Hyperthyroidism: A disease of older cats over 8 years of age. In almost all cases, it is caused by a non-malignant growth of the thyroid gland that causes an increased production of thyroid hormone. Symptoms of hyperthyroidism can include weight loss, enormous appetite, poor hair coat, hyperactivity, vomiting, and diarrhea.

The diagnosis of hyperthyroidism can be confirmed by running blood tests to determine the cat’s thyroid hormone level. Additional blood test should be performed to screen for other health problems that might be present in older cats and might influence treatment choice and prognosis.

Hyperthyroidism is a progressive disease. Symptoms, if untreated, will continue to worsen until the cat dies.

The treatment options discussed below offer advantages and disadvantages, which must be considered for each cat and its owner.


The anti-thyroid drugs can be used to control the signs of hyperthyroidism. These drugs block the production of thyroid hormone. They do not destroy the tumor, and therefore will not cure the disease. However, it can effectively control the signs of hyperthyroidism when given daily. Since therapy requires giving the cat tablets every day (initially 2-3 times a day), we do not recommend this treatment if you can’t give your cat oral medication. It must be given diligently. If drug treatment is interrupted, even for a day or two, hormone levels will increase and signs will return.

Surgery can be performed to remove overly active thyroid glands, thereby curing the hyperthyroid state. Since your cat is older, you may be worried about increased surgical risk. However, in the majority of cats without concurrent medical disease, the risk is minimal. The newest surgical protocol calls for removal of one gland and then re-testing the cat after 30 days to determine if the other side must also be removed. In many cases, only one side will require removal.

Radiation therapy can also be used to destroy the thyroid tissue.