identifed increased levels of all contaminants in cats with hypersomatotrophism* The dust of these chemicals may be particularly relevant in cats due to being indoors and grooming habits_Genetic:_A single non-conservative SNP of the AIP gene has been identifed in 2 of 10 cats with acromegaly. 40% of humans with acromegaly have similar mutations.The AIP gene produces a tumour suppressor proteinAIP also has a range of effects, including the activation of xenobiotic metabolizing enzymes - therefore a mutation may lead to reduced activity and increased xenobiotic concentrations

Question 6

Describe the early and chronic signs of hypersomatotropism in cats.Note the associated clinical signs and relevant clinicopathological findings in each case

Answer 6

Early signs:* Insulin resistance is the first and most clinically relevant sign of acromegaly* Signs of polyuria/polydipsia and polyphagia have been reported. * Solitary polyphagia may be evident in cats with hypersomatotrophism without diabetes mellitus_Chronic signs:_* Upper respiratory tract stridor due to tissue overgrowth* Increased head width * Increased interdental spacing - most evident for the incisors* PU/PD/PP mostly due to poor diabetes control* PP may be independent of DM control* Prognathia inferior - protrusion of the mandible* Abdominal organomegaly - enlarged liver and kidneys* Heart murmur and CHF* CNS signs with a macroadenoma

Question 7

List the diagnostic tests that may assist in the diagnosis of feline hypersomatotropism.Note the potential limitations of each test.

Answer 7

• IGF-1 assay is the test of choice as it has a non-pulsatile secretion profile, unlike GH * As IGF-1 levels may be suppressed in the absence of portal insulin, a diagnosis may be missed in the newly diagnosed diabetic cat ~ 9% of the time * IGF-1 assays are not widely available * A result > 1000 ng/ml has a positive predictive value of 95%* Advanced Imaging * Useful to document a macroadenoma when investigating for hypersomatropism * A functional microadenoma could be missed * CT can be useful in documenting TMJ abnormalities, prognathia inferior and soft tissue overgrowth* Alternative blood tests: * Serum type III polypeptide - increased in FeHS due to increased collagen turnover * Ghrelin is usually reduced, likely due to GH inhibition of release (as ghrelin normally stimulates GH release) * Ghrelin was not different in FeHS when compared to cats with primary DM

Question 8

List the various treatment options for feline hypersomatotropism.What is the expected response rate to each treatment with regards to diabetes mellitus control? What are the potential complications and prognosis?

Answer 8

• Surgery - Hypophysectomy * Low dose treatment with corticosteroid, levothyroxine and desmopressin acetate is necessary * Access to experienced surgeon is a problem * Perioperative mortality of ~ 10% in experienced hands * 15% have good glycaemic control with usual insulin doses * 85% enter diabetic remission within 1-2 months* Medical management: * Pasireotide - a long acting somatostatin analogue with high binding affinity * Has shown to reduce IGF-1 and insulin resistance * 3/8 cats enetered diabetic remission with treatment and insulin requirements markedly dropped in the others* Radiation therapy * May be used, but the response is unpredictable * IGF-1 fails to normalise in most cats * Diabetic control may improve but FeHS changes can continue to progress* DM treatment only * Suggested if definitive treatment is declined or not available * Prepare for management of other co-morbidities - renal insufficiency, cardiac disease, arthropathy, CNS signs (rare)

Question 9

Describe the role of ghrelin in growth hormone regulationWhat is the major role of ghrelin in young dogs?

Answer 9

• Ghrelin is a potent stimulator of GH release* Ghrelin is produced and release from the stomach in the interdigestive period. * Also present in the duodenum, pancreas, lungs, adrenal glands and the CNS* Ghrelin stimulates appetite and levels are reduced following eating and with gastric distensionGhrelin is the major stimulator of growth hormone secretion in young dogs. Moreso than growth hormone releasing hormone. Ghrelin binds to non-GHRH receptors in the pituitary

Question 10

Describe the available diagnostic tests for acromegaly in dogs.Why is IGF-1 a more reliable test than GH?

Answer 10

• GH is expected to be increased with hypersomatotropism, however due to the pulsatile nature of the hormone, levels may be normal.* IGF-1 is more reliably increased with hypersomatotropism, with levels more stable due to non-pulsatile secretion and protein binding in circulation* IGF-1 is highly correlated with body size, so breed specific reference ranges are recommended* Lack of suppression after administration of somatostatin fails to suppress GH levels or GHRH fails to stimulate GH levels. Unfortunately, GH assays are not widely available.

Question 11

What are the treatment options for the various causes of acromegaly in dogs?

Answer 11

1. Progesterone induced acromegaly causes excessive growth hormone release from the mammary ductular epithelium * Ovohysterectomy can effectively manage this form of acromegaly * Progestagen induced acromegaly - stop administration of the medication2. Hypothyroidism induced acromegaly * Levothyroxine treatment should normalise serum GH levels3. For dogs with a somatotroph adenoma, medical, radiation or surgical management are options. * Minimal surgical experience for this specific disease described * Radiation may not be effective and recurrence is common * Medical management via somatotroph analoges (octreotide) - described to normalise plasma IGF-1 and reduce tumour size in ~ 50% of people * GH receptor antagonists are available for people

Question 12

Describe the common deficiencies in congenital pituitary dwarphism in the German Shepherd Dog.What normal pituitary function is typically maintained?

Answer 12

• Growth hormone, TSH and prolactin are typically reduced in the German Shepherd with pituitary dwarphism* ACTH levels are typically maintained and adrenal function is normal* The neurohypophysis function is also maintained. * Normal ADH release and oxytocin* GnRH is also reduced

Question 13

Describe the main clinical features of pituitary dwarphism in dogs.

Answer 13

• Proportionate growth retardation* Retention of the puppy coat (lanugo hairs)* Lack or primary guard hairs* Males are often cryptorchid due to insufficient GnRH* Females are often infertile and fail to ovulate due to absence of an LH surge* Secondary hypothyroidism tends not to occur until 2-3 years of age

Question 14

Describe the growth hormone stimulation test procedure.How are the results interpreted, including sensitivity and specificity?

Answer 14

• Basal GH should be measured immediately prior to administration of a GH secretagogue * GHRH can be used if available * Clonidine or xylazine - both alpha adrenergic drugs can also be used to stimulate GH release * Ghrelin may also be used * GH should be measured at 20-30 minutes after IV administration of the secretagogue* A normal dog should have an increase in GH concentration of 2-4 fold.* A minimal rise suggests deficiency* Partial GH deficiency may return a normal result on stimulation testing

Question 15

Describe the role of advanced imaging in the diagnosis of pituitary dwarphism in dogs?

Answer 15

• Advanced imaging is rarely needed to confirm a diagnosis of congenital pituitary dwarphism* When performed a cystic structure in Rathke’s pouch is often present* Despite the presence of cysts, the pituitary is typically very small, consistent with hypoplasia

Question 16

Describe the utility and use of genetic testing for pituitary dwarphism in dogs.

Answer 16

1. Pituitary dwarphism has been associated with a mutation in the LHX3 gene, a mutation that is common in German Shepherd dogs.2. The condition is caused by an autosomal recessive single gene abnormality * Due to the presence of other genetic deficits, the mutations are often incompatible with life, hence the low incidence of the phenotype despite significant prevalence of the genotype.3. The LHX3 mutation genetic test can be used to confirm the presence of the mutation in dogs suspected of having the disease - confirmatory test4. The genetic test can be used to identify carrier individuals for a responsible breeding program.

Question 17

Describe the treatment options for pituitary dwarphismWhat are the potential risks and complications of each treatment option?

Answer 17

• Porcine GH can be administered as a SC injection three times weekly. * Monitoring for hyperglycaemia is useful to assess for overdose * Use of recombinant human GH results in antibody development* Monitoring of GH and glucose is ideal (though measurement of GH is impractical)* Long term monitoring of dose and effectiveness can be achieved with measurement of IGF-1* Progestagens can be used to stimulate mammary secretion of GH * Medroxyprogesterone acetate * Improvements in hair coat and some increases in body size can be seen with 3-weekly injections * IGF-1 increased sharply, while GH did not exceed normal reference ranges. * Less risk of GH excess and DM * Risks: pyoderma, pyometra, skeletal maldevelopment, mammary tumour, acromegaly* Thyroxine * Started once hypothyroidism is evident * Minimal risks with levothyroxine supplementation

Question 18

Describe the process of storage and release of vasopressin. Include the pathway of release and organ involvement

Answer 18

• Pre-pro-vasopressin is produced within the hypothalamus* The signal peptide is cleaved and pro-vasopressin is transported to the endoplasmic reticulum* The pro-hormone is transported via the hypothalamic-neurohypophyseal tract to be stored within the posterior pituitary gland* During transport and storage, pro-vasopressin is cleaved into the constituent peptides: * ADH / vasopressin * Neurophysin 2 - necessary for correct cleavage and to prevent enzymatic degradation * Copeptin (Glycoprotein) - role unknown

Question 19

Note the factors that cause stimulation of vasopressin release.What provides inhibitory inputs to vasopressin release?

Answer 19

• The major stimulus for release is an increase in plasma osmolality* Circumventricular organs act as osmoreceptors and can alter vasopressin release and directly affect thirst* High pressure baroreceptors in the aortic arch and low pressure receptors in the atria can inhibit vasopressin release * Acts via the glosopharyngeal and vagus nerves respectively* Note: the half life of vasopressin is ~ 6 minutes, so after correction of osmolality, the plasma concentration drops quickly* Other factors that can influence vasopressin release include: * stress * nausea * pain * structural brain disease * drugs / medications * hypoglycaemia * Exercise

Question 20

Describe the process by which ADH effects water resorption in the collecting duct

Answer 20

• Vasopressin binds to V2 receptors on the basolateral surface of the collecting duct epithelial cells within the kidney* V2 binds activates G-protein pathways, increasing cAMP and activates protein kinase A* This activation leads to binding of vasicles containing aquaporin 2 to the apical surface * Increased expression of the aquaporin 2 receptor * Increased passive movement of free water from the hypotonic lumen to the isotonic cortex or hypertonic medullary interstitium

Question 21

Vasopressin’s primary function is considered the mediation of water resorption within the collection duct.Describe the other functions of ADH on various target organs

Answer 21

1. Release of vWF2. Stimulation of NO release into the circulation3. Increase concentrations of Factor VIII4. Smooth muscle contraction * Via V1a receptors5. Glycogenolysis6. Augmentation of ACTH release * V1b receptors in the anterior pituitary7. Release of catecholamine and insulin * V1b receptors in the adrenal gland and pancreas8. Neurotransmitter within the brain - numerous effects

Question 22

What are the potential causes of diabetes insipidus in dogs and cats?

Answer 22

1. Central diabetes insipidus * Lack of ADH production * Congenital * Head trauma (most common in cats) * Post-surgery - hypophysectomy * Neoplasia with posterior pituitary damage (most common in dogs * Idiopathic, infection and inflammatory causes have been suggested2. Nephrogenic diabetes insipidus * Decreased action of AVP at the collecting duct * Lack of or ineffective V2 receptors * Likely to be the most common * X-linked in people and a litter or Siberian Huskies (males affected) * 10x less binding affinity between V2 and ADH * Lack of aquaporin 2 channel expression

Question 23

Describe the pathophysiology of nephrogenic diabetes insipidus in dogs.

Answer 23

• Nephrogenic diabetes is caused by decreased expression in the aquaporin 2 channel * The decreased expression could be due to defects in the channel production * Most commonly there is decreased expression of the channel due to abnormalities in the vinding of ADH to the V2 receptor * V2 receptor mutations result in decreased ADH binding afinity* Nephrogenic diabetes insipidus causes increase water loss from the kidney causing primary polyuria* Secondary polydipsia occurs due to increased plasma osmolality

Question 24

What are the common clinical signs of diabetes insipidus?What are the pathophysiological causes for the potential neurological signs?

Answer 24

• Severe polyuria and polydipsia is the most common clinical sign* Water may be ingested in preference to food leading to weight loss* Excessive drinking may be followed by vomiting* Nocturia and incontinence may occur secondary to the production of vast quantities of urine* Neurological signs in association with DI is common in dogs - common secondary to neoplasia causing destruction or compression of the pituitary gland/hypothalamus* Variations in access to water can cause neurological signs * Water restriction - rapid hypertonic dehydration - osmotic demyelination * Access to water after restriction - rapid change / decrease in osmolality can lead to cerebral oedema* Other endocrine deficiencies may also be present with neoplasia

Question 25

What abnormalities may be seen on routine clinicopathological testing in dogs or cats with diabetes insipidus?

Answer 25

• No abnormalites may be seen if water has not been restricted* Low BUN due to medullary solute washout* With water restriction, hyperosmolar dehydration * Increase sodium and HCT +/- increased chloride*

Question 26

What are the difficulties in differentiating central diabetes insipidus and primary polydipsia?

Answer 26

• The clinical signs and initial clinicopathological testing can be identical* There are similarites in the pathophysiology of two conditions, with AVP release* AVP is absent in CDI* AVP is present, but regulation can be abnormal in PP* People with PP have been shown to have altered rates and set points for AVP release* Dogs with primary polydipsia should concentrate urine with a modified water deprivation test alone

Question 27

Briefly describe the MWDT specifically noting the risks.Why is the ADH response test more appropriate and at least as uesful?

Answer 27

• 3-5 days of slow and gradual water deprivation. * Dry food only * Water spread out over the day in small quantities * Day 1: 120-150 ml/kg * Day 2: 80-100 ml/kg * Day 3: 60-80 ml/kg * This should improve the medullary concentrating gradient* Remove water - hospitalise - monitor body weight after emptying the bladder * Measure USG, HCT, TP, sodium and urea* Monitor above parameters q 1-2 hours* After loss of 5% of body weight, administer desmopressin IV * Recheck USG +/- HCT/TP q 30 minutes for 2 hours, then each hour for 8 hours.Interpretation:* Increase USG with water deprivvation alone - primary polydipsia* Increase in USG after ADH - central diabetes insipidus* No response to ADH - nephrogenic diabetes insipidus

Question 28

Apart from the water deprivation test and desmopressin response test, how else can a potential diagnosis of DI be investigated?

Answer 28

• MRI of the pituitary * Absence of a hyperintense signal in the sella turcica suggests absence of ADH * Hyperintense signal reflects the phospholipid of the secretory granules within the neurohypophysis* Measurement of serum copeptin * Cleavage product of the pro-AVP molecule * Not been assessed in dogs/cats, but useful in humans

Question 29

What is the calcium sensing receptor and where are these receptors located

Answer 29

• The calcium sensing receptor is responsible for monitoring and regulating calcium homeostasis* They are located within the parathyroid gland, kidney and bone (and cartilage)* The calcium sensing receptor interacts with the parathyroid gland to regulate the release of PTH

Question 30

What is iCa?What is the difference between total calcium and iCa?Discuss the relevance in clinical testing

Answer 30

• iCa is the biologically active form of calcium in the serum* The total calcium is the combination of ionised or free calcium (~50% of total), calcium bound to albumin (~40%) and the calcium bound to other anions such as bicarbonate or citrate (~ 10%)* The ratio between total calcium and ionised calcium is generally maintained, so testing of either can be appropriate for diagnosing a calcium abnormality* Total calcium can be lowered by haemolysis, lipemia and hypoalbuminaemia* Ionised calcium concentration can be altered by sample pH, either in disease or due to storage/handling * A reduced pH will cause increase in the iCa * An increase in pH will cause a reduction in iCa

Question 31

Describe the synthesis and secretion pattern of parathyroid hormone.By what mechanisms does high or low serum calcium affect the release PTH

Answer 31

• PTH is continuously synthesised within the chief cells of the parathyroid gland. PTH is then stored in secretory granules within the cytoplasm* PTH is metabolised quickly and continuously* When the calcium levels are low, the rate of granule degradation is low* When calcium levels are high, degradation is accellerated* Vitamin D also influences PTH gene transcription * High Vitamin D slows PTH transcription* Elevated phosphorus slows PTH degradation and enhances secretion* High calcium is sensed by the calcium sensing receptor causing activation. The CaSR is G-protein coupled to gene expression * Activation of the CaSR causes a decrease in PTH production* Low calcium causes inactivation of the calcium sensing receptor * Increased PTH gene expression, and parathyroid gland proliferation

Question 32

Briefly describe the pathway of vitamin D synthesis.

Answer 32

• Cholecalciferol is synthesised in the skin during UVB exposured* Cholecalciferol is hydroxylated to 25-hydroxycholecalciferol by 25-hydroxylase witrhin the liver * This step is not regulated and 25-OH-vitamin D3 levels largely reflect the production of cholecalciferol* 25-OH-cholecalciferol is further hydroxylated to active calcitriol (1,25-OH-cholecalciferol) by alpha-1-hydroxylase. * This conversion occurs within the kidney and is regulated by PTH (increased) and phosphate (supressed)

Question 33

How does vitamin D help to regulate PTH secretion and calcium metabolism?

Answer 33

• The active form of vitamin D, calcitriol, exerts the majority of its effect by enhancing the absorption of calcium from the GIT. Increased calcium due to increased GIT uptake is only one of 3 major components of calcium homeostasis and PTH function* PTH mediates: * Production of calcitriol within the kidney * Enhances or suppresses the activity of osteoclasts * The resoption of calcium in the distal convoluted tubule and thick ascending loop of Henle

Question 34

Calcitonin helps to regulate calcium levels.Briefly describe the production, release and role of calcitonin in calcium homeostasis

Answer 34

• Calcitonin is produced by the C cells in the thyroid gland* Calcitonin has a role in limiting GIT calcium absorption in the post-prandial period, enhance calciuresis and decrease osteoclast activity* Calcitonin can counteract the effects of PTH and can reduce calcium levelsNote:* Medullary thyroid tumours of the C cells, producing excessive calcitonin does not disrupt calcium homeostasis.* The role of calcitonin is relatively minor

Question 35

What are the three major mechanisms of action of parathyroid hormone that work synergistically to increase serum calcium concentration?

Answer 35

1. Increase calcium reabsorption in the distal convoluted tubule and ascending loop of Henle2. Enhance the hydroxylation of 25,OH,vitamin D to calcitriol in the kidney * Calcitriol then enhances calcium absorption in the gut3. Enchance the number of osteoclasts on bone surfaces, thus increasing bone resorption.

Question 36

What is the mechanism of polyuria and polydipsia with primary hyperparathyroidism?

Answer 36

• Hyperparathyroidism leads to hypercalcaemia* High calcium antagonizes the effects of ADH on the collecting ducts* HyperCa inhibits tubular uptake of sodium and chloride, inhibiting urinary concentrating mechanisms* Polyuria is the primary process, with secondary polydipsia

Question 37

Describe the reasons why urinary calculi are common in dogs with primary hyperparathyroidism.

Answer 37

• PTH causes an increased uptake of calcium from the renal tubules, however, dogs with high PTH and hypercalcaemia still excrete increased amounts of calcium in the urine (calciuresis)* Phosphate excretion is increased by PTH* Supersaturation of the urine with calcium and phosphorus can lead to crystal and stone formation * calcium phosphate stones* Increased calcium absorption from the gut lumen also enhances absorption of oxalate * Increased oxalate absorption leads to increased oxalate excretion in the urine * calcium oxalate stones

Question 38

List and discuss the various treatment options for management of hypercalcaemia (PHPT or hypercalcaemia of malignancy)

Answer 38

1. Fluid therapy * saline fluid therapy enhances calciuresis * sodium competes with calcium ions, reducing tubular reuptake2. Frusemide * Use once hydrated3. Glucocorticoids * No effect for PHPT * Reduce PTHrP by inducing tumour lysis or reduced sysnthesis and secretion * Increase renal calcium loss, decrease GIT absorption, decrease bone resorption4. Bisphosphonates * Inhibit osteoclastic activity and induce osteoclast apoptosis * Primarily used for HOM or for bone pain associated with osteolytic bone lesions.5. Calcitonin * inhibits osteoclastic activity and inhibits renal reabsorption of calcium6. Surgery * Parathyroid tumour removal will reduced calcium levels rapidly

Question 39

What are the common aetiologies for hypoparathyroidism in dogs and cats?

Answer 39

• Post cervical trauma or surgery* Immune mediated - majority of cases - presumed due to presence of lymphocytic +/- plasmacytic infiltrate on necropsy* Idiopathic

Question 40

Describe the clinical signs expected with hypoparathyroidismWhat is the typical time frame leading to clinical presentation?

Answer 40

• The clinical signs of hypoparathyroidism are primarily caused by persistently low calcium levels.* Neuromuscular signs tend to predominate* Signs can include: * muscle cramping, fasciculations and pain * may affect behaviour * Tremors / shaking * Stiff or stilted gait * Jaw champing * Facial pruritis * Seizures* The clinical signs are often present for days to weeks prior to presentation. Median ~ 2 weeks. Clinical signs may be acute in onset and occasionally have been present for > 1 year prior to presentation

Question 41

What are the differentials for hypocalcaemia - common and rare?

Answer 41

1. Eclampsia2. Hypoparathyroidism * post surgery, trauma or immune mediated/idiopathic3. AKI4. CKD5. Pancreatitis6. Diabetes mellitus7. Malabsorption syndromes8. Urinary tract obstruction9. Phosphorus containing enemas

Question 42

Discuss the acute management of dogs with clinical signs and hypocalcaemia

Answer 42

• If signs are severe, then treatment with an IV calcium gluconate bolus is recommended (together with IV fluids) * Followed by a Ca Gluconate CRI* Monitor the ECG during the initial bolus - over 10-15 minutes.* Continue calcium infusion for 1-3 days with ~ q 12 hour calcium monitoring* Oral calcium supplementation can be provided* Oral calcitriol supplementation should be commenced * Calcitriol is then used medium to long term depending on the inciting cause for the hypocalcaemia and if the condition is permanent.

Question 43

What are the chronic treatment options for dogs or cats with hypoparathyroidism?Briefly note the mechanism of action of treatment?

Answer 43

• Complete lack of PTH leads to hypocalcaemia due to the following mechanisms: * reduced bone resorption * reduced calcitriol formation in the kidney - reduced GIT absorption * Reduced Ca reabsorption fro, the kidney* Supplementation with Vitamin D3 (calcitriol) increased the absorption of calcium from the GIT * Initially calcium may be supplemented, but generally this is not required longer term * For most dogs/cats, compounded calcitriol needs to be used to avoid inaccurate dosing * Consistent choice of experienced pharmacy is important* Aim for calcium to be low normal or just below the reference range * Avoids hypocalcaemia clinical signs * Minimises the risk of vitamin D toxicosis (hyperCa, HyperPO4 and AKI) * Low phosphate diets would be recommended as lack of PTH causes increased PO4 resorption in the kidney

Question 44

What are the treatment goals for hypoparathyroidism and how are they monitored?

Answer 44

• Treatment is directed at maintaining calcium levels above the level that could result in clinical signs.* A target in the low normal range or just below the normal range is ideal* The target range aims to reduce calciuresis and the risk of calcium containing urolith formation* Aiming for a low end calcium level also reduces the total supplementation of Vitamin D, limiting the risk of vitamin D toxicosis

Question 45

Describ the basic production and regulation of thyroid hormone

Answer 45

• Iodide enters the body in food.* Iodide is transported into the follicular cells via the Na⁺/I^- symporter* Iodide is then transported into the colloid across the apical membrane* Thyroid peroxidases catalyses the majority of reactions within the follicular cell including: * Oxidation of to iodide to iodine * Iodine reacts with thyroglobulin to form iodotyrosines * TPO also catalyses reactions to form the thyroid hormones T4, T3, * Thyroglobulin is cleaved to form either mono- or di-iodotyrosines * These then combine to form either T3 or T4* The thyroid hormones are release from the thyroid gland after stimulation by TSH from the pituitary* TSH is under the control of TRH, released from the hypothalamus* Negative feedback occurs at all levels, with increased T3 and T4 reducing secretion of both TSH and TRH

Question 46

Hypothyroidism has been described in dogs with lymphocytic thyroiditis and thyroid atrophy in ~equal amounts?Explain the histological findings in each

Answer 46

• Thyroid atrophy is an “end stage” type change with marked reduction in the number of thyroid follicles and follicular cells* There is gradual replacement of the normal thyroid tissue with adipose and collagen* Lymphocytic thyroiditis is a destructive immune process with a marked cellular infiltrate with lymphocytes, macrophages and plasma cells.* With progression of the inflammatory process, there is replacement of normal thyroid cells with fibrous connective tissue

Question 47

Describe the presence of thyroglobulin auto-antibodies in dogs.How does the presence of autoantibodies correlate with clinical hypothyroidism?

Answer 47

• Thyroglobulin auto-antibodies are present when the adaptive immune system identifies thyroglobulin as non-self.* The presence of thyroglobulin auto-antibodies +/- antibodies directed against T3 or T4 are indicative of an auto-immune thyroiditis* These antibodies are present in ~ 50% of dogs with clinical hypothyroidism.* However, the presence of auto-antibodies does not necessarily mean hypothyroidism will develop * Only ~ 20% of euthyroid dogs with circulating auto-antibodies will go on to develop hormonal evidence of thyroid dysfunction * Only 5% become clinically hypothyroid

Question 48

List and briefly describe the potential clinical signs with hypothyroidism

Answer 48

1. Dermatological signs * Very common and reported in ~80% of hypothyroid dogs2. Cardiovascular effects * Theoretically reduced cardiac systolic function3. Neuromuscular abnormalities * Many are difficult to prove a direct link * Cranial nerve and peripheral nerve abnormalities have tenuous direct link and would be difficult to prove causation even with treatment. eg. megaoesophagus4. Ophthalmic changes * Minimal evidence but may be associated with KCS5. Reproductive changes * gestation may be increased, birth weights reduced and pup survival decrease * no change in the inter-oestrus period

Question 49

What are the common abnormalities on routine clinicopathological testing of dogs with hypothyroidism?

Answer 49

• Low total and free T4* Anaemia - usually mild and non-regenerative * Likely due to combined decrease in EPO and reduced marrow stimulation* Hypercholesterolaemia (75%) * Reduced degradation of lipids/lipoproteins* High triglycerides * in conjunction with elevated cholesterol* Increased CK * mild increases only and may be due to low grade myopathy +/- reduced renal clearance* (Increased fructosamine) * Due to reduced protein turnover, rather than prolonged hyperglycaemia

Question 50

What assay methadologies are commercially available to assess thyroid hormone levels.Briefly note the limitations of each methodology

Answer 50

1. Radioimmunoassay (RIA) * Considered gold-standard * Requires the use of radio-isotopes and cannot be fully automated2. Liquid chromatography mass spectrometry * Research utilised3. Chemiluminescence * Most commonly used by commercial laboratories * Rapid and fully automated testing possible * Auto-antibodies may cause spuriously increased results4. Enzymatic methods * Least accurate, rarely used5. In-house ELISA have show variable and often innacurate results in multiple studies

Question 51

What drugs have been associated with changes in thyroid hormone concentrations?Briefly note the changes that may be seen.

Answer 51

1. Prednisolone * Reduced to normal TT4 and free T4 * Normal to increased TSH2. Phenobarbital * Decreased to normal TT4 and Free T4 * Normal to mild increase in TSH3. Sulfonamides * Decreases in TT4 and free T4 * Increased TSH4. Clomipramine * Decreased TT4 and free T4 * No change to TSH5. Aspirin * Decreased TT4 with decreased to normal free T4 * No change in TSH6. Carprofen * As for prednisolone

Question 52

Hypothyroidism in cats is most often iatrogenic and caused by treatment with I131.How can feline iatrogenic hypothyroidism be identified and confirmed?

Answer 52

• The majority of cats receiving I 131 are likely to develop transient hypothyroidism. * New research is underway in an attempt to determine I 131 dose from tumour volume as determined by scintigraphy * Individualised I 131 doses should reduce the incidence of post-treatment hypothyroidism* Low TT4 should be present in all hypothyroid cats* Free T4 measurement may not add significantly to information from TT4 alone* cTSH can be measured and should be increased - more for identification of spontaneous hypoT4* Scintigraphy can assist in confirming hypothyroidism and differentiate from sick euthyroid syndrome (non-thyroidal illness syndrome)

Question 53

When and how should iatrogenic hypothyroidism be managed in cats following I131 treatment?

Answer 53

• Iatrogenic hypothyroidism is common if not expected in a majority of cats following I 131 treatment* If renal function is adequate, then treatment is rarely required.* It is recommended to wait and monitor for 3-6 months as long as renal function is adequate and there are no overt clinical signs of hypothyroidism * It must be noted that some of the clinical signs of hypothyroidism overlap with the changes seen after successful management of hyperthyroidism * weight gain, reduced drinking, reduced activity* If there is an azotaemia (IRIS stage II-III) noted post treatment, then L-thyroxin supplementation is essential

Question 54

What are the proposed causes of hyperthyroidism in cats?

Answer 54

1. TSH receptor mutation * Leads to high constitutive secretion of T42. cAMP activating G protein alpha subunit mutation * Increased activation and release3. Decreases in expression of inhibitory G proteins * Decreased ability to inhibit cAMP production resulting in sustained release of T44. Canned foods implicated but not proven5. PBDE’s (polybrominated diphenyl ethers) implicated but not proven

Question 55

Describe the effects of thyroid hormone on the feline cardiovascular system

Answer 55

Most effects are regulated byT3* Positive chronotropic effect * AV conduction time is reduced * Upregulation of myocardial beta adrenergic receptors* Positive inotropic effect * Regulated through ion channel alterations that ultimately enhance the activity of myosin* Myocardial hypertrophy due to upregulation of myocardial protein expression +/- increased blood pressure

Question 56

Hyperthyroidism may or may not cause elevated blood pressure.Discuss the relevant evidence

Answer 56

• Early studies (1990) identified a high prevalence of hypertension in cats with hyperthyroidism - 87% * The normal was not stated in the study and hypertension was likely adjudged at a lower level that accepted today * White coat effect in “stress intolerant” hyperthyroid cats is real and difficult to quantify* Later reports have identified hypertension in 5-20% of cats * Small abstracts identified no improvement in blood pressure with appropriate treatment of hyperthyroidism * No cats had BP >180 mmHg* Humans with thyrotoxicosis rarely have elevated blood pressure. * When present, it is usually only systolic hypertension as thyroid hormone causes peripheral vasodilatation* There does appear to be an association between hyperthyroidism and hypertension, but cause and effect proof is not been established

Question 57

Discuss the effects of hyperthyroidism on the urinary system

Answer 57

• Polyuria and polydipsia * Primary polydipsia may be present due to an exaggerated thirst response to changes in osmolarity * Thyroid hormone excess may cause down-regulation of aquaporin channels* Thyroid hormone decreases peripheral vascular resistance - relaxation of the capilliary smooth muscle * This leads to increased renal blood flow * Nitric oxide synthase activity is increased –> increased NO –> augmented vasodilation * Increased beta-adrenergic receptors also assists vasodilatation* RAAS may be activated by the vasodilation and increased renal blood flow. Afferent arteriolar vasoconstriction increased glomerular filtration pressures and GFR* Tubular effects * Upregulation of chloride channels - enhanced resorption of chloride, lower chloride in the macula densa, increased tubuloglomerular feedback and increased GFR * Increase sodium potassium ATPase and Na+/H+ exchange * enhanced Ca++/Na+ exchange and calcium resorption

Question 58

Describe the process and interpretation of the T3 suppression testWhen may this test be indicated?

Answer 58

1. Blood is collected, serum separated and frozen2. The following day, T3 is administered at 25 mg/cat PO q 8 hours for 7 total doses3. Blood is drawn on the morning of the third day, after the final T3 dose4. Both assays are run together, to mitigate inter-assay variation_Interpretation:_* ​​​Low T3 in the second sample - poor owner compliance* Low T4, High T3: not hyperthyroid* High T3 no or blunted decrease in T4: HyperthyroidIndicated for assessment of suspected hyperthyroid cats with normal basal hormone results

Question 59

Describe the mechanism of action of methimazole for the treatment of feline hyperthyroidism

Answer 59

• Methimazole (and the pro-drug carbimazole) inhibit thyroid follicular cell thyroperoxidases* Inhibition of iodination of tyrosyl residues into thyroglobin and coupling of tyrosyl residues into T3 and T4* The drug contributes to a decrease in thyroid hormone production depleting stores

Question 60

Briefly note the major differences and considerations when using transdermal methimazole in place of the oral medication

Answer 60

• Gloves must be worn as the drug can traverse human skin* It may be slower to reach euthyroidism, therefore monitoring can start at ~ 4 weeks* Response can be less consistent than with the oral drug, and overall, transdermal methimazole is likely to be less effective* Reduced GIT adverse effects should be expected with the transdermal formulation* All other adverse effects from the drug - neutropenia, hepatotoxicity and facial excoriation are similar

Question 61

Discuss the potential role of TSH in thyroid tumour development in dogs?

Answer 61

• TSH directly stimulates thyroid cell mitogenesis (cell division) and hormone synthesis* TSH has been shown to induce angiogenesis in thyroid tumour cell lines* Excessive TSH has been shown to cause benign toxic nodular goiter in humans* ~ 50% of a beagle colony developed thyroid tumours after the induction of lymphocytic thyroiditis - which leads to decreased negative feedback and persistently elevated TSH.* VEGF is a target for TSH activation fo angiogenesis* Mutations in the TSH receptor leading to constitutive G protein signalling has also been linked to hyperthyroidism

Question 62

Describe the clinical staging of canine thyroid carcinoma

Answer 62

• Tumours are staged based on: * Tumour size * Presence of local lymph node involvement * Presence of distant metastasis* Stage 1: < 2 cm, no mets* Stage 2: < 2 cm and ipsilateral local LN involvement OR 2-5 cm without LN involvement* Stage 3: > 5 cm with local node involvement, NO distant metastasis* Stage 4: > 5 cm, local and distant metastatic disease

Question 63

List the treatment options for thyroid carcinoma in order of relevance. Note the specific indications and contra-indications for each treatment modality

Answer 63

1. Surgery: treatment of choice for mobile tumours without distant metastatic disease. Local lymph nodes should be removed if considered abnormal on staging2. Radiation therapy: Primarily used when surgery is contraindicated or if the tumour is large and adhered to underlying structures.3. ¹³¹Iodine: primarily used in humans following surgery. Limited studies in dogs with variable study protocols, so comparison is not straighforward. Appears best utilised post-surgery in dogs with stage II-IV disease. Large doses mat be of safety concern4. Chemotherapy: limited data available, but cisplatin/carboplatin single agent therapy may improve survival. Best used as an adjunct post-surgery in dogs at risk of metastasis or recurrence.

Question 64

What is the prognosis for dogs with thyroid carcinoma?

Answer 64

• Prognosis is highly dependent on stage of disease, histopathology (especially evidence of invasiveness) and treatment modality* Prognosis for grade I and II tumours following complete surgical excision is generally good at ~ 3 years for unilateral disease or 30-39 months for bilateral disease. Ectopic thyroid carcinoma may be less amenable to surgery* Radiation treatment of stage I-III tumours carries a favourable prognosis with 1 and 3 year progression free survival of 80% and 72% respectively.* Palliation or local therapy alone in the presence of metastatic disease, survival of 6-8 months has been reported

Question 65

Discuss the base pathology for insulinoma and describe the staging of such tumours

Answer 65

• Most insulin secreting tumours are malignant carcinomas* Most pancreatic tumours arise from the B cells within the islet of Langerhans (insulin producing cells)* ~80% are solitary and metastasis to the local lymph nodes and liver are common at the time of diagnosis (~45-64%)_Tumour Staging:_1. Solitary pancreatic tumour2. Pancreatic tumour with lymph node metastasis3. Pancreatic tumour with distant organ metastasis +/- lymph node metastasis.

Question 66

Describe the physiological pathway of insulin secretion including the regulatory mechanisms.What changes occur to this pathway with insulinoma?

Answer 66

• Glucose is the primary regulator of insulin secretion. ie. increased serum glucose will lead to insulin secretion via exocytosis* Glucose enters the cell via GLUT transporter* Glucose is phosphorylated to pyruvate and enters the citric acid cycle generating ATP* ATP closes ATP-sensitive K+ channel resulting in an increased membrane polarity and subsequent depolarisation* Membrane depolarisation activates calcium channels and there is an influx of calcium ions* Calcium plays a role in vesicle docking to the cell membrane and exocytosis of insulin_Insulinoma:_Constitutive production and release of insulin without glucose as a trigger. In normal cells, insulin release is completely inhibited by serum glucose levels below 80 mg/dL (4.4 mmol/L)

Question 67

What are the counter-regulatory hormones secreted in response to hypoglycaemia

Answer 67

1. Glucagon2. Catecholamines3. Glucocorticoids4. Growth hormone

Question 68

Discuss the mechanism of action of the 4 counter-regulatory hormones released in response to hypoglycaemia

Answer 68

1. Glucagon * Increased gluconeogenesis in the liver * Decreased glycolysis and increased glycogenolysis * Decreased lipogenesis and increased lipolysis - conversion of fatty acids to acetyl CoA or ketones2. Catecholamines * Essentially similar effects on glucose regulation as glucagon * effects primarily mediated within the liver3. Corticosteroids * Effects are widespread - liver, pancreas, muscle and fat * Decreased GLUT2 and glucokinase - reduced insulin uptake and metabolism by beta cells * Decreased insulin secretion * Liver - Increased gluconeogenesis and lipogenesis and decreased insulin sensitivity * Increased insulin sensitivty in adipose tissues with inhibition of lipolysis4. Growth Hormone * Direct antagonism of insulin effects * Other effects are similar to that of glucagon. * Stimulates IGF-1 which stimulates the insulin receptor and provides direct counter-regulation to the effects of GH

Question 69

What clinical signs are most common with insulinoma?Why are these clinical signs most evident?

Answer 69

• The clinical signs of insulinoma relate directly to the presence of hypoglycaemia* Neurological signs predoiminante with weakness, ataxia, collapse, seizures, disorientation and dullness * The brain does not store significant carbohydrates, nor is it able to utilise fuels other than glucose well for energy generation. Thus the brain is most sensitive to the effects of hypoglycaemia* Counter regulatory mechanisms including increased catecholamines may also result in clinical signs * nervousness, tremours, hunger, weakness

Question 70

Consider and note the various mechanisms of hypoglycaemia.Provide examples of each cause for hypoglycaemia

Answer 70

1. Increased insulin or insulin-like factor production * Insulinoma * extra-pancreatic tumour * beta-cell hyperplasia2. Decreased glucose production * Hepatic insufficiency * Hypoadrenocorticism * GH deficiency * Hypopituitarism * Glycogen storage diseases * Neonates and toy breeds3. Excessive glucose consumption * Sepsis * Hunting dogs / extreme exercise4. Drug related * sulfonamides, oral hypoglycaemics, insulin, salicylates, acetaminophen, ethanol, monoamine oxidase inhibitors, ACEIs, tetracycline5. Spurious / testing error * storage with red blood cells * polycythaemia

Question 71

What is the expected prognosis following surgery for insulinoma in dogs?

Answer 71

• Approximately 80% of dogs become euglycaemic immediately following surgery* Median period of normoglycaemia is ~ 14 months* Median survival in a recent retrospective study was 2 years / 746 days* ~ 8 month survival following hypoglycaemia relapse* Stage is the major predictor of survival

Question 72

Discuss the use of imaging in the diagnosis of insulinoma

Answer 72

• Radiographs are typically normal or unremarkable.* Abdominal ultrasound has a variable sensitivity and specificity in the detection of insulinoma. * Old reports suggest a sensitivity of ~ 50-60% with abdominal ultrasound * Nodular hyperplasia may be over-interpreted * Metastatic disease may or may not be identified * US guided fine needle aspiration can be useful at identifying metastatic disease or primary insulinoma* MRI has been described with good sensitivty but variable appearance of insulinoma* CT - Triple phase CT is recommended as there is variable hpyo- and hyper-attenuation during the arterial and pancreatic/tissue phases. Sensitivity is very good but correct identification of left or right lobe is not perfect.* As there is no pathognomonic imaging findings for canine insulinoma, the interpretation must incorporate the clinical and clinicopathological findings.

Question 73

Discuss the acute management of clinical hypoglycaemia secondary to insulinoma

Answer 73

• Clinical hypoglycaemia typically manifests with neurological signs including weakness, collapse, mental dullness and seizure * The signs are primarily caused by neuroglyopenia * The problem is most often slowly progressive and chronic* An initial dextrose bolus followed by a CRI of 2.5-5% dextrose is typically effective at rapidly resolving the clinical signs* The dextrose administration should stop with resolution of clinical signs even if mild hypoglycaemia persists* As neoplastic beta cells continue to response to glucose normally, provision of glucose can lead to increased insulin release and paradoxical hypoglycaemia may ensue* Glucagon CRI following a glucagon bolus can be effective - single case report in 2000, 20 dogs reported more recently across 2 studies.* If dextrose fails to resolve signs, then dexamethasone or octreotide may be beneficial. Glucagon is likely indicated prior to octreotide

Question 74

Discuss the definitive and longer term management of dogs with insulinoma

Answer 74

• Surgery is recommended for all dogs with stage I and II disease. Surgery is likely to be beneficial for dogs with stage III disease also, though the prognosis is worse and return to eugylcaemia less likely.* ~ 10% of dogs will develop diabetes mellitus for a variable period of time following surgery. Insulin therapy is required for a variable period as a result* Persistent or recurrent hypoglycaemia is most often managed initially with prednisolone starting at a dose of ~ 0.5 mg/kg/day* Dietary management with small frequent meals and a high protein, moderate fat and high complex carbohydrates (low simple carbohydrates) is recommended* Other medications of potential benefit or to reduce the prednisolone requirement include: * diazoxide * octreotide

Question 75

Describe the mechanism of action, indications and adverse effects of diazoxide.

Answer 75

• Diazoxide is a benzothiadiazine derivative* Inhibits closure of beta-cell ATP-dependent K+ channels * Inhibits cellular depolarisation * Inhibits opening of the calcium channel* Reduced calcium influx reduces the rate of insulin exocytosis* Diazoxide also increased glycogenolysis and gluconeogenesis within the liver* Tissue uptake of glucose is inhibited* Responses are variable with ~70% of dogs responding to doses of 10-40 mg/kg/day* Adverse effects are uncommon and include vomiting anorexia and ptyalism.

Question 76

Discuss the potential primary causes of diabetes mellitus in dogs

Answer 76

• The underlying cause of beta cell dysfunction in dogs is not fully established and many possible causes have been identified* Protective and susceptible genotypes have been identified * Mutations in the MHC II genes - DLA (dog leukocyte antigen) have been identified* Mutations have been identified in the canine insulin gene* Most often the histological changes include: * Reduced beta cell number within islets * Reduced pancreatic islet size and number * Beta cell enlargement, vacuolisation and degeneration * Occasionally lymphocytic infiltates have been reported together with antibodies directed against islet cells, insulin and proinsulin.* The role of autoimmunity has not been fully established

Question 77

Note the potential causes of secondary diabetes mellitus in dogs.Include the mechanism of action of each cause in the answer

Answer 77

• The most common cause of secondary pancreatitis is the destruction of normal pancreatic islets by acute or chronic severe pancreatitis * Pancreatits has been identified in 30-40% of newly diagnosed diabetic dogs * Though, this may be more a trigger for DKA, than the initiating event causing pancreatits* Diestrus elevations in progesterone can contribute to diabetes mellitus * Diestrus results in elevated progesterone * Progesterone stimulates release of GH from the mammary tissue * Both Progesterone and GH antagonise the effects of insulin * This condition of insulin resistance can lead to overt DM * Reversiblity of DM relies on early diagnosis and removal of the source of progesterone (ovohysterectomy)* Glucocorticoid use can result in transient insulin resistance and diabetes. This is usually transient and resolves with cessation of the exogenous glucocorticoids

Question 78

Describe the pathophysiological effects of insulin deficiency

Answer 78

• Decreased tissue utilisation of glucose, amino acids and fatty acids* Accelerated glyogenolysis and gluconeogenesis * End result is hyperglycaemia* Catabolic state with increased proteolysis and muscle breakdown* As glucose absorption continues as normal, hyperglycaemia leads to glycosuria as the renal resorption of glucose threshold is reached. * Osmotic diuresis and water loss * Secondary increases in osmolarity and stimulation of thirst receptors in the hypothalamus (mediated by ATII)* Increased catabolism and decreased energy utilisation triggers hunger resulting in polyphagia* Leptin levels are decreased in the absence of insulin, releasing hunger inhibition

Question 79

Note the pathophysiological consequences of insulin deficiency

Answer 79

• Cachexia due to catabolism* Increased hunger as a result of negative energy balance and inadequate utilisation of calories* Increased lipolysis * increased free fatty acids undergo metabolism to form acetyl CoA to supply citric acid cycle * increased ketones via metabolism of excess acetyl-CoA* Increased fatty acid synthesis leads to increased hepatic uptake of fatty acids * Increase synthesis of triglycerides and VLDL * Hyperlipidaemia and hepatic lipidosis

Question 80

Discuss the progression from mild transient hyperglycaemia through to diabetic ketoacidosis in newly diabetic dogs.Include reference to urinalysis results in the answer

Answer 80

• Stress induced hyperglycaemia is rare in dogs* Mild hyperglycaemia can be caused by DM or concurrent disease that antagonises the effect of insulin* Mild to moderate and persistent hyperglycaemia is necessary to establish a diagnosis * Glycosuria must be present to establish the diagnosis * Glycosuria in the absence of hyperglycaemia suggests a renal tubular disease * Fanconi syndrome, primary renal glyucosuria, AKI, nephrotoxin* With persistent moderate hyperglycaemia and low insulin, ketogenesis occurs and ketosis will be seen. Ketosis will lead to ketouria * Low to moderate 3-beta hydroxybutyrate suggests diabetic ketosis * Moderate to high 3BH together with ketouria and a metabolic acidosis is consistent with DKA

Question 81

Discuss the essential diagnostic tests to establish a diagnosis of diabetes mellitus.What additional tests may be useful to completely investigate a newly diagnosed diabetic dog?

Answer 81

Diagnosis:* Hyperglycaemia and glucosuria are necessary to establish a diagnosis * Other changes on biochemical testing may be seen due to secondary hepatic vacuolar change or with concurrent disease_Additional Tests:_* The need for additional testing largely depends on the clinical presentation of the dog. An otherwise healthy dog with PU/PD or an “incidental” diagnosis on routine testing may not require further tests* Urine culture is generally recommended at the outset* Abdominal ultrasound is indicated when DKA is present or with additional systemic signs of illness (vomiting/diarrhoea, abdominal pain)* cPL may be recommended with suspected pancreatitis* Special considerations should be given to intact females - assessment for pyometra, ovarian cysts, increased progesterone

Question 82

Discuss the dietary recommendations for dogs with diabetes mellitus

Answer 82

1. Feed two equal sized meals at 12 hour intervals around the time of insulin administration * timing of insulin administration can be variable and dependent on individual appetites and eating behaviour2. Meals must be complete and balanced and CONSISTENT3. Aim for a caloric intake that maintains a near ideal weight * weight loss diets should be considered for obese dogs to help minimise insulin resistance4. Majority of calories from complex carbohydrate and protein5. Minimise fat content of diet to limit increases in cholesterol, free fatty acids, triglycerides and glycerol * Fat restricted diets are essential if there is a pancreatitis history6. Some studies suggest high fibre diets help to regulate glycaemic control7. The diet should also consider concurrent conditions and balanced appropriately - eg. food intolerance/IBD, renal disease

Question 83

Briefly describe the recommended exercise regime for dogs with diabetes mellitus, noting the physiological reasons for benefit.What should be avoided?If strenuous exercise is expected, should the insulin dose be adjusted?

Answer 83

• Regular and consistent exercise is to be encouraged.* Exercise helps with glycaemic control in many ways * increases glucose utilisation, limiting hyperglycaemia * Increases blood flow and thus distribution of insulin * Stimulates translocation of the glucose transporter GLUT-4 in muscle cells* Sporadic and strenuous exercise should be avoided if possible - ie. no walks during the week, and hard work on the weekend* Owners should consider reducing the insulin dose if strenuous exercise is anticipated. Monitor for signs of hypoglycaemia or PU/PD.

Question 84

List the various insulin types that may be utilised for canine diabetes mellitus.Identify the duration of effect of each

Answer 84

1. Lente insulin - Porcine - intermediate acting * Caninsulin: 8-14 hours2. NPH - recombinant human - intermediate acting * Humulin * Duration of effect: 4-10 hours3. Protamine Zinc Insulin (PZI) - recombinant human - long acting * ProZinc * Duration of effect: 10-16 hours4. Glargine / Detemir - recombinant human - long acting * Duration of effect: 8-16 hours * Designed to maintain basal insulin requirements in humans * A few small studies have suggested adequate control with a flat or peakless curve

Question 85

Describe the mechanism of action that enables prolonged activity of both insulin glargine and insulin detemir

Answer 85

Glargine:* Structure: asparagine replaces glycine at A21 and two positively charged arginine molecules are added to C-terminus of the B chain * Complete solubility at pH 4, low aqueous solubility at neutral pH* SC injection - neutral milieu - forms insulin analogue microprecipitates - slows absorption* pH dependent, so cannot be mixed / diluted_Detemir:_* Structure threonine removed from B30 and a 14 carbon fatty acid bound to the B29 lysine* The added fatty acid chain allows the insulin to bind reversibly to albumin* Albumin binding slows the absorption and prolongs the metabolic activity of the insulin* Significantly more potent (~4 x) than other insulin preparations in dogs

Question 86

Discuss the mechanisms by which insulin may be less effective in the presence of concurrent conditionsNote an example of a condition that could contribute to each

Answer 86

1. Alterations in insulin metabolism (pre-receptor) * Development of insulin auto-antibodies2. Alterations in insulin receptor binding (receptor) * Reduced binding affinity * Reduced receptor number * Obesity - in normal animals results in a state of hyperinsulinism and subsequent down-regulation of insulin receptors. However maximal insulin effect is maintained until the insulin receptor number is reduced > 90% from normal3. Interference with the signalling cascade (post-receptor) * The same antagonistic conditions that result in reduced receptor number likely also affect insulin signal transduction

Question 87

Discuss the interaction between and inhibitory effects of growth hormone on the effects of insulin

Answer 87

• The interaction between growth hormone and insulin is complex* GH induces IGF-1 release, which has insulin like activities * lack of GH causes an insulin resistance type state due to lack of IGF-1 effects* GH suppresses the expression of GLUT4 transporter on the cell membrane * via upregulation of a molecule that inhibits translocation* GH increases lipolysis and free fatty acid production * FFA inhibit the effect of the insulin receptor * FFA promotes hepatic lipid oxidation and production of acetyl-CoA * Precursor to gluconeogenesis via citric acid cycle * Lipid intermediates from triglyceride production (from FFA) impede insulin signalling pathways

Question 88

What is fructosamine?How do fructosamine levels relate to glycaemic control?How is fructosamine helpful in monitoring diabetes mellitus in dogs?

Answer 88

• Fructosamines are glycated proteins produced by irreversible, non-enzymatic reactions between glucose and plasma proteins.* Fructosamines are therefore a marker of the average blood glucose over the lifetime of the particular plasma protein* The variable binding proteins lead to variable estimates on the time frame reflected by fructosamine* An estimate of 1-3 weeks is often suggested* Low fructosamine suggests prolonged periods of hypoglycaemia over the previous 1-3 weeks* Elevated fructosamine suggests prolonged periods of hyperglycaemia over the previous 1-3 weeks* Fructosamine can be used in place of blood glucose curves in stressed patients that do not tolerate repeated blood draw.* I would suggest that glucose sensor information reduces the issues of hospital / needle associated stress and reduces the utility of fructosamine monitoring.

Question 89

List the causes for ineffective control of diabetes mellitus

Answer 89

1. Insulin / technical issues * Denatured insulin / incorrect handling * Draw up / injection technique2. Insulin underdose3. insulin overdose and Somogyi response * Caused by either hypoglycaemia or a rapid drop in blood glucose4. Short duration of insulin effect5. Prolonged duration of insulin effect6. Development of insulin antibodies * Uncommon with canine/porcine/human insulin * Common with beef based insulin7. Concurrent disorder causing insulin resistance

Question 90

List the common concurrent disorders that contibute to insulin resistance and poor glycaemic control

Answer 90

Any inflammatory, infectious, neoplastic or endocrine disorder could cause insulin resistance1. Hyperadrenocorticism * Including iatrogenic HAC due to iatrogenic corticosteroid use2. Diestrus3. Hypothyroidism4. Chronic pancreatitis5. Chronic kidney disease6. Infection * urinary tract / oral7. Neoplasia8. Hyperlipidaemia9. Obesity

Question 91

List the potential mechanisms for development of Type 2 DM in cats

Answer 91

1. Insulin resistance * Obestiy if the major cause of insulin resistance in cats * Insulin sensitivity is to some degree genetically determined * Cats with DM are ~ 6 times less sensitive to insulin - increased hepatic glucose production, decreases tissue utilisation of glucose2. Reduced insulin secretion * beta cell failure may be related to the chronic inflammatory state of obesity - immune cells can be found in tissue involved in energy homeostasis (liver, pancreas, muscle, fat) * Increased expression of IL-1b has been documented3. Islet amyloid polypeptide (IAPP or Amylin) * Amylin is secreted with and modulates the effects of insulin * Increase amylin secretion in idividuals with insulin resistance * Amylin may fold into beta-pleated sheets forming toxic intracellular aggregates and fibrils * Trigger for inflammation and beta cell death4. Oxidative damage * Pancreatic beta cells have limited antioxidant activity and are susceptible to oxidative damage * Glucotoxicity and lipotoxicity increase cellular respiration in beta cells leading to increased production of reactive oxygen species * Oxidative stress down regulates insulin and amylin and upregultes inflammatory or apoptotic pathways

Question 92

Describe the glucose tolerance test together with interpretation of test results

Answer 92

• In humans, there are well established guidelines for assessment of blood glucose following oral glucose bolus * The test is indicated when routine BG and urine screening are not consistent with a diagnosis of DM* In cats, an IV glucose bolus (0.5 mg/kg or 1.0 mg/kg) is administered with monitoring of the blood glucose at 2 and 3 hours post administration depending on the size of the glucose bolus* Normal cats should have a BG < 9.8 mmol/L at 2 hours* Normal cats should have a BG < 6.5 mmol/L at 3 hoursElevated results can suggest pre-diabetes.

Question 93

Define the difference between DK and DKA.At what point do DK and DKA become evident after the onset of overt diabetes mellitus in cats

Answer 93

• DK is the presence of increased blood ketones - b-hydroxybutyrate with normal blood pH. Ketouria may or may not be present.* DKA is the progression from a state of DK to metabolic acidosis. This is most often triggered by a concurrent disease process* Ketosis will generally develop within 10-30 days of a cat becoming insulinopenic (diabetic) * Ketosis will not develop unless the BG is > 17 mmol/L* Ketouria developed after a meas of 23 days of persistent marked hyperglycaemia in one study with wide variation

Question 94

Diabetic remission is the hope when initiating treatment of a new diabetic cat.What factors have been associated with an increased chance of achieving diabetic remission?

Answer 94

1. Use of a low carbohydrate diet2. Use of long acting insulin - glargine or detemir3. Tight glycaemic control within first 6 months of tx4. Older cat (likely slower progression to over disease)5. Recent corticosteroid tx (likely unmaksing pre-diabetic condition6. Absence of neuropathy7. Lower mean blood glucose (milder disease)8. Lower cholesterol

Question 95

Discuss the dietary risk factors for the development of diabetes mellitus in cats.

Answer 95

• Obesity is the single largest risk factor for the development of diabetes in cats * Cats fed high fat diets (eg. wet food) had a 3 times greater risk of diabetes * Cats fed a high carbohydrate diet (>50% ME from carb) had a 2 times greater risk of diabetes* High carbohydrate diets carry the risk of significant post-prandial hyperglycaemia * Significant increase with 47% carbohydrate food versus high protein (46% protein, 27% carbohydrate* Insulin levels follow the post-prandial hyperglycaemia * Regularly elevated glucose and insulin can lead to obesity, insulin resistance, and decreased beta cell function* The magnitude and duration of post-prandial hyperglycaemia is exacerbated by weight gain - ie. obesity contributes to insulin resistance and persistent hyperglycaemia*

Question 96

Discuss the recommendations for maintenance of cats at risk of diabetes or in a pre-DM state

Answer 96

• In a young, healthy and lean adult cat, a moderate carbohydrate diet is likely to be safe and well tolerated. Excessive intake leading to obesity should obviously be avoided* High carbohydrate diets (>50%) are unlikely to contain sufficient protein and are not recommended* For cats with an increased risk of diabetes, carbohydrate content should be < 12%* Older cats or cats with conditions that could predispose to diabetes (breed, corticosteroid use, obesity) would also benefit for low carbohydrate diets

Question 97

Discuss the dietary recommendations for cats with diabetes mellitus with reference to the aim of achieving diabetic remission

Answer 97

• The highest remission rates (~80%) have been achieved in cats fed a fiet where < 6% of metabolizable energy was derived from carbohydrates.* Remission rates between cats fed a 12% CHO (low fibre) food and 26% CHO (moderate fibre) food were 68% and 41% respectively - Bennet et al 2006* Systematic review in 2014 suggested CHO restriction was beneficial but no single factor was identified that predicted diabetic remission

Question 98

List the basic mechanisms of action of oral hypoglycemic medications

Answer 98

1. Reduce or slow glucose absorption from the intestine2. Increase insulin secretion3. Increase insulin sensitivity

Question 99

List the classes of oral hypoglycemic drugs the have been assessed in cats.Give an example of each

Answer 99

1. Sulfonylureas * Glipizide * Binds to ATP dependent potassium channel - closure of which triggers depolarisation and calcium influx - insulin release2. Meglitinides * Nateglinide * Similar action to the sulfonylureas but bind to different sites on the ATPase3. Biguanides * Metformin * Insulin sensitizer, mechanism of action not fully known4. Thiazolidinedione * Darglitazone * Binds in the nucleus and improves insulin sensitivity in muscle, adipose and liver. Reduces gluconeogenesis5. Alpha-glucosidase inhibitors * Acarbose * Inhibits membrane bound brush border enzymes in the SI * Useful when administered with a high carbohydrate food, but this combination was not as good as a low carbohydrate food alone.

Question 100

What are incretins?

Answer 100

• Incretins are hormones produced within the gastrointestinal tract that are rapidly release in response to food intake. * Glucagon-like peptide * Gastric inhibitory peptide* They have numerous effects that synergise to stimulate insulin release, reduce hyperglycaemia and limit glucose intake

Question 101

What are three potential therapeutic options to optimise incretin activity

Answer 101

• Primary incretin therapy - often utilised in an extended release form due to rapid degradation by dipeptidylpeptidases (DPP-4)* GLP-1 agonist drugs - resistant to DPP-4 degradation* DPP-2 inhibitors - extend the activity of naturally occuring GLP-1

Question 102

Discuss the evidence for the use of extended release exenatide in cats with newly diagnosed diabetes mellitus

Answer 102

• The drug can be administered as a long-acting once weekly or new once monthly SC injection * Previous short acting versions were twice daily* The drug is a GLP-1 receptor agonist* Shown to reduce weight gain after commencing diabetes treatment with insulin* Reduced the dose of insulin required for adequate glycemic control* Generally well tolerated, though vomiting and decreased appetite was reported * Both would be expected given the mechanism of action - increases nausea and satiety* A placebo controlled study in 2016 revealed a tendency for increased weight loss in obese cats, but insulin requirement unchanged.

Question 103

What factors stimulate or suppress the release of CRH from the hypothalamus?

Answer 103

Stimulation:* Cytokines * IL-1, IL-6, TNF-a* Leptin* Dopamine* Vasopressin* Angiotensin II_Inhibition:_* Corticosteroids* Somatostatin

Question 104

POMC is the precursor molecule for ACTH.Other biologically active metabolites are also produced when POMC is cleaved. What are these substances and briefly note their effect

Answer 104

1. beta-lipotropin * mechanism of action is unknwon2. beta-endorphin * Likely act as an endogenous opiate with a role in pain alleviation3. Melanocte stimulating hormone * Involved in the secretion of melanin in the skin4. CLIP (corticotropin-like intermediate peptide) * Insulin secretagogue * Also increases exocrine pancreatic function

Question 105

What are the three zones of the adrenal cortex?What substances do each of the three regions secrete?

Answer 105

1. Zona glomerulosa - outer most * Mineralocorticoids2. Zona fasiculata - middle3. Zona reticularis - inner most * Both the fasciulata and reticularis contain 17-a hydroxylase necessary for synthesis of cortisol and androgens

Question 106

The pituitary theory is most supported as the mechanism of development of PDHWhat is the pituitary theory and describe the support for the theory

Answer 106

• The pituitary theory is most likely accurate* Somatic mutation of a corticotroph produces a tumour clone* The majority of dogs with PDH have reduced CRH production * Likely due to negative feedback from consistently elevated ACTH and cortisol levels* Most pituitary adenomas are monnoclonal suggesting they arise from a single cell* Most adenomas contain stem cells that drive growth and most are likely the result of a multi-stage process of tumourogenesis

Question 107

Functional Adrenal Tumour have a variable response to ACTH.With reference to the literature, discuss why and how adrenal tumours may respond to ACTH

Answer 107

• Functional adrenal tumours tend to secrete cortisol autonomously and excessively * Circulating ACTH and CRH levels are suppressed* The role of ACTH receptors in the function of functional adrenal tumours varies.* Human studies suggest that steroidogenesis requires ACTH even in adrenal tumours* With regards to the ACTH stimulation test: * Some tumours do not respond at all to ACTH * Some have a normal response * Some tumours show an exaggerated response as may be expected with PDH* Upregulation of ACTH receptors does not appear to involved in the pathogenesis, but down regulation may be involved in malignancy * Malignant FAT’s may be less likely to respond to ACTH due to receptor down-regulation

Question 108

What is the proposed mechanism of polyuria and polydipsia in dogs with hyperadrenocorticism?

Answer 108

• Corticosteroids potentiate sodium and chloride retention* This leads to expansion of the effective circulating volume * Increased blood pressure * Increased glomeurlar filtration rate* Cortisol appears to exert a negative feedback effect on ADH release, similarly to the negative effect on other hypothalamic / pituitary hormones* Cortisol also antagonises the effect of ADH at the collecting duct, reducing free water resorption, causing primary polyuria* Hypernatremia can also stimulate primary polydipsia

Question 109

UTI is common in dogs with uncontrolled HAC - what is the reported incidence?What mechanisms may lead to the development of UTI in dogs?

Answer 109

• The most often quoted incidence of UTI at the time of diagnosis comes from a 1990 paper that assessed 42 dogs with HAC, DM or both. * This paper reported an incidence of 40-50% UTI* A large cohort controlled study (1519 HAC dogs from 1984-2004) assess the presence of UTI at the time of death * They reported a 6.6% incidence, significantly higher than dogs without HAC* Poor urine concentration - reduced bactericidal properties* Immunosuppressive effects of elevated cortisol levels* Urine retention * Due to increased urine production * Decreased bladder tone / function* Urine incontinence

Question 110

What are the potential causes for excessive panting in dogs with hyperadrenocorticism

Answer 110

• Respiratory muscle weakness* Reduced pulmonary compliance* Secondary to pulmonary hypertension / vasocontriction* Cortisole has a direct effects on the respiratory centre * Stress and fear have been show to increase tidal volume in humans with activation of the thalamus and other central regions of the brain.

Question 111

What are the common clinicopathological abnormalities commonly seen with newly diagnosed HAC in dogs?

Answer 111

​Haematology:* Lymphopenia - steroid lympholysis* Eosinopenia - bone marrow sequestration* Neutrophilia / monocytosis - steroid enhanced capilliary margination* Thrombocytosis - marrow stimulation* Erythrocytosis_Biochemistry:_* Elevated liver enzymes * ALKP (85-95%) - primarily due to induction * ALT mild increases likely secondary to vacuolar changes or blood flow changes * Cholesterol and triglycerides - increased lipolysis* Normal or mildly elevated glucose * Gluconeogenesis and insulin resistance* BUN - 30-50% low due to diuresis* Phosphate - High or low * Low due to renal excretion * High with renal secondary hyperparathyroidism due to calciuresis* Bile acids and cPLi may be increased - potentially not specifically due to hepatic dysfunction or overt pancreatitis

Question 112

Discuss the reasons for a hypercoagulable state in dogs with uncontrolled hyperadrenocorticism

Answer 112

• Numerous small studies have assessed coagulability in dogs with newly diagnosed and controlled HAC* Most studies indicate hypercoagulability in ~ 80-90% of dogs with newly diagnosed HAC and these persist despite adequate treatment_Documented abnormalities:_* Increased fibrinogen* Increased thrombin-anti-thrombin complexes* Decreased anti-thrombin (possibly due to urinary protein loss - or utilisation by thrombin binding)* Increased platelet aggregation* TEG - Increased angle and MA and reduced K * Suggests more rapid and larger/stronger clot formation * Some dogs show reduced fibrinolysis parameters* Thrombocytosis may play a roleNo specific coagulation abnormality is able to predict others

Question 113

Discuss the relationship and relevance of urinary protein loss in hyperadrenocorticism

Answer 113

• ~ 70% of dogs have an elevated UPC (>0.5) at the time of diagnosis* ~ 45% are increased above 1.0 * Note that the presence of urinary tract infection may spuriously increase these results, though the sediment is usually inactive* The UP:C may be increased due to elevated GFR (~60% of dogs) - may be caused by the increased ECV and BP* The GFR reduces with treatment, so UP:C should also reduce with treatment_Clinical relevance:_* To date, there are no studies that suggest a specific clinical relevance to the elevated UPC* No specific treatment is recommended for the elevated UP:C

Question 114

List the screening tests used to investigate for hyperadrenocorticism in dogs with a strong suspicion of diseaseIndicate the sensitivity and specificity of each test

Answer 114

1. Urinary cortisol:creatinine ratio * Very high sensitivity - approaching 100% * Low specificity of ~ 20-77% * PPV of 90% when the UCCR is >100 (ref. < 10)2. ACTH stimulation test * Sensitivity of ~85% for PDH and 60% for AT * Specificity is 85-90% for both3. LDDST * Sensitivity is ~ 90-95% for PDH and ~100% for AT * Specificity is highly variable and dependent on the patient population * With low prevalence, then specificity may be as low as 40-50% * With a high prevalence, the specificity is much improved4. UCCR/LDDST combined

Question 115

Discuss the interpretation of LDDST results.What criteria are used to diagnose HAC?What criteria can be used for distinguishing PDH from AT?

Answer 115

• Blood is collected for cortisol readings at baseline, 4 hours and 8 hours post IV dexamethasone administration* Normal dog will have cortisol results < cut-off for 24-48 hours post ACTH* 4 or 8 hour results above the laboratory cut off are supportive of HACPDH can be supported by any of the following: 60-65% of dogs with PDH will display one or more of the following.1. Suppression at 4 hours (below cut-off) with loss of suppression at 8 hours result > lab cut-off = HAC2. 4 hour cortisol < 50% of baseline3. 8 hour cortisol < 50% of baseline

Question 116

List the available tests that can differentiate between PDH and a functional AT as cause for hyperadrenocorticism in dogs.Describe the interpretation and utility of the tests

Answer 116

1. HDDST * ~ an additional 10% of dogs that do not suppress with the LDDST will suppress with HDDST. * Lack of suppression does not exclude PDH or prove AT2. Endogenous ACTH * Should be elevatated with PDH and undetectable with a functional AT * ~ 20% will be non-diagnostic (between low and elevated) * Sample handling can result in spuriously low results3. Imaging - abdominal US * PDH - normal to hypertrophied, bilaterally symmetrical or not. * Nodular hyperplasia maybe difficult to interpret * AT - mass present, contralateral gland < 5 mm4. Advanced imaging * ~ 50% of adrenal microadenomas are not detected on advanced imaging (3-4 mm tumour size) * Most indicated when there is evidence of PDH and concurrent neurological signs * Larger tumours can be detected by either modality, though MRI better estimates size and impact on surrounding structures

Question 117

What is the mechanism of action of trilostane?

Answer 117

• Trilostane is a synthetic steroid analogue* Acts as a competitive inhibitor of 3-beta-hydroxysteroid dehydrogenase* Blocks conversion of pregnenelone to progesterone* Blocks the conversion of 17-hydroxy pregnenolone to 17-hydroxy progesterone also* Peak action occurs at ~ 2 hours with complete metabolism within 10-18 hours * Hence twice daily dosing being preferred* 11 beta hydroxylase may also be inhibited, reducing conversion of cortisone to cortisol

Question 118

Discuss the indications for radiation therapy and the expected outcomes

Answer 118

• Radiation therapy is generally reserved for dogs with a macroadenoma.* Tumour size > 8 mm has been suggested as an indicated for radiation treatment* While plasma endogenous ACTH is expected to reduce in all dogs, the clinical improvement in signs of HAC may only improve in ~ 50% * One study of 6 dogs revealed resolution of HAC signs in 3/6 dogs, but return of signs in 2 of those dogs. * While the tumours do consistently reduce in size, the functionality of a smaller tumour can persist* Radiation is most useful at preventing the signs associated with a pituitary mass* Requirement for medical management of the hyperadrenocorticism should be expected.* Radiation takes months to cause a reduced tumour size

Question 119

Contrast and compare the clinical signs of HAC in cats with those in dogs

Answer 119

• The majority of cats diagnosed with HAC have concurrent diabetes mellitus (79%)* PU/PD/PP are the most common metabolic signs, similar to dog * However, these signs are most likely to be attributed to the poor control of diabetes * PU/PD does not appear to be a feature of the disease unless there is concurrent DM or CKD * Low USG is not common unless associated with DM or CKD* Pendulous abdomen and muscle loss are common, as for dogs* Skin fragility syndrome appears to be an extreme presentation dermal atrophy that is not seen in dogs * Calcinosis cutis has not been reported in cats * Alopecia is uncommon

Question 120

List the endocrine tests available to investigate hyperadrenocorticism in cats.Note the utility, sensitivity and specificity of each

Answer 120

1. Baseline cortisol - not useful * Normal values less established that for dogs2. ACTH stimulation test * Timing of peak varies, so post-ACTH samples should be collected at 60 and 90 minutes * Dose of 5 mcg/kg or 0.125 mcg/cat * Sensitivity from 56-80% * post-ACTH levels of 400-600 are “grey” but could be positive with other supportive clinical signs3. Dexamethasone suppression test * Normal canine dose 0.01 mg/kg fails to suppress 20% of normal cats (reduced specificity) * Higher dose of 0.1 mg/kg is recommended, but some cats with PDH remain suppressed at 8 hours (reduced sensitivity)4. UCCR * Expected to be a similar test as for dogs with excellent sensitivity and poor specificity * Assay variabilities may limit the usefulness

Question 121

Discuss the diagnostic approach and consideration of differential diagnoses after identifying an incidental adrenal mass

Answer 121

• An incidental adrenal mass may be identified by either US or CT and may or may not be the reason for the initial imaging investigation * Benign or malignant non-functional tumours are unlikely to cause clinical signs unless they invade or compress adjacent vasculature, rupture or become necrotic * Functional tumours may or may not cause clinical signs or be high on the differential diagnosis list and be the reason for imaging investigation* The differentials for an adrenal mass include both cortical (carcinoma, adenoma, aldosteronoma) and medullary (phaeochromocytoma) tumoours. These may be functional or non-functional* Further investigations may include blood and urine testing to assess electrolytes and renal function for possible aldosterone producing tumours or plasma or urine catecholamines for phaeochromocytoma* Fine needle aspiration could be considered pending size* Thoracic imaging may be recommended* Repeat US monitoring in 1-3 months if the mass was truly incidental and non-clinical* All investigations would need to consider the liklihood that the adrenal mass is the reason for initial investigation. ie. consideration of and the importance of concurrent conditions must be taken into account.

Question 122

Discuss the role and regulation of aldosterone

Answer 122

• Aldosterone is produced by the zona glomerulosa in the outer most layer of the adrenal cortex. * Also produced in the heart, brain and vascular tissues where it likely acts in a paracrine manner* Aldosterone primarily acts to balance sodium and potassium concentrations and preserve ECV​Regulation:* Aldosterone is released directly in response to an increase in potassium - most potent stimulator* Activation of the RAAS via reduced ECV or BP will trigger aldosterone production (increase Angiotensin II)* Reduced atrial stretch can trigger aldosterone release_Specific Actions:_* Aldosterone blocks potassium resorption and triggers sodium reabsorption in the distal convoluted tubule and collecting duct - acts via the Na+/K+ pump on the basolateral surface. * creates a potassium concentration gradient for diffusion into the luminal space* Increases ATP dependent H+ secretion in the collecting duct* Sodium preservation in the gut, sweat, saliva * Increases in sodium indirectly cause increased free water preservation and expansion of the ECV

Question 123

What are the potential causes of hyperaldosteronism in cats and dogs?How might primary and secondary hyperaldosteronism be differentiated?

Answer 123

• Hyperaldosteronism can be primary or secondary_Primary:* Aldosterone producing tumour - autonomous secretion * High aldosterone, low renin_Secondary:* Congestive heart failure - reduced ECV, systemic hypotension* Chronic kidney disease * High aldosterone, high renin

Question 124

Describe the history and clinical signs in cats with primary hyperaldosteronism

Answer 124

• Most cats are older with a median age of 13 y.* Clinical signs relate to the presence of low potassium and or high blood pressure_Low potassium:* ​Muscle weakness, especially cervical ventroflexion* Episodic or acute signs* Difficulty jumping* Listlessness and ataxia* Limb rigidity - uncommon* Dysphagia - uncommon_High blood pressure:* Acute blindness or ocular colour change (due to hyphema) is the most common sign* Neruological signs - ataxia, seizure, mentation changes may be seen with intracerebral vascular accident (haemorrhage or ischaemia)

Question 125

Describe the typical clinicopathological findings in cats with primary or secondary hyperaldosteronism

Answer 125

Primary:* Primary hyperaldosteronism is characterised by a low potassium* Increased CK is common due to hypokalaemic myopathy* Metabolic alkalosis is common due to aldosterone mediated increased H+ excretion (via ATP dependent excretion in the distal convoluted tubule)* Elevated BUN and creatinine can be seen secondary to primary hyperaldosteronism* Increased serum aldosterone on specific testing_Secondary:_* Elevated BUN and creatinine (and SDMA) in cats with primary renal disease * Hyperaldosteronism will result with potassium retention or mineralocorticoid receptor bloackade * Hyperaldosteronism can also increase due to dehydration and reduction in the ECV

Question 126

Describe the interpretation of the urinary aldosterone to creatinine ratio.

Answer 126

• This test theoretically assesses urinary excretion of aldosterone over time (compared with creatinine) and may be a more accurate reflection of aldosterone levels than a single serum sample* However, aldosterone is primarily metabolised within the liver and urinary excretion is minimal. * only 3 of 9 cats with primary hyperaldosteronism had elevated UACR in one study (JVIM 2013)

Question 127

Discuss the merit of the fludrocortisone suppression test in cats suspected of having primary hyperaldosteronism.What are the risks and limitations of the test?

Answer 127

• As primary hyperaldosteronism is a condition of autonomous aldosterone secretion, confimation via a suppression test is ideal* Fludrocortisone promotes retention of sodium and water, increasing ECV * This effect would normally suppress release of aldosterone in the absence of hyperkalaemia (which would be a contraindication for the test in the first place)* The suppression test requires oral administration of fludrocortisone orally q 12 hours for 4 days* In normal cats, this should reduce UACR by > 50%.* With primary hyperaldosteronism, the UACR should be unchanged, though 4/9 were suppressed in one study_Risks/limitations:_* Plasma basal aldosterone was more definitive in one small study* Fludrocortisone can enhance hypokalaemia in cats with the disease or induce hypokalaemia in normal cats* Minimal studies have been performed, so the most appropriate protocol has not been established

Question 128

Surgery is the treatment of choice for most cats with primary hyperaldosteronism.Discuss the role of medical management in cats with primary hyperaldosteronism

Answer 128

• Medical management can be utilised as sole treatment or to help stabilise a cat with PHA prior to surgery* Treatment is aimed at minimising the effects of increased aldosterone, high blood pressure and low potassium_Treatment:_* Potassium supplementation * 2-6 mEq PO q 12 hours* Spironolactone (aldosterone receptor blockade) * 2-4 mg/kg PO q 24 hours* Amlodipine (calcium channel blocker) * 0.625-2.5 mg mg PO q 24 hours * To help lower blood pressure with dose titrated to effect starting at the low end* Monitoring involves measurements of potassium, blood pressure and intermittent ultrasound to evaluate the adrenal mass. As renal function can be affected by high aldosterone levels, renal function should also be monitored and management with a renal diet commenced initially.

Question 129

Describe the eitiology of phaeochromocytoma

Answer 129

• Phaeochromocytoma is a tumour of the chromaffin cells within the adrenal medulla* The tumour is of neuroendocrine cells* Aetiology in dogs is largely unknown* In humans, phaeochromocytoma has been associated with a missense mutation in the succinate dehydrogenase subunit D * There is high homology between SDHD in humans and dogs

Question 130

Describe the pathway of catecholamine production within the adrenal medulla.

Answer 130

• Tyrosine enters the chromaffin cells* Tyrosine converted to L-dihydroxyphenylalanine (L-DOPA) * Tyrosine hydroxylase * This is the rate limiting step and norepinephrine exerts negative feedback on tyrosine hydroxylase* L-DOPA is converted to dopamine * DOPA debarboxylase* Dopamine is converted to noradrenaline * Dopamine beta-hydroxylase* In the adrenal medulla, PNMT converts noradrenaline to adrenaline * Note that PNMT is only present in the adrenal medulla and gene expression is influenced by cortisol

Question 131

Describe the process of catecholamine release from the adrenal gland

Answer 131

• Catecholamines are stored within vesicles within the medullary chromaffin cells* They are in an active equilibrium with catecholamines in the surrounding cytoplasm* Catecholamines “leak” from their storage granules and a methylated within the cytoplasm and small amounts can leak into the circulation as free metanephrine and free normetanephrine * The methylated catecholamines are produced within the adrenal gland* Norepinephrine and epinephrine are released into the circulation via exocytosis from storage vesicles after stimulation by the sympathetic nervous system - via acetylcholine

Question 132

How can catecholamine levels be measured?Which test is most reliable in the diagnosis of a phaeochromocytoma

Answer 132

• Catecholamines can be measured either within plasma or urine * The catecholamine metabolites NMN and MN are primarily excreted within the urine * NMN and MN are more reliable and consistent measures of catecholamine production than the parent compounds* Plasma metaneprhine and normetanephine levels more accurately reflect tumour mass * epinephrine and norepinephrine are secreted in an episodic manner and are rapidly metabolised* Urinary metanephrine and normetanephrine should be compared with creatinine within the same sample* In both urinary and plasma measurements, normetaneprhine is a more reliable indicator than metanephine, * This suggests that most produce and secrete norepinephine preferentially or in excess of epinephrine

Question 133

What are the potential clinical manifestations of a phaeochromocytoma

Answer 133

• The clinical signs either relate to the space occupying mass or the physiological action of increased catecholamines* Hormone secretion is sporadic and inconsistent, therefore the presenting signs also vary* Despite this, the majority of dogs have an unremarkable physical examination_Catecholamine effects:* Cardiovascular * Hypertension, tachycardia, panting, tachpnoea, arrhythmia, collapse, pallor * Secondary to hypertension - retinal detachment, hyphaemia, vascular accident* Neuromuscular * Weakness, tremors, anxiety, pacing, seizures* Non-specific * PU/PD, vomiting, inappetance, weigh loss, vomiting, diarrhoea, abdominal pain_Abdominal mass* Invasion of the CVC - ascites, hind limb oedema* Invasion of the aorta - cold hind limbs, claudication/pain* Spontaneous rupture - haemabdomen

Question 134

Describe the mechanism of action of phenoxybenzamine.How and why is the drug used in the management of phaeochromocytoma?

Answer 134

• Phenoxybenzamine is a long-acting alpha-adrenergic antagonist.* This blocks the actions of norepinephrine and epinephrine at the alpha receptors * Alpha 1 adrenergic receptors mediate: * Vasoconstriction - increased peripheral resistance * Increased caridac output * Increased blood pressure * Stimulates gluconeogenesis * Alpha-2 adrenergic receptors: * Inhibit release of norepinephrine, acetycholine and insulin* Pheochromocytoma is a tumour that increases release of epinephine and norepinephrine, the effects of which are mediated via both alpha and beta receptors* The alpha effects are potentially deliterious and significantly increase anaesthetic risk

Question 135

In humans, antibodies against 21-hydroxylase have been documented in most patients with hypoadrenocorticism.What reactions does the 21-hydroxylase enzyme catalyse?

Answer 135

• 21-hydroxylase catalyses the conversion of progesterone to deoxy-corticosterone. * Deoxycorticosterone is subsequently converted to corticosterone by 11b-hydroxylase and aldosterone (aldosterone synthase)* 21-hydroxylase also catalyses the conversion of 17-hydroxy progesterone to 11-deoxycortisol, which is subsequently converted to cortisol by 11b-hydroxylase* Absence of the 21-hydroxylase enxyme therefore leads to reduction in both cortisol and aldosterone and clinical signs consistent with adrenal gland failure

Question 136

Discuss the known or suspected genetic basis to the disease in dogs

Answer 136

• Certain breeds have been shown to have an increased risk * Great dane, Standard Poodle, Bearded Collie, Portugese water dog* Various studies have revealed variable answers* In poodles and the portugese water dog, it is most likely autosomal recessive with a single locus having a major effect* The MHC II coding for DLA may be involved* The cytotxic T-lymphocyte associated protein 4 may also be involved* There is no link as yet between any of the genetic variations / associations and the development of auto-antibodies directed against the adrenal gland or it’s component enzymes

Question 137

What is atypical hypoadrenocorticism?Note the natural history and link to “typical” hypoadrenocorticism

Answer 137

• Atypical hypoadrenocorticism occurs when there is a marked reduction in cortisol with maintenance of normal sodium and potassium concentrations.* In some dogs, destruction of the zona glomeurlosa lags behind the loss of the fasiculata and reticularis meanding residual aldosterone production is lost after cortisol. In this scenario, atypical hypoA becomes typical over time.* The zona glomerulosa contains less 21-hydroxylase that the other 2 layers: The immune destruction may be slower in this layer* In some dogs with atypical hypoA, aldosterone levels are reduced suggesting Na and K concentrations are not entirely dependent on aldosterone for maintenance.

Question 138

What is secondary hypoadrenocorticism?What are the potential causes of secondatry hypoadrenocorticism?

Answer 138

• Secondary hypoadrenocorticism results from defective release of ACTH from the pituitary (tertiary if there is CRH failure from the hypothalamus)* As aldosterone release is largely driven by the RAAS and alterations in potassium concentration, ACTH deficiency generally causes solitary cortisol deficiency* Causes: Any condition that causes damage to the pituitary could lead to secondary hypoA * Neoplasia * Inflammation * Infection * Infarction * Trauma * Idiopathic* Note: with most of the above, neurological signs and other hormone deficienceis would be likely.

Question 139

Discuss the most commonly identified clinical signs associated with hypoadrenocorticismExplain the cause for the signs and reason for variability

Answer 139

The signs of dogs with HypoA are often vague initially and can eventually lead to acute or sudden severe decompensation.* Anorexia* Vomiting* Lethargy / weakness / shaking* Diarrhoea* CollapseMost clinical signs are attributable to reduced cortisolWaxing and waning over time may be due to gradual loss of cortisol production over time.Eventual collapse and decompensation may or will occur with stressful conditions or if the gland eventually fails completelyWith decompensation and collapse, then secondary signs due to electrolyte imbalance and cortisol deficiency become evident (bradycardia, severe dehydration / hypovolaemia,

Question 140

Discuss the physiological abnormalities that occur secondary to mineralocorticoid and glucocorticoid deficiency

Answer 140

Glucocorticoids:* Gastric mucosal barrier function * Defective barrier function can lead to inappetance, vomiting, diarrhoea and eventual weight loss * Note: ulceration is uncommon, with haematemesis or melena not typically observed with HypoA* Blood pressure* Body temperature* Glucose concentrations * Reduced gluconeogenesis, reduced lipolysis and protein catabolism * May in part be the cause for weakness, lethargy and shaking_Mineralocorticoid:_* lack of sodium resorption - sodium loss in the urine* Increase sodium loss into the urine causes osmotic diuresis* Compensatory PD* Aldosterone triggers the Na+/K+ exchanger, so reduced sodium resoprtion is coupled to reduced potassium excretion and hyperkalaemia

Question 141

Why do we see bradycardia in dogs with acute collapse and severe decompensation due to hypoadrenocorticism

Answer 141

• Aldosterone deficiency leads to increased sodium excretion and reduced potassium excretion with resultant hyperkalaemia* Elevated potassium in the extracellular fluid reduces the concentration gradietn between intracellular and extracellular potassium in the cardiac pacemaker cells.* The outward K current is thus reduced or slowed* The outward K current is necessary for phase 3 of the action potential - rapid repolarisation* Slowing of rapid repolarisation leads to bradycardia, despite the sympathetic drive due to hypovolaemia and illness.

Question 142

What are the major findings in acutely unwell dogs with hypoadrenocorticism that may help differentiate from acutely unwell dogs with normal adrenal gland function

Answer 142

• A lack of cortisol, leads to a lack of the normal or expected stress/inflammatory response in a sick dog.* Dogs with hypoadrenocorticism have * significantly elevated lymphocyte and eosinophil counts (often within the normal range though) * Significantly lower neutrophil counts, again often within the normal range

Question 143

Whatis the utility of sodium, potassium and the sodium:potassium ratio in the diagnosis of hypoadrenocorticism

Answer 143

• Hyponatremia and Hyperkalaemia have been identified in > 80% of dogs with hypoA * HyperK - 98% * Hyponatraemia - 83%* The ratio of Na to K has been shown to be of both high sensitivity and specificity * Care to avoid over-interpretation in the face of significant haemolysis* Adler et al 2007 reported the sodium potassium ratio of 27 or 28 as a cut off with good sensitivity and specificity * 27: sensitivity 89%, specificity 97% * 28: sensitivity 93%, specificity 96%* Levels below 27 have reduced sensitivity whereas values of 29 or above have specificity < 90%* The median ratio of 73 dogs with hypoA, was 22.9. * While a ratio of 23 had a specificity ~ 100%, the sensitivity was only 74%

Question 144

List the potential biochemistry changes seen with hypoadrenocorticism.Note the incidence of each and likely cause

Answer 144

• Azotemia - increased BUN and creatinine are common * Particularly common in dogs with acute or decompensated illness, consistent with an AKI * Generally 100% reversible with rapid treatment* Acidemia * ~60% * Due to reduced H⁺ excretion, normally mediated by aldosterone (Na⁺ / H⁺ exchanger in the distal convoluted tubule)* Hypochloraemia * Reduced resorption with sodium in the kidney * Vomiting* Hypercalcaemia * Reduced calciuresis, normally mediated by cortisol * Increased liberation of Ca⁺⁺ from albumin due to acidosis (H⁺ binding to albumin)* Hypoglycaemia * ~ 20% of dogs * Likely due to cortisol deficiency, reduced gluconeogenesis and glycogenolysis * Increased insulin release does not appear to contribute * Reduced caloric intake and GIT disturbances mnay contribute* Hypoalbuminemia and hypocholesterolemia (uncommon) * Reduced GIT intake or increased GIT loss

Question 145

List the testing options for assessment of adrenal function in dogsNote the utility of each test

Answer 145

• ACTH stimulation tests * Can assess cortisol and other hormones* Endogenous ACTH assay * Typically elevated in dogs with hypoA * More often used to differentiate PDH and AT in dogs with hyperadrenocorticism* Endogenous cortisol assay * Cortisol levels display random fluctuation and some dogs without hypoA can have low baseline cortisol with normal adrenal function * Can be used to screen for hypoA - normal result excludes the diagnosis* Cortisol:endogenous ACTH ratio * Has been shown to be significantly reduced in dogs with hypoA with no overlap in the ratio with normal dogs

Question 146

Note the treatment priorities in dogs during an acute Addisonian crisis

Answer 146

• IV fluid therapy * To correct dehydration * Correction of electolyes and acid base status * Sodium chloride is generally recommended despite the low pH * Acidosis generally resolves after ~12-24 hours of diuresis * Care must be taken not to increase hyponatremia too quickly when the abnormality is severe - hypertonic saline is absolutely contraindicated* Glucose / dextrose * Indicated if hypoglycaemia is present at initiation of treatment * Can be administered if hyperkalaemia persists after 6-8 hours of fluid therapy* Insulin can be utilised if hyperkalaemia persists after glucose/dextrose and blood glucose is > 11 mmol/L* Glucocorticoids * Typically reserved until an ACTH stimulation test has been performed * While hydrocortisone is sufficient to manage cortisol deficiency, a low dose of dexamethasone is often used

Question 147

What is DOCP and how is it used in the management of hypoadrenocorticism

Answer 147

• Desoxycorticosterone pivalate is an analogue of deoxycorticosterone * Deoxycorticosterone is converted to corticosterone and finally aldosterone within the zonal glomerulosa of the adrenal gland* DOCP is available as a long-acting injection* Administered initially at 2.2 mg/kg SC each 25 days.* Electrolytes should be monitored at 14 days, with a 10% dose reduction if the Na:K is > 32* If the Na:K is > 32 on day 25, then the next injection can be delayed 5 days* The target Na:K is 29-32 and the dose can be reduced or interval extended when >32. The opposite occurs if the Na:K is < 28

Question 148

Where is secretin synthesised and released?What is the target organ?What is the target effect?

Answer 148

• Secretin is produced by the S cells located principally in the duodenum* Secretin is released in response to H⁺ ions in the gastric secretions_Target Organs:* Secretin targets the biliary tract epithelium and pancreatic ductal cells* Secretin also targets the gastric epithelium_Effects:* Increased bicarbonate synthesis from the pancreas* Increased bile production within the biliary tree* Decreased/inhibition of gastric acid secretion* Inhibition of gastric motility

Question 149

Where is glucagon synthesised and released?What is the target organ?What is the target effect?

Answer 149

• Glucagon is secreted by the pancreatic alpha islet cells* Transcription of the same gene that leads to glucagon release, produces GIP and GLP from the gastrointestinal L cells_Target Organ:* Glucagon primarily acts within the liver_Effect:* Glucagon is the primary hormone stimulus for gluconeogenesis and glycogenolysis* Overall effect is to counter the actions of insulin

Question 150

Where is glucagon-like peptide (GLP) synthesised and released?What is the target organ?What is the target effect?

Answer 150

• GLP (GLP-1 and GLP-2) are produced within the L cells in the small intestine and colon* GLP may also be secreted by neurons in the solitary tract of the brain stem* The peptide is released into the portal system_Target Organ:* Pancreas and gastrointestinal tract* Also has effects widely throughout the body * Liver - decreased gluconeogenesis * Adipose - increased glucose uptake and lipogenesis * Heart - increased glucose uptake and function * Muscle - increased glucose uptake * Brain - increase satiety and decrease appetite_Effects:* GLP primary acts to promote insulin secretion in a glucose dependent manner * Increased glucose sensitivity * Increased b-cell proliferation * Decreased glucose sensitivity of the b-cells (reduces negative feedback)* Reduced gastric motility and emptying* Other as noted above

Question 151

Where is gastric inhibitory peptide synthesised and released?What is the target organ?What is the target effect?

Answer 151

• Gastric Inhibitory Peptide or Glucose dependent Insulinotropic Polypeptide (GIP) is produced by K cells within the duodenum and jejunum_Target Organs:_Similar to GLP, GIP is an incretin with the major physiological effect exerted via stimulation of the pancreatic b-cells to release insulin_Effect:_The effects of the incretins are numerous and include:* Increased insulin production, increased b-cell proliferation, decreased b-cell apoptosis* Increased glucose utilisation in the muscle and heart* Satiety and hunger regulation in the brain with influences on hippocampal memory formation* Positive effect on bone remodelling and improved bone quality and density

Question 152

Where is gastrin synthesised and released?What is the target organ?What is the target effect?

Answer 152

• Gastrin is secreted by the neuroendocrine G cells in the gastric antrum and duodenum and pancreas* Secreted in response to gastric stretch and the presence of ingested protein_Target Organ:* Gastrin targets the parietal cells in the gastric fundus and body_Effect:* Gastrin stimulates histamine release from enterochromaffin-like cells* Stimulate the insertion of the K⁺/H⁺ pump in the apical surface of the parietal cell * Release of H⁺ ions into the gastric lumen

Question 153

Where is cholecystokinin synthesised and released?What is the target organ?What is the target effect?

Answer 153

• Synthesized in I cells located in the duodenum and jejunum* Released in response to various substances entering the duodenal lumen, namely fatty acids, H⁺ and amino acids_Target Organ:* Peptide neurotransmitter in the enteric nervous system * stimulates pre-synaptic cholinergic neurons * Effects on the gallbladder and pancreas are mediated by acetylcholine release from these enteric neurons* Gallbladder* Pancreas_Effect:* Gallbladder contraction* Release of pancreatic enzymes* Overall co-ordination of digestion * regulation of fluid secretion * inhibition of gastric emptying * Sphincter of Oddi relaxation * Stimulation of pancreatic growth

Question 154

Where is somatostatin synthesised and released?What is the target organ?What is the target effect?

Answer 154

• Somatostatin is produced within the: * Hypothalamus * Delta cells within the * Pancreatic islet cells * Subsets of neurons in the CNS and enteric nervous system* Instestinal D cells release somatostatin in response to fats, protein and bile within the gut lumen_Target Organs / effect:_ Overall inhibition of the following:* Gastric acid production* Pepsinogen* Gallbladder contraction* Insulin secretion* Exocrine pancreatic function* GIT motility* Nutrient absorption* Also inhibits the release of growth hormone, TSH and prolactin in the hypothalamus and pituitary.The overall effect of somatostatin is to slow the digestive process and reduce growth / metabolism

Question 155

Where is motilin synthesed and released?What is the target organ?What is the target effect?

Answer 155

• Synthesised in GI cells and structurally related to ghrelin* Secretion is dependent on the fasted or fed state, with the majority of action occuring during the fasted state* Release due to gastric acid or lipd entering the small intestine_Target:* Gastrointestinal tract* Pancreas* Biliary tract_Effect:* Initiates and coordinates migrating motility complexes* Help to clear the GIT in the interdigestive state* Regulates gastric, pancreatic and biliary secretions during the fed state

Question 156

Where is ghrelin synthesised and released?What is the target organ?What is the target effect?

Answer 156

• Ghrelin is synthesised and secreted by enteroendocrine cells within the stomach and pancreas* Ghrelin concentration is at its highest with an empty stomach and prior to a meal_Target:* The primary action is within the gastrointestinal tract* Also has a direct effect on the pituitary gland and hypothalamus_Effect:* Binds to L cells and augments the secretion of GLP-1 * enhanced GLP-1 secretion and similar effects* Simulates growth hormone synthesis and release* Activates the central cholinergic-dopaminergic reward system for food and addictive drugs/alcohol

Question 157

Where is serotonin synthesed and released?What is the target organ?What is the target effect?

Answer 157

• Released from the enteric neurons and enterochromaffin cells (90%) within the gastrointestinal tract* 8% is found within platelets* 1-2% within the CNS_Target / effect:_* Acts with an endocrine and paracrine function to stimulate smooth muscle contraction and intestinal secretion* In the brain, serotonin helps to regulate mood, appetite and sleep* Released from platelets, contributes to either vasoconstriction or vasodilation (via mediation of nitric oxide release)

Question 158

Describe the clinical syndrome caused by a gastrin-producing tumour

Answer 158

• The syndrome caused by gastrin hypersecretion in humans is called Zollinger-Ellison syndrome* Gastrin stimulates hypertrophy of the gastric / pyloric antrum* Gastric hyperacidity can lead to gastric ulceration and if associated with vomiting / reflux, oesophagitis may also be seen* When severe, oesophagitis may lead to regurgitation in combination with the vomiting* Diarrhoea may results due to gastrin’s inhibitory effect on intestinal water absorption

Question 159

Describe the clinical findings of dogs with gastrinoma

Answer 159

• Clinical signs largely depend on the stage of disease* Vomiting, diarrhoea and weight loss are the most common owner observations* Abdominal pain, regurgitation, GI bleeding and polydipsia may also be seen* Signs could be as for any dog with chronic vomiting, but may manifest as more severe

Question 160

Describe the potential clinicopathological findings in dogs with gastrinoma

Answer 160

• There are no specific changes on routine CBC and biochemistry testing* Leukocytosis, neutrophilia, left shift, anaemia and low protein may be seen with gastric ulceration and inflammation* Hypokalaemia, hypochloraemia and hyponatraemia may be seen with chronic vomiting* Metabolic alkalosis may be present due to loss of H⁺ in gastric secretions and vomit* Elevated liver enzymes - non-specific* Elevated bilirubin if the pancreatic mass causes biliary obstruction

Question 161

Discuss the utility of imaging, endoscopy and specific lab tests for the diagnosis of gastrinoma in dogs

Answer 161

• Radiographs - unhelpful* Ultrasound: * May identify pancreatic tumour * May identify metastasis to local lymph nodes, liver or mesentery * Gastric antral hypertrophy may be visualised* Endoscopy: can identify the secondary changes due to hypergastrinemia. Even with biopsy, the changes are not specific for gastrinoma diagnosis * Antral hypertrophy * Ulceration * Oesophagitis * Duodenal inflammation * Increased gastric luminal fluid* Gastrin levels (often paired with gastric fluid pH) * Not extensively investigated and no cut-off has been identified * Should be several-fold above the upper reference range * Finding of high gastrin, low gastric pH with clinical suspicion should be diagnostic * Gastrin can be elevated with renal disease, gastropathies, hepatopathies and with the use of acid blocking drugs* Provocative testing with secretin, calcium or both. * Either or both should lead to a two-fold increase in gastrin levels with gastrinoma

Question 162

Provide a brief overview of the clinical findings and diagnostic tests that could confirm a diagnosis of glucagonomaNote the important differentials or diseases that need to be excluded when investigating for possible glucagonoma

Answer 162

• Dogs with glucagonoma may present with insulin-resistant diabetes mellitus, lethargy, decreased appetite. Necrolytic migratory erythema is commonly reported and often considered diagnostic in people with glucagonoma. * NME is more often seen with liver disease in dogs * Increased glucagon –> increased gluconeogenesis and amino acid utilisation –> amino acid deficiency * Both metabolic liver disease responsible for so-called hepatocutaneous syndrome and glucagonoma cause amino acid deficiency, likely to contribute to NME* Ultrasound - the primary pancreatic tumour has only rarely been identified. Hepatic changes as seen in hepatocutaneous syndrome may be identified/excluded* Glucagon should be elevated, but there is no commercially available canine assay.* Amino acids have been uniformly decreased - arginine, histidine, lysine

Question 163

Explain the structure of the thyroid gland

Answer 163

Note the thyroid follocles which are cuboidal epithelial cells surrounding colloid.

C-cells are also present (responsible for calcitonin secretion)

Highly vascularised

Question 164

Outline thyroid hormone synthesis

Answer 164

1. Iodine enters the thyroid gland, co-transported with sodium via NIS
   a - The concentration gradient for sodium is generated via ATPase
2. Iodine diffuses through the cell and into the colloid mediated by counter-transport with chloride through pendrin (Chloride-iodide anti porter)
3. Iodine is coupled to thyroglobulin, a glycopeptide containing tyrosine residues produced by the sER/Golgi
4. Iodide is oxidised by thyroid peroxidase using H2O2
5. Organification (coupling of Iodine to tyrosine) occurs mediated by thyroid peroxidase
6. Pinocytosis into the cell occurs and proteases breakdown the colloid droplet to allow active thyroid hormones to be released into the blood.
7. Deiodinases release tyrosine and iodine from inactive products to allow recycling of these compounds

Question 165

Outline the different stages of thyroid iodination and number of iodine residues for each molecule

Answer 165

Question 166

What is the majority of thyroid hormone bound to in the blood?

Answer 166

Thyroxine binding globulin, it can also bind to pre-albumin and albumin

Question 167

Why does T4 likely have a longer duration of action than the more potent T3?

Answer 167

It takes longer for T4 to unbind from binding proteins in the blood.

Question 168

How does thyroid hormone expert its effect?

Answer 168

Through binding to nuclear thyroid hormone receptors which combine with the retinoid x receptor and then bind to thyroid response elements.

Question 169

What are the effects of TSH on thyroid production (5 points)

Answer 169

1. Increased proteolysis of TG
2. Increased NIS activity
3. Increased iodination of tyrosine
4. Thyroid hypertrophy
5. Increased number of thyroid cells

Question 170

What is the effect of ACTH on the adrenal cortex that results in increased steroid hormone production?

Answer 170

It enhances the activity of cholesterol desolate which is the first step in steroid synthesis converting cholesterol to pregnenolone.

Question 171

Which steps of the steroidogenic pathway does trilostane have an effect on?

Answer 171

3B hydroxysteroid dehydrogenase

Question 172

What is the role of 11BHSD?

Answer 172

It converts cortisol, which has some MC activity, into cortisone (which does not) and is present in the renal epithelium. This is why cortisol, in health, does not have a significant MC effect.

Note that 11 beta-HSD 1 has the opposite effect

Question 173

What food item may inhibit 11BHSD2?

Answer 173

Licorice, as it contains glycorrhetinic acid

Question 174

Outline the effects of MC activation and drugs that may impact on some of these effects?

Answer 174

1. Aldosterone binds to the MR which then stimulates production of NA/KATPase, ENaC and ROMK.
2. Spironolactone inhibits the MR
3. Ameloride prevents Na entry into renal cells by antagonising ENaC

Question 175

Factors and hormones leading to increased vs. decreased insulin secretion

Answer 175

Question 176

At what level of serum phosphate is all filtered PO4 re-absorbed by the kidney?

Answer 176

<1mmol/L

Question 177

What treatments have been used/described for the management of hyposomatotropism in dogs/cats?

Answer 177

Use of porcine or human GH products but a problem with these treatments is that antibody development may occur.

Progestin use to stimulate mammary GH production has not been effective in cats.

Question 178

What is the prevalence of hypersomatotropism in diabetic cats?

Answer 178

18 - 32%

Question 179

What is the gold standard for diagnosis of HS in people?

Answer 179

Oral glucose administration followed by GH measurement - this has not been described in cats. The normal response is a suppression of GH secretion in response to testing.

Question 180

Testing, other than GH/IGF-1 that may be used in diagnosis of fHS?

Answer 180

PIIIP - serum type III pro collagen polypeptide, may be up to 5x higher in fHS DM cats compared to DM cats

Gherlin - as a GH secretagogue it is lower in fHS than in healthy cats but may not be lower that DM cats. It has also been shown to increase in response to radiation therapy even when IGF-1 does not change.

GHRG stimulation/SST suppression tests (dogs)
- 1mcg/kg GHRH will NOT stimulate GH release if it is mammary driven (discriminatory test between these types)
- 10mcg/kg SST should suppress GH production in normal animals

Question 181

What drug may reduce polyphagia in fHS cats?

Answer 181

Fluoxitine

Question 182

In what scenario is gherlin an important secretagogue for GH release?

Answer 182

Possibly more important in young animals, compared with GHRH

Question 183

What phase of reproduction is GH secreted from mammary epithelium in dogs?

Answer 183

dioestrus, more often 3 - 5 weeks following oestrus

Question 184

Treatment for canine acromegaly?

Answer 184

MAMMARY DRIVEN:

OVH
Aglepristone

PITUITARY

As for cats
Pegvisomant (GH receptor antagonist)

Question 185

Which canine breed is predisposed to pituitary dwarfism? What other hormonal deficiencies are noted in this clinical scenario?

Answer 185

GSD - autosomal recessive

Will typically have combined GH, TSH and PRL deficiency, however, ACTH production is preserved

Other breeds:
- Czechoslovakian wolfdog
- Karelian Bear Dog
- Lapland reindeer dog
- Lapponian herder
- Saarloos Wolfdog
- Tibetan Terrier
- White Swiss Shepherd

Question 186

Clinical presentation of hyposomatotropism in dogs

Answer 186

Morphological Abnormalities
* Proportionate growth retardation
* Coat changes:
○ Retention of secondary (languno) hairs
○ Lack of primary guard hairs
○ Truncal alopecia (due to loss of languno hairs), sparing trunk and extremities.
○ Hyperpigmentation
○ Scale
○ Secondary bacterial infections
* Pointed muzzle
* LHX3 gene also associated with atlantoaxial joint malformation +/- instability

Reproductive Abnormalities
* Cryptorchidism (due to impaired gonadotrophs)
* Failure to ovulate

Other

May develop reduced demeanour at 2-3 years old due to hypothyroidism and impaired renal function.

Question 187

What laboratory parameter is possible in congenital hyposomatotropism in dogs?

Answer 187

Increased creatinine

Question 188

Testing to confirm a diagnosis of congenital hyposomatotropism

Answer 188

Basal hormone testing:
- IGF-1 (GH)
- TSH, TT4
Stimulation testing:
Determine basal GH and then re-measure 20 - 30 minutes after:
- GHRH
- Clonidine or xylaxine
- Gherlin

Other:
TRH stimulation
GnRH stimulation

Genetic testing: LHX3 mutation

Question 189

What advanced imaging characteristic may be seen in cases with pituitary dwarfism?

Answer 189

Pituitary cysts or hypoplasia

Question 190

Treatment of canine pituitary dwarfism

Answer 190

Porcine GH (human GH results in antibody formation)
Progestagens (to stimulate mammary production)

Question 191

Vasopressin structure and sub-hormones

Answer 191

Neurophysin II - involved in the correct processing, transport and cleavage of AVP

Copeptin - unknown function

n.b. provasopressin is the molecule that is transported down the HPA axons to the PP

Question 192

Half life of AVP

Answer 192

6 minutes

Question 193

Main stimulators for release of AVP

Answer 193

Osmoreceptor stimulation: 1% increase in Osm can stimulate release
Sodium - via magnocellular neuron stimulation

High pressure baroreceptors - normally inhibit release via glossopharyngeal and vagus nerves
Low pressure volume receptors
- A drop in blood volume by 10 - 15% will remove inhibition from these systems and result in AVP release.

Question 194

AVP actions based on receptors (renal V2, other V2, V1a, V1b, brain)

Answer 194

Question 195

What breed has CDI been described in a familial fashion?

Answer 195

Afgan Hounds

Question 196

DAMNIT V for central and nephrogenic DI in dogs and cats

Answer 196

Idiopathic
Traumatic
Inflammation/Infection
Cysts
Congenital defect
Neoplasia:
- Carniophayngioma
- Meningioma
- Chromophobe adenoma/carcinoma
- Lymphoma
- Metastatic neoplasia

Question 197

What percentage of dogs undergoing hypophysectomy will develop prolonged CDI?

Answer 197

53%, 42% of which will need permanent therapy

Question 198

What breed has primary NDI been reported in and what is the basis of this condition genetically?

Answer 198

Has been suspected in a litter of Siberian Huskeys in which it was suspected to be due to a X-linked mutation (as only males affected?). All the affected dogs had 10x lower binding affinity for AVP. It is thought to be due to reduced AQ2 expression.

Question 199

Clinicopathologic features of DI in dogs and cats?

Answer 199

Low urea, hyposthenuria
Dehydration (if water has been restricted)

Question 200

Outline how to perform a MWDT (see printed notes)

Answer 200

(See printed notes)

Question 201

What is the potential risk of performing a DDAVP trial in an animal with psychogenic polydipsia?

Answer 201

Water intoxication and hyponatraemia

Question 202

Indications for and how to perform HH test

Answer 202

Indication: useful to distinguish primary polydipsia from medullary washout from NDI after negative water deprivation and exogenous ADH testing

Procedure

1. Instil 20ml/kg water via stomach tube and measure urine flow via urinary catheter (ml/min)
2. Infuse 2.5% NaCl at a rate of 0.25ml/kg/min over 45 minutes
3. Measure urine volume every 15 minutes during infusion followed by every 15 minutes for 45 minutes afterwards

Interpretation

Normal = Decrease in urine production due to stimulation of ADH release from increase serum osmolality

NDI = no real change or even potentially ain increase in urine production

PP = Should have a normal response as the hypertonic saline should have

Risks

* Risky in patients with CHF
* May cause hypernatraemia in animals that cannot excrete a sodium load

Question 203

What tests other than DDAVP response, MWDT, HH and ADH administration have been described in diagnosis of DI in dogs/cats/humans?

Answer 203

AVP Measurement

* Generally not useful as it is pulsatile
* Handing is challending
* No widely available assays

AQ2 Expression

* Not been used clinically although should reflect AVP exposure (experimentally)

Copeptin Measurement (Plasma)

* Shown to reflect AVP concentrations in people but not evaluated in small animals. 

Advanced Imaging

* Useful if a structural disease process is suspected
* Intensity of the PP on T1w MRI has been shown to be proportional to AVP content in dogs (and has been sued to represent phospholipid or secretory granules in people) but has not been evaluated clinically.

Question 204

Treatment for CDI?

Answer 204

DDAVP, although may not always work that well

Question 205

Treatment for NDI

Answer 205

Diet: reduce solute load through low sodium and low protein diets
Thiazide diuretics
* Decrease Na absorption from distal tubules which will decrease blood volume and therefore increase PCT resorption of Na and water (due to slowed GFR)
* Possible that there is also a tubuloglomerular feedback which results in:
* Upregulation of AQ2
* Upregulation of DCT Na transporters

NSAIDs

May have a synergistic effect with thiazide diuretics but side effects are common.

Potassium-Sparing Diuretics

More for avoiding iatrogenic hypokalaemia

Chlorpropamide

* Usually for patial CDI
* Unknown MoA (may upregulate AVP receptors)

Question 206

What is the reported incidence of iatrogenic hypothyroidism following RAI in cats?

Answer 206

5 - 83%

Question 207

What is a risk factor for development of iatrogenic hypothyroidism following RAI in cats?

Answer 207

Pertechnetiate uptake from both thyroid glands prior to treatment

Question 208

What types of congenital hypothyroidism have and have not been reported in cats?

Answer 208

• Two categories:
  ○ Dyshormonogenesis
  ○ Dysmorphogenesis (hypo/aplasia)
  • Both are likely due to autosomal recessive traits (known to be the case in people)
  • Dyshormonogenesis is a problem of hormone synthesis therefore there is a lack of negative feedback on the pituitary, TSH is high and the gland enlarges.
    ○ Defective thyroid peroxidase and impaired iodine organification has been found in related DSH and Abyssinian cats
  • Dsymorphogenesis does not cause gland enlargement, it has been documented in related cats with an inheritance pattern consistent with an autosomal recessive trait.
  • There has been a case of hypothyroidism associated with TSH resistance in a family of Japanese cats.
  • Central congenital hypothyroidism (Hypothalamic or pituitary deficiencies) have not been reported in cats.

Question 209

What dermatologic change associated with hypothyroidism in dogs does not often develop in cats?

Answer 209

Alopecia, however, cats can get coat changes such as a sebbhoraeic coat

Question 210

What is the radiographic feature of congenital hypothyroidism?

Answer 210

Delayed growth plate closure

Question 211

How are cats tested for hypothyroidism.

Answer 211

Both TT4 and fT4 can be used to rule out the condition but given that they can be low for a variety of non-thyroidal reasons they cannot confirm the diagnosis.

cTSH can be measured alongside one of the above for diagnosis of iatrogenic hypothyroidism.

TSH stimulation testing can be used to rule out euthyroid sick syndrome in cats treated with RAI.
- Measure basal TT4
- Administer 25mcg rhTSH IV
- Measure TT4 6 hours later

Question 212

Outline how the use of scintigraphy can be useful in the diagnosis of feline hypothyroidism (iatrogenic vs. congenital forms).

Answer 212

• Sensitive and specific
  
  • Reduced or absent pertechnetate (99m TcO4-) uptake supports the diagnosis
  • Use of I123 can help determine between dysmorphogenesis and dyshormonogenesis.
    ○ Dysmorphogenesis - absent uptake
    ○ Dyshormonogenesis - may be normal, but since organification is deficient, administration of perchlorate will cause the I123 to be discharged from the gland (perchlorate discharge test)

Question 213

What is the prevalence of hyperthyroidism in cats?

Answer 213

2 - 4 %, although this may rise to >6% in cats >9 years old

Question 214

What is the prevalence of ectopic thyroid tissue and thyroid carcinoma in cats?

Answer 214

Ectopic tissue in <4% cases
Prevalence of thyroid carcinoma 1 - 2%

Question 215

What condition should you consider in an older unwell cat with erythrocytosis?

Answer 215

HyperTH - as most other conditions would be expected to cause a mild non-regenerative anaemia

Question 216

What would be expected of TT4 and fT4 concentrations in a cat with euthyroid sick syndrome?

Answer 216

May expect TT4 to be low and fT4 to be high

Question 217

How to perform a T3 suppression test

Answer 217

• d0 Collect a blood sample and separate serum before storing frozen or refridgerated
  
  • d1-2 initiate T3 administration 25mcg/cat q8h PO for 2 days (6 doses)
  • d3 morning give 7th dose of T3 before the cat returns to the clinic for repeat blood sampling
  • Measure tT3 and tT4
    ○ T3 should be higher in the second sample if the owner has been administering it properly (this is why you are measuring this)
    ○ T4 should be suppressed if the cat is not affected, there is little overlap in results between FHT and unaffected cats so it can be a useful test

Question 218

Indications for and interpretation of thyroid scintigraphy

Answer 218

• Uptake of pertechnetate or radioactive iodine
  ○ Can calculate pertechnetate uptake or thyroid-to-salivary ratio
  ○ 99% of FHT cats will have a thyroid to salivary ratio of >1.5
  ○ Healthy cats have a ratio <1
  
  • Thyroid-to-background ratio is less sensitive than the thyroid-to-salivary ratio.
  • Another indication is pre-thyroidectomy to determine unilateral vs. bilateral disease and whether there is thyroid tissue elsewhere, and to increase level of suspicion for thyroid carcinoma.

Considered a GOLD STANDARD. It is very useful in animals that are not responding well to anti-thyroid medication as it can detect ectopic thyroid tissue. Uptake of radioactive iodine is compared to that of the salivary gland.

Question 219

Disadvantages of TRH stimulation testing

Answer 219

○ Commercial TRH is not available
○ Lab-grade TRH cannot be recommended for clinical patients
○ Possibly limited usefulness in cats with oncurrent illness
○ Can cause cholinergic and CNS reactions e.g.
§ Salivation
§ Tachypnoea
§ Micturition
§ Nausea
§ Vomiting
Diarrhoea