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Hyperthyroidism in Cats

What is it?

Hyperthyroidism occurs when the thyroid gland, a small paired gland in the neck, overproduces thyroid hormone.  Thyroid hormone regulates the metabolism in the body, and when it is over-produced, the metabolism speeds up, resulting in weight loss and a rapid heart rate, and sometimes in excess drinking, urination and diarrhea.  The good news is that it is a treatable condition in cats.  There are three treatment options:  lifetime medication, surgery to remove the thyroid, or injection with radioactive iodine.  A consideration with any of these treatment options is that sometimes these cats, who tend to be elderly, have underlying kidney disease.  When the body is in a hyperthyroid state, the blood pressure tends to be higher, which brings more blood to the kidneys, masking any kidney disease.  If we return the thyroid level to normal, if there is underlying kidney disease, the cat may start to show signs of problems with the kidneys (vomiting, poor appetite, etc.)

Medication with Tapazole

Tapazole (methimazole) is a medication that reduces the production of thyroid hormone.  The pros of medication with tapazole are that we can stop administering it if the cat does experience any kidney problems.  The cost is about $30 per month.  Side effects sometimes include problems with the liver, reduced appetite or vomiting, although most cats do not experience side effects.  The cons include having to administer the pills twice daily, and over time, the cat will need an increasing amount of tapazole because the underlying thyroid condition is not definitively treated and the gland will continue to produce more and more hormone.  The cost of the medication and rechecking bloodwork (to make sure the cat’s system is tolerating the medicine and that the dose is correct) can add up. 

Surgery to remove the thyroid

Surgery can be performed to remove the abnormal thyroid gland.  Prior to any surgery, tapazole will need to be administered for a few weeks and the kidneys rechecked to make sure that the cat’s kidneys are healthy before surgery.  Surgery is a good option because it definitively treats the problem by removing the diseased gland, and usually medication will not need to be administered on a daily basis afterward.  However, there are a number of concerns with having the surgery, including the risks of anesthesia and surgery on an older patient.  Other complications include that some cats will actually have a LOW thyroid after surgery and will still need to be pilled daily with thyroid supplement.  The parathyroid gland, which is located on the thyroid gland, is sometimes inadvertently removed causing problems with calcium regulation after the surgery.  The cost is about $400 for the surgery.

Radioactive Iodine Treatment

If the cat’s kidneys are normal and the cat is otherwise healthy, the cat may be a good candidate for radioactive iodine treatment.  This treatment involves going to Cornell University where an injection of radioactively tagged iodine is given.  This chemical is taken up only by the abnormal cells in the thyroid that are overproducing hormone.  These cells are killed, leaving behind the healthy cells which can still function normally.  The treatment is very safe and carries few side effects.  The only drawback is that the cats need to be boarded at the University for approximately five days, to minimize human exposure to the radioactive chemical.  After discharge from the University, some precautions will need to be taken with the kitty litter for a time after the treatment.  Some of these cats may require re-treatment years later if they re-develop the condition.  The cost is approximately $800-1300.

For more information, please visit these links:


Feline Hyperthyroidism

Winn Feline Foundation on Hyperthyroidism