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Hyperthyroidism in cats
Last reviewed: 08.07.2025

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The following information is not intended to replace regular visits to the veterinarian. If you think your cat may have hyperthyroidism, contact your veterinarian immediately. Remember, do not give your cat any medications without first consulting your veterinarian.
What is hyperthyroidism?
Hyperthyroidism is the most common thyroid disorder in cats. It is most often caused by excessive levels of circulating thyroxine, a thyroid hormone more commonly known as T4, in the bloodstream.
What are the symptoms of hyperthyroidism?
Weight loss and increased appetite are among the most common clinical signs of this condition. Weight loss is seen in 95-98% of cases of feline hyperthyroidism, and increased appetite is seen in 67-81% of cases. Excessive thirst, increased urination, hyperactivity, unkempt appearance, panting, diarrhea, and increased hair loss have also been reported. Vomiting is seen in approximately 50% of affected cats. The clinical signs are a result of the effects of elevated T4 on various organ systems.
What cat breeds (and ages) are prone to hyperthyroidism?
Hyperthyroidism can occur in any breed of cat, male or female, but it occurs almost exclusively in older animals. Less than 6% of cases occur in cats under 10 years of age. The average age of onset is 12 to 13 years.
How is hyperthyroidism diagnosed?
Because some common diseases in older cats, such as diabetes, inflammatory bowel disease, intestinal cancer, and chronic renal failure, share some of the same clinical signs with hyperthyroidism, a number of tests are needed. A complete blood count, blood chemistry panel, and urinalysis alone will not diagnose hyperthyroidism, but they can definitely rule out diabetes and renal failure. Cats with hyperthyroidism may have normal complete blood counts and urinalysis results, but blood chemistry panels often show elevated levels of certain liver enzymes.
In the vast majority of cases, a definitive diagnosis of hyperthyroidism is based on a simple blood test that shows elevated T4 levels in the bloodstream. Unfortunately, 2-10% of cats with hyperthyroidism have normal T4 levels. One possible explanation is that in mild cases, T4 levels may rise and fall within the normal range. Another explanation is that an underlying medical condition is keeping T4 levels down to the normal range or upper limit of normal, leading the veterinarian to mistakenly believe that the cat's thyroid status is normal. Because these are older cats, underlying medical conditions are common, and diagnosing hyperthyroidism in these cats can be challenging.
How is hyperthyroidism treated?
There are several treatment options for hyperthyroidism, each with its own advantages and disadvantages.
- Oral administration of an antithyroid drug. Methimazole (trade name TapazoleTM) has long been the mainstay of drug therapy for hyperthyroidism in cats. It is very effective in treating the condition, often producing results in 2 to 3 weeks. Unfortunately, 15 to 20% of cats experience side effects such as loss of appetite, vomiting, lethargy, bleeding disorders, jaundice, itching of the head and face, and sometimes blood cell changes. Most side effects are mild and eventually resolve, although sometimes the drug may need to be discontinued. Lifelong daily medication is required, which is a disadvantage for owners whose cats are resistant to taking the pills. Complete blood counts and T4 levels should be repeated regularly for the rest of the cat’s life.
- Surgical removal of the thyroid gland. Hyperthyroidism is usually caused by a benign tumor called a thyroid adenoma that involves one or, more commonly, both lobes of the thyroid gland. Fortunately, most cats with hyperthyroidism have benign, well-encapsulated tumors that are easily removed. Surgery is usually curative, but anesthesia may be undesirable in older patients because their disease may have affected the heart and other organs. Although surgery may seem expensive, it is often less expensive than years of oral medications and regular blood tests.
- Radioactive iodine therapy. This is the best and most complex treatment option. Radioactive iodine, which is injected (usually under the skin), concentrates in the thyroid gland, where it is irradiated and destroys the hyperfunctioning tissue. No anesthesia or surgery is required, and usually only one course of treatment is needed to achieve a cure. Radioactive iodine treatment used to be done only in specialized, licensed facilities, but there are now many private treatment facilities. Hospitalization may be extended. Depending on local or state regulations, cats may need to stay in the facility for 10 to 14 days until the radioactivity in the urine and feces has decreased to acceptable levels. Radioactive iodine therapy is also expensive. The cost has dropped from about $1,200 to $500 to $800, but it is still prohibitive for most cat owners.