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Hyperthyroidism (Thyroid Cancer) in Cats

Hyperthyroidism in cats is almost always associated with a cancer-a benign adenoma (more common) or a malignant adenocarcinoma. These cancers tend to occur in older cats and the major effect is the increased thyroid hormone production they stimulate. Exposure to secondhand cigarette smoke may be a factor. Himalayans and Siamese have a lower risk for this problem.

The first signs may be dramatic. The increase in thyroid hormones tends to cause an increase in appetite-the finicky eater now wolfs everything down and may even open cupboards looking for more-and an increase in activity. The sedate older cat who used to spend his days soaking in the sun on his window seat is now flying around the house like a wild kitten. Weight loss, vomiting, and panting may all be noted. Careful palpation of the area under the chin when the head is lifted may reveal a small lump. Thyroid tumors may appear only on one side, but they can be bilateral.

Blood work is important to see if thyroid hormone levels are truly elevated. It is important to check kidney function, because the increased metabolic rate may hide kidney failure. It is also important to thoroughly evaluate the heart, which can be damaged from the increased metabolic rate. High blood pressure or hypertension is often seen in hyperthyroid cats.


Treatment tends to start with an oral medication called methimazole, which lowers hormone production. This must be given daily and manufacturers are working on palatable and easy-to-administer forms for cats, including a compounded version that is applied to the ear and absorbed through the skin. Medical treatment enables your veterinarian to evaluate the cat’s kidney and cardiac status prior to the more definitive treatment of surgery or radioactive iodine.

Surgery is another option, with the cancerous gland removed. Care must be taken not to damage or remove the parathyroid glands that regulate calcium metabolism. If both thyroid glands are removed, the cat will need supplemental thyroid for the rest of his life.

A third and popular option is to use radioactive iodine to destroy the cancerous tissue. Cats who undergo this therapy must stay at a treatment center for 7 to 25 days while the radioactivity shed in the cat’s urine and feces decreases to a safe level. These cats may need supplemental thyroid for the rest of their lives.

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