Hyperthyroidism

Hyperthyroidism is a common endocrine disorder of middle-aged and older cats.

Diabetes mellitus and hyperthyroidism may present concurrently.

Pathophysiology and clinical signs
A thyroid adenoma causes secretion of increased amounts of thyroid hormones (tri-iodothyronine (T3) and thyroxine (T4)).

Hyperthyroidism is a multi-system disorder. Clinical signs include:

• weight loss
• polyphagia
• polydipsia
• polyuria
• hyperactivity
• diarrhoea

The high levels of thyroid hormones can cause the development of heart disease, and these patients may have:

• a heart murmur
• difficulty breathing
• tachycardia
• arrhythmias.

Diagnosis
Hyperthyroidism must be differentiated from diabetes mellitus. This is usually done using laboratory measurement of T4 concentration.

In a few cats the diagnosis may need to be confirmed by:

1. A follow-up total T4 measurement 3 or more weeks later or
2. Free T4 measurement (by equilibrium dialysis) or
3. A T3 suppression test.

Treatment
Once hyperthyroidism has been confirmed, there are several treatment options, including:

• surgical removal of the gland
• radioactive iodine
• anti-thyroid medications

The initial choice of treatment is often guided by concern about the cat's kidney function, since treatment may precipitate renal failure. For hyperthyroid cats that are assessed with normal kidney function, surgery or radioactive iodine treatment are most frequently recommended.