An overactive thyroid, which vets call hyperthyroidism, means the thyroid gland makes too much thyroid hormone, so the cat's body runs too fast. The FDA calls it the most common hormone disease of cats older than 8 and says a benign growth is the cause in almost 99 percent of cases. With treatment, Cornell University's Feline Health Center says, the outlook is generally good, and without it, the FDA says, the disease is almost 100 percent fatal.
The thyroid gland sits in the neck, and the FDA describes its job as regulating the body's engine. In hyperthyroidism an enlarged gland makes too much of its hormones, T3 and T4. Cornell says the usual cause is a growth that is not cancer, called an adenoma. The Merck Veterinary Manual says both halves of the gland are affected in about 70% of cases.
The American Association of Feline Practitioners says there is no clear picture yet of what causes it, and puts the disease at up to 10% of US cats older than 10.
The FDA lists weight loss despite an increased appetite, vomiting, diarrhea, hyperactivity, drinking and urinating more than normal, and an unkempt coat. The disease comes on gradually, so the signs are easy to miss at first. Rarely, the Merck manual's owner pages say, a hyperthyroid cat eats less and turns quiet.
Cornell says thyroid hormones affect nearly all of the organs. The FDA says the heart has to pump faster and harder, its muscle thickens, and in time the heart can fail. Blood pressure rises too, and in some cats it climbs so high that the retina comes away from the back of the eye, which causes sudden blindness.
| Test | What it measures | What the result tells the vet |
|---|---|---|
| Feeling the neck | Whether the thyroid gland is enlarged. | The Merck manual says a lump in the gland can often be felt. |
| Total T4, in blood | The level of the main thyroid hormone. | Cornell says most hyperthyroid cats have a high T4. The Merck manual says about 5 to 10% of affected cats have a normal one, so a normal result does not rule the disease out. |
| Further thyroid tests | Free T4, a T3 suppression test or a thyroid scan. | They are for the cat that has the signs but a normal T4. |
| Blood count, chemistry panel and urine test | The blood cells, the kidneys and the liver. | The FDA says high thyroid hormone increases blood flow to the kidneys and may hide kidney disease. The blood count is the starting point before the medicine, which lowers blood cells in a few cats. |
| Blood pressure and heart rate | The strain on the heart and the blood vessels. | The FDA says kidney disease and hyperthyroidism are the two most common causes of high blood pressure in cats. |
More on the samples: blood tests and urine samples.
The feline practitioners recommend treating every hyperthyroid cat. The Merck manual's owner pages list four ways.
| Treatment | What happens | What to weigh |
|---|---|---|
| Medicine (methimazole) | A tablet or liquid by mouth blocks the making of thyroid hormone. It is given for life, or to steady the cat before another treatment. | It controls the disease but leaves the growth in place. Blood is tested before it starts, after 3 and 6 weeks, and then every 3 months. |
| Radioactive iodine | The cat gets one injection. The thyroid takes up the iodine, and the radiation destroys the overactive tissue. | Cornell calls it the treatment of choice where it is available. It needs no anesthetic, and in about 95% of cases the disease is gone within three months. The cat stays at a licensed hospital, usually for three to five days, and most hospitals allow no visitors. Up to half of cats end up with thyroid levels that are too low, and about a third of those show signs and may need thyroid hormone. |
| Surgery | The overactive gland is removed under a general anesthetic. | Cornell says it is rarely chosen, because the medicine and radioactive iodine work as well and are less invasive. The small glands beside the thyroid, which control blood calcium, can be damaged. |
| Iodine-restricted food | A prescription food with too little iodine for the gland to make excess hormone. | The Merck manual says the cat must eat only this food for the rest of its life, and that other food, treats or prey make it ineffective. The feline practitioners say that is difficult in a home with several cats or for a cat that goes outdoors. |
The FDA approved Felimazole coated tablets in May 2009, a liquid called Felanorm in July 2024 and a generic coated tablet in July 2025. All three contain methimazole and are given by mouth. Cornell also mentions a gel for the skin, which is not one of those three. The Felimazole label is on What is Felimazole?
The FDA's list of side effects starts with a poor appetite, vomiting and low energy, and includes an itchy face, head and neck. The serious ones are liver disease and low blood cell counts. The Merck manual says serious effects develop in fewer than 5% of cats, and its owner pages say most side effects develop in the first three months.
Whatever the treatment, the kidneys are watched, because lowering the thyroid hormone lets hidden kidney disease show. The feline practitioners say a trial of the medicine before radioactive iodine or surgery may predict how the kidneys will cope.
The label carries warnings for people. Wash your hands after giving the medicine, do not break or crush the tablets, and wear gloves to handle the litter, urine, stool or vomit of a treated cat. Women who are pregnant, may become pregnant or are nursing should wear gloves to handle the tablets too. The FDA says methimazole can cause birth defects.
Cornell says the outlook is generally good with treatment, and that trouble in other organs can make it worse. The feline practitioners' guideline says cats without kidney disease have a median survival of up to 5.3 years. Cornell adds that the heart changes and the high blood pressure often improve once the thyroid is under control.
None of the sources read for this page gives a national price for any of the four treatments. Cornell calls the medicine relatively inexpensive, but it comes with blood tests for life: CareCredit's national average for a cat's chemistry panel is $144, from a 2025 to 2026 cost study by Synchrony. Radioactive iodine is done only at specially licensed clinics and includes a hospital stay. Ask for both totals in writing. More figures are on What does a vet bill cost? Help is on When you cannot pay the vet bill.
Go now if a hyperthyroid cat suddenly cannot see, or breathes with its mouth open or with effort.
Call the vet the same day if a cat on methimazole stops eating, vomits, scratches its face or neck, has a fever or seems very dull. The label lists these among the signs of illness to watch for, and warns that the medicine can cause bleeding.
All the signs are on Cats: is this an emergency?
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Sources read on 2 October 2026; every number is as the source gives it.