Stray cat sitting on the street.

Hyperthyroidism in Cats

A condition of hyperthyroidism in cats is caused by an increase in circulating levels of thyroid hormones, namely, thyroxine (T3) and triiodothyronine (T4). When it occurs, hyperthyroidism is most commonly seen in cats greater than 8 years of age, usually as a result of a tumor involving the thyroid gland.

Because thyroid hormone helps regulate the body’s metabolism, the clinical signs seen with hyperthyroidism can be directly related to the exaggerated increase in the cat’s metabolic rate.

These symptoms may be mild to severe depending on the amount of excess hormone being secreted. Signs typically include noticeable weight loss in the presence of a voracious appetite, nervousness and hyperactive behavior, and a rough, unkempt haircoat.

Other less common signs seen include increased water consumption, regurgitation (due to rapid overeating), panting, and breathing difficulties, especially if the thyroid glands are grossly enlarged.

In addition, many cats with elevated thyroid levels also suffer from inflammatory bowel disease (IBD). As a result, vomiting and/or diarrhea related to this may be seen in the hyperthyroid feline as well.

Diagnosis of hyperthyroidism is made through the evaluation of clinical signs, physical exam findings, and special laboratory tests. On physical examination, nodules can usually be palpated in the neck region because of glandular enlargement.

In addition, a rapid heart rate and pulse are often detected because of the effects of the thyroid hormones on the heart. This cardiac affect can be especially dangerous in cats suffering from concurrent cardiomyopathy.

A diagnosis of hyperthyroidism can also be verified through the use of special tests designed to detect levels of thyroid hormone in the blood. If this condition is definitively diagnosed, a number of treatment options exist.

The type of treatment chosen will depend on the severity of the thyroid hormone elevation and other underlying disease factors (such as the presence of heart disease or kidney disease).

Medical treatment for hyperthyroidism involves the administration of special drugs designed to inhibit production of thyroid hormone by the thyroid gland, thereby controlling clinical signs.

Side effects from giving such drugs can include anemia, immune cell suppression, decreased appetite, vomiting, weakness, and itching. Since most cats must stay on this medication for the remainder of their lives, close monitoring by and periodic communication with a veterinarian is essential.

Yet another form of medical therapy that yields successive results in hyperthyroid felines is called radioactive iodine therapy. This type of therapy selectively destroys malfunctioning thyroid cells using radiation.

Most cats suffering from hyperthyroidism will respond favorably to medical therapy. However, if drug therapy fails to resolve the disorder and radioactive iodine therapy is unavailable, surgical removal of the thyroid gland (partial or complete thyroidectomy) must be performed.

If extensive tumor involvement necessitates the removal of the thyroid, then daily thyroid hormone supplementation will be required for the remainder of the cat’s life. Felines placed on such supplementation should have blood thyroid levels checked every 6 to 8 months to ensure that adequate levels are being given.

An inherent risk associated with the surgical removal of the thyroid gland in cats is a complication known as hypoparathyroidism. This condition, characterized by low blood calcium levels, is caused by the inadvertent removal of the parathyroid gland (tightly adhered to the thyroid gland) when the thyroid tissue is removed.

Signs of low blood calcium, which normally arise within 3 days of parathyroid gland removal, include profound weakness, muscle tremors and spasms, and in some cases, seizures.

Felines suffering from this postsurgical complication require prompt treatment with calcium supplements. These supplements, as well as vitamin D tablets, will be required for life to help maintain proper calcium levels within the body.