Endocrine - medicine Flashcards
What are the roles of thyroid hormones on the body?
Increase metabolism
Increase heart rate/contractility
Increase activity levels - responsiveness to catecholamines
Regulate other hormones
What make up thyroid hormones?
Iodine containing amino acids (require dietary iodide for production)
Where is the thyroid gland located?
Two lobes either side of the trachea
What are the fractions of thyroid hormone?
Mostly in protein bound state - reservoir
Metabolically active free portion - T3 and T4
What are the names of T3 and T4?
T4 - total thyroxine
T3 - Triiodothyronine
What is the difference between T4 and T3?
T4 - major secretory product of the thyroid gland
T3 - converted to this in periphery, better uptake into cells so mroe rapid action
What is the regulatory axis of thyroid hormones?
The hypothalamic-pituitary-thyroid axis
TRH - thyrotropin releasing hormone
TSH - thyroid stimulating hormone (thyrotropin)
What is non-thyroidal illness/euthyroid sick syndrome?
Disease elsewhere in the body suppresses T4 production
What animal tends to get hypothyroidism and why?
DOgs - canine hypothyroidism is the most commonly acquired disease of adult dogs
Immune mediated lymphocytic infiltrate causes thyroiditis and progresses to idiopathic atrophy
What are the most common clinical sign of canine hypothyroidism?
Dermatological changes - alopecia, hyperpigmentation, skin thickening
Other the dermatological changes, what other clinical signs does hypothyroidism cause?
Lethargy, weight gain
Bradycardia
Neuromuscular weakness
When do you test for hypothyroidism?
Only test for hypothyroidism in presence of CLINICAL SUSPICION - non-thyroidal illness/euthyroid sick syndrome
What can be seen on biochem/haematology to support clinical suspicion to diagnose hypothyroidism?
Haematology - mild non-regenerative anaemia
Biochem - hyperlipaemia (hypercholesterolaemia and/or hypertriglyceridaemia) after fasting
What is tested for on a thyroid panel?
Total T4
TSH
What is seen on a thyroid panel to suggest hypothyroidism?
Low total T4 and high TSH
What does it mean if TSH is high but T4 is normal on a thyroid panel?
Recovering from a non-thyroidal illness
May be early hypothyroidism - retest in 1-3 months
What does it mean if T4 is low but TSH is normal?
Non-thyroidal illness
Possibly hypothyroid - retest in 1-3 months
What other tests for hypothyroidism can you get from the lab other than a tyroid panel?
Utility of free T4
Thyroglobulin antibody assay
When and why would you use a test for utility of free T4 when suspect hypothyroidism?
Free T4 less affected by non-thyroidal illness so more accurately reflect thyroid function
Use if suspicious of hypothyroidism but thyroid panel inconclusive
What does a thyroglobulin antibody assay tell you?
Tells you that antibodies that are released during lymphocytic thyroiditis are present - immune mediated destruction of thyroid gland
Antibodies = thyroiditis (not hypothyroidism)
What breed have naturally lower T4 than others?
Greyhounds
What drugs can reduce thyroid hormone levels?
Glucocorticoids
NSAIDS
Trimethoprim sulphonamide
Phenobarbitone
How do you treat hypothyroidism?
Lifelong twice daily supplementation with synthetic levothyroxine sodium
Ideally without food
How do you monitor levothyroxine replacement?
If twice daily = Peak T4/TSH - about 3hrs after administration
If once daily = trough T4/TSH - should be low normal
When do clinical signs of hypothyroidism resolve after treatment/
Can take weeks to months
What are some factors that cause treatment failure in hypothyroidism?
Incorrect diagnosis
Insufficient time
Expired drug
Inadequate dosing, not giving every day
Obesity
Concurrent disease
What is the name for a crisis and collapse due to hypothyroidism and concurrent disease?
Myxoedema coma
What is cretinism?
Congenital hypothyroidism
What does congenital hypothyroidism cause?
Disproportionate dwarfism
(growth hormone deficiency causes proportionate dwarfism)
Impaired mental development
What does canine thyroid neoplasia present as? What does it cause?
NON-secretory - not associated with hyperthyroidism
Usually unilateral
Usually malignant - carcinoma
How do you treat canine thyroid neoplasia?
Surgical resection +/- adjunctive radiation therapy
Histology of excisional biopsy
How does canine thyroid neoplasia differ from feline thyroid neoplasia?
Canine - non-secretory, malignant, rare
Feline - benign, cause hypERthyroidism, very common
What causes feline hypothyrodism?
Naturally occurring - very rare
Most commonly iatrogenic - secondary to treatment of hyperthyroidism
What is the most common feline endocrinopathy?
Hyperthyroidism
What are the two presentations of feline hyperthyroidism?
Multinodular adenomatous hyperplasia/adenomas - autonomously functioning follicles
Functional thyroid carcinoma - less common
What are the main risk factors for developing hyperthyroidism?
Increasing age
Female
Canned food - iodine deficiency/excess?
Indoor?
Litter tray use?
Exposure to chemical products - thyroid disruptors eg. flea/pest control, garden/household products
What are the most commonly seen clinical signs of hyperthyroidism in cats?
Weight loss
Polyphagia
Hyperactivity
(PUPD, V+/D+, CV, Resp signs)
May have palpable goitre
Poor coat condition
What is apathetic hyperthyroidism?
Weight loss
Inappetence/anorexia
Lethargy
But same diagnosis
What are the common differential diagnoses for polyphagia (increased appetite) with weight loss?
Hyperthyroidism
Diabetes mellitus
Exocrine pancreatic insufficiency
SI GI disease
What are the common differential diagnoses for PUPD with weight loss?
Hyperthyroidism
Diabetes mellitus
Chronic kidney disease
What are the common differential diagnoses for polyphagia (increased appetite) with PUPD?
Hyperthyroidism
Diabetes mellitus
What are the common differential diagnoses for vomiting, diarrhoea, inappetence with weight loss?
Hyperthyroidism
Chronic enteropathies
How many hyperthyroid cats get a palpable goitre?
70% of hyperthyroid cats
Anywhere from base of tongue to base of heart - can drop into thoracic inlet
How do you diagnose hyperthyroidism?
Increased total T4 - very straight forward as very high sensitivity and specificity
What adjunctive diagnostics can you use in hyperthyroidism?
BP and retinal exam - hypertension common
Haem/biochem
Urinalysis
Echocardiography - can cause hypertrophic cardiomyopathy and heart failure in cats
What is seen on haem/biochem in hyperthyroidism?
High haematocrit- T4 stimulates erythropoeitin
Increased liver enzymes - reactive hepatopathy
Hyperphosphataemia - increased bone turnover
What drug do you give to medically manage hyperthyroidism? What do they do?
Carbimazole (pro-drug) (Cats are like “carbs-in-ma-hole” cos they’re hungry heheh)
Methimazole
Have same effect - reversible inhibits thyroid hormone synthesis (if stop then will become hyperthyroid again)
What good effects on the body does hyperthyroid medical management have?
Rapid effective control of hyperthyroidism
Reverses adverse systemic effects
Improves patient morbidity and QOL
Unmasks concurrent renal disease - enable evaluation
How does medical management of hyperthyroidism unmask concurrent renal disease?
Hyperthyroidism causes increased glomerular filtration rate - increases blood flow to the kidneys which reduces the amount of creatinine in the blood
When reduce T4 then reduce GFR to normal which increases the creatinine in the cats
The drugs do not cause kidney disease, they just reduce the pathologically high GFR back to normal so can see previously hidden kidney disease in these cats
Need to continue the drug treatment - high GFR for a long time can speed up kidney disease so it makes it worse if you stop the drugs even though it looks like it is causing kidney disease
What is the aim of hyperthyroid medical therapy?
Total T4 in the LOWER HALF of the reference interval - regular monitoring in 3 week/3 month intervals (if stable)
What are the adverse clinical signs/side effects of medical management of hyperthyroidism?
Anorexia
Vomiting
Lethargy
Facial excoriation
Usually within first 1-2 months
What are 3 ways you can deal with the adverse clinical signs of medical management of hyperthyroidism?
Surgical management
Discontinue and restart at lower dose in a week
Give drugs transdermally rather that orally
What adverse lab findings can be found in medical management of hyperthyroidism? When should you discontinue treatment?
Thrombocytopaenia, neutropenia - discontinue treatment
Acute toxic hepatopathy - discontinue treatment
Azotemia - from unmasking CKD, continue treatment
What are the advantages of long term medical management of hyperthyroidism?
Usually effective
Reversible
No anaesthesia or hospitalisation needed
No lump sum cost - incremental yearly
What are the disadvantages of long term medical management of hyperthyroidism?
Non curative - dose escalation overtime, tumour worsens
Twice daily administration
Regular monitoring needed
Side effects
What are the two permanent treatments of hyperthyroidism?
Radioiodine - gold standard
Surgery - semicurative
How does radioiodine cure hyperthyroidism in cats?
Subcut administration of radioisotope of iodine I131
This concentrates in the thyroid glands and radiation causes follicular cell death
What are the advantages of radioactive iodine treatment of hyperthyroidism?
Curative - in 95% of cases
Dont need lifelong treatment/monitoring
No anaesthesia
What are the disadvantages of radioactive iodine treatment of hyperthyroidism?
Expensive - £3500
Limited availability - only certain centres do it
Period of isolation/handling restrictions
Irreversible - may cause hypothyroidism
What are the advantages of surgical management of hyperthyroidism?
Often curative
Readily available - offered in general practice
No ongoing treatment/monitoring
What are the disadvantages of surgical management of hyperthyroidism?
Short term expense (£1500-3400)
Anaesthesia/hospitalisation
Risk of surgical trauma
Risk of post-op hypoparathyroidism
Irreversible - risk of hypothyroidism
Why can surgical management of hyperthyroidism potentially cause hypoparathyroidism?
Parathyroid glands close to thyroid glands
PTH maintains serum calcium
Risk of surgical trauma/bruising
What are the principles of dietary management of hyperthyroidism?
Feed exclusively iodine restricted diet - limit thyroid hormone production
What are the pros and cons of dietary management of hyperthyroidism?
No pills, no surgery , no isolation
Affordable
But takes longer to respond to treatment, doesnt reduce T4 to lower half of reference range
Submaximal clinical improvement
Must feed diet exclusively
What can cause canine hyperthyroidism?
Naturally occurring hyperthyroidism - very rare
But increasing from raw-fed diets - particularly feeding cow goitres
Where is cortisol produced?
Adrenal glands - zona fasiculata
What regulates cortisol secretion?
Hypothalamic-pituitary-adrenal axis - CRH, ACTH
What are the most common causes of naturally occurring hyperadrenocorticism?
Pituitary dependent - most common 85% (usually smaller dogs)
Adrenal tumour - carcinoma (usually larger dogs)
What changes to the adrenal glands do pituitary dependent and adrenal dependent hyperadrenocorticism cause?
Pituitary dependent - bilateral hypertrophy
Adrenal dependent - contralateral adrenal atrophy (not needed)
What is an iatrogenic cause of hyperadrenocorticism?
Chronic glucocorticoid (steroid) use