5.Feline hyperthyroidism 5. Feline hyperthyroidism

Hyperthyroidism, or thyrotoxicosis is a common condition of older cats (6-20 years) with a reported incidence of 1/300 cats. There is no breed or sex predilection.

Gross pathology
Cats with hyperthyroidism are usually thin or emaciated. They have unior bilateral enlargement of the thyroid glands.

Benign functional adenoma (adenomatous hyperplasia) of the thyroid gland. Thyroid carcinomas rarely cause hyperthyroidism in the cat.


Cardiac changes in hyperthyroidism are due to the direct effects of increased thyroid hormone secretion on the heart, and increased adrestimulation. These changes include increased heart rate, conejection fraction (at rest but not during exercise), pulse pressure and cardiac output.

Secondary hypertrophic cardiomyopathy may occur, or the condition may progress to congestive heart failure.

Clinical findings
The following are most frequently noted:
(1) weight loss
(2) polyphagia
(3) hyperactivity
(4) tachycardia
(5) polydipsia/polyuria
(6) cardiac murmur
(7) vomiting
(8) diarrhoea.

Enlargement of the thyroid gland(s) can usually be palpated and may be unilateral or bilateral. A chain of thyroid masses may extend down the neck and through the thoracic inlet.

On ECG sinus tachycardia and large R waves are seen in lead II (>0.9mV). Arrhythmias are also often present.

Chest radiographs reveal left-sided cardiac enlargement with other signs of congestive heart failure, e.g. pulmonary oedema and/or pleural effusion. Echocardiography demonstrates left ventricular dilatation, hyperand increased contractility. These changes are reversible once the hyperthyroidism is corrected.

Radionuclide imaging is helpful to confirm whether both lobes of the thyroid are involved, to identify small adenomatous changes, to detect intrathoracic remnants and to identify metastases.

Laboratory findings
Elevated T4 concentrations and usually elevated T3 as well.

These cases also may have elevated alanine aminotransferase (ALT), aspartate aminotransferase (AST), serum alkaline phosphatase (SAP) and lactate dehydrogenase (LDH) concentrations. Hyperphosphataemia sometimes occurs.

Leukocytosis, eosinopenia and increased packed cell volume are commonly found.

Clinical findings, elevated T4 concentrations, palpation of enlarged thyroid lobe(s).

Primary objective is to create a euthyroid state:

(1) thyroidectomy - treatment of choice
(2) antithyroid drugs - carbimazole 10-15 mg daily in divided doses for 1-3 weeks
(3) radioactive iodine therapy.

Specific treatment for the secondary cardiac disease should only be given if needed:

(1) diuretics (frusemide 1 mg/kg, b.i.d. to t.i.d.) if oedema or pleural effusion is severe
(2) oral propranolol if tachydysrhythmias are severe at a dose of 2.5 mg b.i.d. to t.i.d. up to 6kg body weight (for cats over 6kg body weight give Smg b.i.d. to t.i.d.).