General Information:

The thyroid is a two-lobed gland located in the neck on either side of the windpipe. The thyroid produces thyroid hormone, a substance that controls the rate at which cells function. Too much thyroid hormone makes cells work very fast, while too little hormone causes cell function to slow down. An overactive thyroid is relatively common in older cats, although quite rare in the dog.

Typical symptoms of hyperthyroidism are related to an increase in overall metabolism. Weight loss, increased appetite, patchy hair loss, unkempt haircoat, increased activity/restlessness, vomiting, and diarrhea are all common symptoms.

The cause of hyperthyroidism is not known. About 15% of hyperthyroid cats have a thyroid tumor (benign) on just one lobe of the thyroid gland, about 80% have excess activity in both lobes of the thyroid gland, and about 5% have a malignant (cancerous) tumor.


Diagnosis is relatively straightforward by a simple blood test. The levels of thyroid hormones are measured in the blood. A high level of thyroxine (T4) is considered diagnostic. Because hyperthyroidism is invariably a disease of geriatric cats, we typically recommend a complete geriatric blood profile to test other parameters, including blood sugar and liver and kidney function.


Three treatment options are available for hyperthyroidism. Each treatment has advantages and disadvantages. The doctor will explain the treatment options to you to help you make the best decision for you and your pet.

Methimazole (Tapazole®) is an oral medication given twice daily for the rest of your pet's life. It is relatively inexpensive and highly effective. This medication works by preventing the thyroid gland from producing thyroid hormone.

The affected thyroid gland(s) can be surgically removed. The procedure is relatively straightforward and often achieves a permanent cure without the need for lifelong medication. If only one lobe of the gland is found to be involved at the time of surgery, this is all that is removed; however, the possibility that the other lobe can become affected in the future still exists. Another concern with surgery is that it involves general anesthesia. Because geriatric hyperthyroid cats may have additional health problems, anesthesia may be more risky.

A third treatment is radioactive iodine. This is a very effective treatment option and is considered by many to be the "gold standard." It is given by injection and acts by destroying the abnormal thyroid cells. Radioactive iodine has the advantage of being able to destroy all abnormal thyroid cells — even those not visible during surgery. This procedure must be performed at a facility licensed to handle and administer radioactive substances. Because treated cats must eliminate the radioactive iodine through the urine and feces, they must stay hospitalized at the facility for 3-7 days until determined to be free of radioactivity. This is usually the most expensive treatment option.

It is important to understand that the blood supply to the kidneys will decrease after treatment of hyperthyroidism. We use methimazole before surgery or radioactive iodine because the effect of oral medication is reversible. If kidney function worsens with treatment by methimazole, a permanent form of treatment is avoided. If kidney function remains normal, then permanent treatment is an option.