Introduction
Hyperthyroidism is the most common feline endocrine disorder. In 98% of cases it is the result of benign adenomatous hyperplasia of one or both thyroid lobes and causes multi-systemic disease due to the excessive production of active thyroid hormone. The underlying aetiology of this condition remains unclear, but it predominantly affects middle aged to old cats with the average age of onset around 12-13 years. To date, there is no evidence to support a breed or sex predisposition. Major clinical features are those of polyphagia, weight loss, tachycardia (often with a murmur), PuPd, hyperactivity and intermittent GI signs. A palpable goitre may or may not be present.
Diagnostic tests
Routine clinical pathology
Laboratory findings typically include a reactive polycythaemia and a stress leucogram, with neutrophilia and sometimes lymphopaenia and eosinopaenia. The most common biochemical abnormalities are elevated AP, ALT, mild azotaemia and an increased phosphorus. Blood glucose may be elevated and this is probably a stress response.
Total thyroxine concentration (TT4)
Measurement of total serum thyroxine (TT4) is the preferred screening test for cats suspected of hyperthyroidism. Total T4 levels are also used to monitor the response to therapy. Significant daily variations in T4 concentrations have been shown to occur in hyperthyroid cats. Those in the early stages of disease, or suffering from severe concurrent illness may have T4 values which fall within the reference range (10% of all cases, 40% of cases with mild hyperthyroidism). Repeated T4 estimation 3-6 weeks later or checking the free T4 by equilibrium dialysis may provide a diagnosis.
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Free T4 by equilibrium dialysis (FT4D)
Equilibrium dialysis is the most accurate way to measure free T4 and is preferable to older analogue methods, which have little, if any, diagnostic advantage over total T4. FT4D is generally less affected by non-thyroidal illness and altered protein levels. >98% of all hyperthyroid cats and 95% of hyperthyroid cats with TT4 levels within the reference interval have elevated free T4 levels. Up to 20% euthyroid cats with severe non-thyroidal disease will have elevated FT4D concentrations, therefore results must always be interpreted in accordance with the clinical signs and FT4D is not recommended as an initial screening test in place of TT4. The ability to accurately measure free T4 (FT4D) has, to a large extent, removed the need to rely on dynamic testing.
Therapeutic monitoring
Ideally, cats suffering from hyperthyroidism should ultimately be managed with radioiodine or surgically as these treatments can provide a cure for the underlying pathology. However for a variety of reasons chronic medical management (or dietary iodine restriction) is a realistic option in many cases.
The anti-thyroid drug thiamazole (methimazole) produces consistent, reliable suppression of hormone production. Carbimazole is a pro-drug converted to thiamazole following oral administration. Approximately 10-15% of cats may experience adverse reactions, usually within the first 3 weeks of therapy. These include bouts of vomiting, anorexia and lethargy. Mild haematological abnormalities for example, lymphocytosis, leucopaenia, eosinophilia can also be seen in up to 15% of cases. These are transient side effects in most cases and can usually be managed without withdrawing the drug. More severe adverse reactions include facial excoriation, severe haematological changes such as agranulocytosis and thrombocytopaenia (<5% cases) and hepatopathy (<2% cases) and these reactions usually necessitate withdrawal of therapy.
TT4 should be measured 2-3 weeks after starting therapy or after any dose change until euthyroidism is achieved, 3 months later and then every 6 months or as indicated clinically. Ideally the drug dosage should be adjusted in an attempt to maintain T4 levels in the low-normal range. The timing of sampling in relation to medication is not important as long as regular dosing has been given. AP and ALT decline progressively as cats become euthyroid, but mild elevations can persist. Monitoring haematology is also indicated given the potential for adverse reactions.
Monitoring renal function is particularly important when treating hyperthyroid cats. The increased glomerular filtration rate associated with overt disease may mask concurrent chronic renal failure. As cats approach euthyroidism, significant renal disease may become apparent and in such cases a balance between management of hyperthyroidism and preservation of renal function may be required to optimise quality of life. Cats with pre-existing azotaemia and those with iatrogenic hypothyroidism and post-treatment azotaemia have shorter survival times than those without.
Monitoring of blood pressure is also recommended as part of the routine care of hyperthyroid cats due to the potential for both pre-existing systemic hypertension and that which can develop during treatment.
Therapy has no effect on the underlying pathology and the tumour will continue to increase in size, therefore dosage increases may be required.
Following bilateral thyroidectomy, parathyroid gland can be injured, devascularised or inadvertently excised leading to hypoparathyroidism and subsequent hypocalcaemia. This is expected to occur within 1-5 days of surgery but is rarely permanent, with recovery over the following days to months.