Endocrinology Flashcards
What is the aetiology of feline hyperthyroidism?
No evidence of auto-immune disease in cats
>95% benign adenomatous hyperplasia/adenoma of thyroid tissues
Spontaneous secretion of thyroid hormones, escaping control of hypothalamus and pituitary gland
2/3rds cases bilateral, 1/3rd cases unilateral
What are some potential causes of feline hyperthyroidism?
Nutritional factors (iodine levels, presence of goitrogens) Environmental factors (flea sprays, garden pesticides?) Genetic factors (Siamese/Himalayan cats 10x less likely to be hyperthyroid) Circulating factors (thyroid growth stimulating immunoglobulins)
What are some risk factors for developing feline hyperthyroidism?
Regular use of flea sprays/powders (3-4 fold increase in risk)
Indoor cats (4 fold increase)
Reported exposure to lawn herbicides/fertilisers/pesticides (3-5 fold increase)
Cats fed mainly canned food (3-4 fold increase)
What is the typical signalment for feline hyperthyroidism?
Middle-aged to elderly cats (mean = 10-13 years)
No sex predisposition
Signs vary from mild to severe depending on duration of disease/presence of concurrent diseases
What effect do thyroid hormones have in cats and what does this mean for hyperthyroid cats?
Increased: metabolic rate (skinny), CO (tachycardia), HR, BP (hypertensive), GI motility (V+/D+), CNS activity (altered mentation)
Decreased: sleep (more active), bodyweight (lose weight)
What are the major clinical signs of feline hyperthyroidism?
Palpable enlarged thyroid glands Weight loss Polyphagia Hyperactivity PUPD Tachycardia
What are some minor clinical signs of feline hyperthyroidism?
Lethargy Intermittent anorexia Voice changes Muscle weakness/tremors Congestive heart failure Heat intolerance Mild pyrexia Dyspnoea/tachypnoea
What is apathetic hyperthyroidism in cats?
Small percentage (<10%)
Lethargy, inappetence, weight loss, obtundation
Likely reflecting an underlying comorbidity, e.g. severe cardiac abnormalities often present
How do we handle feline hyperthyroid patients?
Cat-friendly Hands-off approach Put in quiet/dark room to calm Gabapentin (50mg once) 2hrs before travelling, oxygen therapy if required Acclimation period Monitoring RR (risk of heart failure)
How do we diagnose feline hyperthyroidism?
Compatible clinical signs
Screening tests
Confirmatory diagnostic test
What screening tests can we perform for feline hyperthyroidism?
Haematology
Biochemistry - elevated liver enzymes (mild-moderate), concurrent disease? chronic kidney disease?
Urinalysis - chronic kidney disease?
BP measurement
What is the confirmatory diagnostic test for feline hyperthyroidism?
Serum total thyroxine (T4) - gold standard
Elevated in most hyperthyroid cats (>50-60 nmol/L)
May fluctuate
Occasionally high-normal (early disease / non-thyroidal illness)
What are the general treatment options for feline hyperthyroidism?
Medical management - anti-thyroid drugs Iodine-restricted diet Surgery - thyroidectomy Radioactive iodine treatment Medical management should be tried first - stabilise prior to GA if surgery, assess renal function (CKD)
How do we use anti-thyroid drugs for feline hyperthyroid patients?
Methimazole BID (tablets, transdermal gel, oral liquids)
Slow-release carbimazole tablets SID
Normally euthyroid in <2-3 weeks - recheck 2-3 weeks after starting treatment, recheck CBC and biochem in first 3 months of treatment
What are the advantages of medical (anti-thyroid drug) management of feline hyperthyroidism?
Readily available Rapidly effective Inexpensive Practical No GA/hospitalisation
What are some disadvantages of medical (anti-thyroid drug) management of feline hyperthyroidism?
Lifelong
Long-term resistance
Compliance
Side effects
What side effects might we see from anti-thyroid drugs?
Minor, common, transient (10-20%) - e.g. vomiting, anorexia, lethargy
Major, rare, persistent (1-5%) - persistent GI signs, severe leukopenia/anaemia/thrombocytopenia, dermatitis (facial excoriation), hepatopathy, lymphadenomegaly, myasthenia gravis
STOP TREATMENT if major side effects!
How can we feed an iodine-restricted diet to feline hyperthyroid patients?
E.g. Hills y/d Must be fed as sole food Can be euthyroid within 3 weeks Lifelong Less effective and not suitable for severely hyperthyroid cats
What considerations should we have pre-surgical management of feline hyperthyroidism?
Systemic effects of hyperthyroidism
Cardiac disease
Hypertension
Other diseases
Describe a thyroidectomy.
Stabilise with medical management prior to surgery
Removal of one or both thyroid glands
Preservation of parathyroid tissue to avoid post-op complications (hypocalcaemia)
Overall typically achieves euthyroidism in >90% of patients and within 24-48hrs
What are the advantages of a thyroidectomy for feline hyperthyroid patients?
Curative
Rapidly effective
Short hospitalisation period
What are some disadvantages of a thyroidectomy?
GA Skill Location Recurrence Cost Complications
What complications can we see post-thyroidectomy?
Damage to/removal of parathyroid tissue (post-op hypoparathyroidism)
Damage to recurrent laryngeal nerve (laryngeal paralysis - uncommon)
Damage to sympathetic trunk (Horner’s syndrome - uncommon)
Possible recurrence of disease if unilateral
Describe iatrogenic hypoparathyroidism.
Usually bilateral thyroidectomy
Usually transient - recovery of glands/restored function of ectopic tissues, weeks-months to recover
What are the clinical signs of iatrogenic hypoparathyroidism?
Within 2-3 days
Inappetence, weakness, tremors, ptyalism, pawing at face
Progressing to tetany, seizures, coma and death
Monitor serum calcium twice a day if bilateral surgery/concerned
What is the first step of treatment for iatrogenic hypoparathyroidism?
IV 10% calcium gluconate slowly (10-20mins)
Monitor with ECG for arrhythmia and bradycardia
Initial bolus followed by CRI IV
Avoid bicarbonate/lactate/phosphate-containing fluids - precipitate calcium
Subcut admin not recommended - causes skin sloughs
What is the second step of treatment for iatrogenic hypoparathyroidism?
Oral therapy as soon as possible (takes 1-3 days to work)
Calcium - elemental in divided doses, wean off IV drip
Oral vitamin D long-term
Gradually weaned off therapy if possible
Describe radioiodine treatment of feline hyperthyroidism.
Gold standard
Administered systemically but concentrated in thyroid
Beta particles cause local cell death
Gamma rays dangerous - cat isolated for 1-2 weeks
15 centres currently in UK
What are the advantages of radioiodine treatment for feline hyperthyroidism?
Gold standard Curative Simple procedure Higher doses to treat adenocarcinoma No GA Cost
What are the disadvantages of radioiodine treatment?
Limited availability Isolation period Irreversible May take some time to achieve euthyroid Very rarely causes iatrogenic hypothyroidism Cost
How are hyperthyroidism and chronic kidney disease linked?
HT may mask underlying CKD
Treatment may unmask CKD
Careful consideration before treatment
Medical management first before curative treatment of HT
Reassessment once euthyroid
Unmasking of mild azotaemia may not preclude definitive treatment
How should we monitor feline hyperthyroid patients?
Irrespective of treatment regime Recommend 6-mothly check-ups once stabilised Recurrence Hypertension CKS - urea-creatinine, BP, urinalysis
What is the prognosis for feline hyperthyroid patients?
Largely dependent on - severity/presence of concurrent disease, especially heart disease
Uncomplicated hyperthyroid cats have an excellent prognosis following curative treatment
What is canine thyroid neoplasia?
Carcinomas more common, adenomas usually incidental findings Usually large, solid, palpable mass Locally invasive +/- metastasise Most are euthyroid or hypothyroid Only 10% hyperthyroid
What are the clinical signs of canine thyroid neoplasia?
Average age 10yrs
Mass in ventral neck region
+/- cough/dyspnoea
How do we diagnose canine thyroid neoplasia?
Histopathology of mass (care - very vascular)
FNA often blood-contaminated so not diagnostic but may confirm thyroid origin
How do we treat canine thyroid neoplasia?
Surgical removal
Ideally followed by chemotherapy/radiation therapy - seek specialist advice
Radioactive iodine treatment - very high doses required for dogs so less used
What is the prognosis for canine thyroid neoplasia patients?
Depends on size, local invasion, functional status and histological diagnosis (adenoma vs carcinoma)
Large, invasive masses at time of diagnosis - prognosis guarded to poor (6-24months with aggressive treatment)
Prognosis excellent following surgical removal of adenomas / good following removal of small, well-circumscribed carcinomas
Describe primary canine hypothyroidism.
At the level of the thyroid gland - most common form
Lymphocytic thyroiditis = destructive immune-mediated process, infiltration of lymphocytes/macrophages/plasma cells and replacement by fibrous connective tissue, clinical signs occur when 75% of gland is destroyed
OR
Thyroid atrophy = degenerative process with limited inflammation, progressive replacement by adipose and connective tissue, possibly end-stage lymphocytic thyroiditis
Describe secondary canine hypothyroidism.
Rare
Pituitary hypoplasia (congenital - disproportionate dwarfism) or dysfunction (acquired - neoplasia)
Most common cause is suppression of TSH secretion by exogenous glucocorticoid admin and hyperadrenocorticism
What is the signalment for canine hypothyroidism?
Mean age at diagnosis = 7 years
Breeds predisposed to lymphocytic thyroiditis tend to develop hypothyroidism sooner, e.g. English Setter/Golden Retriever/Rhodesian Ridgeback/Cocker Spaniel/Boxer
What are some clinical signs of canine hypothyroidism?
Decreased metabolic rate - weight gain, lethargy
Endocrine alopecia, rat-tail, hair in telogen phase (no regrowth)
Bitches - reproductive signs
Bradycardia
Neuromuscular disease - megaoesophagus, laryngeal paralysis, facial nerve paralysis (more likely concurrent disorders)
Association with myxoedema coma - mental dullness, weakness, hypothermia, hypotension
Ocular/GI signs
How can we diagnose canine hypothyroidism?
Appropriate history and clinical signs
Haematology and biochemistry
Specific thyroid testing - Total T4 (TT4), Canine TSH (cTSH), anti-thyroglobulin antibodies (TgABs)
What are we looking for on haem/biochem tests to diagnose canine hypothyroidism?
Mild non-regenerative anaemia (normocytic and normochromic)
Hypercholesterolaemia
Hypertriglyceridaemia
Hyperlipidaemia
Describe the Total T4 (TT4) test for diagnosing canine hypothyroidism.
Useful initial screening test - excellent sensitivity
Thyroglobulin antibodies can falsely increase TT4
T4 decreases with age/breed/non-thyroidal illness/drug therapy so poor specificity
Never use as single diagnostic test
Describe the canine TSH (cTSH) test for diagnosing canine hypothyroidism.
cTSH increased in hypothyroidism due to lack of negative feedback
Moderate sensitivity - low cTSH in central hypothyroidism/corticosteroid therapy
Good specificity - largely non-affected by non-thyroidal illness or drugs, but elevated in euthyroid dogs if recovery from non-thyroidal illness
Describe the anti-thyroglobulin antibodies (TgABs) test for diagnosing canine hypothyroidism.
No information about thyroid function
If positive, consistent with lymphocytic thyroiditis
Can be present long before hypothyroidism
How do we treat canine hypothyroidism?
Synthetic T4 - physiologic prohormone for active T3
Sodium levothyroxine 0.02mg/kg SID or divided BID
Absorption and metabolism vary between dogs
Bioavailability halved with food - consistency for admin and monitoring
In dogs with cardiac disease/diabetes mellitus/hypoadrenocorticism, start with 25% dose and titrate up
6-8 weeks before evaluating effect
Peak concentration = 3-5hrs post pill
Half-life = 9-15hrs
How do we monitor dogs post-treatment for canine hypothyroidism?
Clinical response
6-8 weeks after starting treatment / 2-4 weeks after altering dose
Measure TT4 - 6hrs post pill for SID and 4-6hrs for BID
Measure fT4 if chronic prednisolone admin
Measure cTSH
Aim = TT4 upper half of reference and TSH normal
What are the potential complications after treatment for canine hypothyroidism?
Thyrotoxicosis - rare, secondary to drug overdose
Clinical signs = panting, anxiety/aggression, PUPD, weight loss, polyphagia
Treatment = reduce dose/discontinuation
Myxoedema coma - rare
Treatment = supportive care, IV levothyroxine
What is the prognosis for canine hypothyroidism patients?
Good - adult dogs with primary hypothyroidism
Guarded - secondary hypothyroidism
What is the prognosis for canine hypothyroidism patients?
Good - adult dogs with primary hypothyroidism
Guarded - secondary hypothyroidism
What is the alternative name for primary canine hypoadrenocorticism?
Addison’s disease
Describe primary hypoadrenocorticism.
Most common
Lack of glucocorticoids and mineralocorticoids
Atypical hypoadrenocorticism = lack of glucocorticoids but normal mineralocorticoids
Suspected immune-mediated destruction of adrenal cortex
Describe secondary canine hypoadrenocorticism.
Central cause - neoplasia, inflammation, infection, infarct, iatrogenic
Only glucocorticoid deficiency as aldosterone secretion is regulated by RAAS
Neurological signs
What are the clinical features of Addison’s disease?
Typically young/middle-aged female dogs
Breed disposition e.g. Standard Poodles/Bearded Collies/Nova Scotia Duck Toller/Great Dane
Often vague, waxing and waning history
Lack of cortisol = weakness, vomiting, diarrhoea, anorexia (esp. at times of stress)
Lack of aldosterone (not in atypical cases) = PUPD due to low Na
How does an Addisonian crisis present?
Collapse, severe dehydration and hypovolaemia, pre-renal azotaemia and cardiac arrhythmias due to hyperkalaemia (bradycardia)
Emergency!
