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Hyperthyroidism in Cats—There’s an FDA-Approved Drug to Treat It

What is hyperthyroidism?
What are the signs of hyperthyroidism in cats?
How is hyperthyroidism in cats diagnosed?
How is hyperthyroidism in cats treated?
How does Felimazole work?
Do cats on Felimazole need to be monitored?
What are the side effects of Felimazole?
Are there any warnings for people who handle Felimazole?
Resources for You


What is hyperthyroidism?

Hyperthyroidism results when the thyroid gland over-produces thyroid hormones. It’s the most common endocrine disease of cats older than 8-years-old. (An endocrine disease is a disease of the organs that make and secrete hormones.)

In almost 99 percent of the cases, hyperthyroidism is caused by a benign (non-cancerous) tumor of the thyroid gland. The butterfly-shaped thyroid gland is located in the neck, with one lobe on each side. It plays an important role in regulating the body’s “engine,” or metabolic rate. When the thyroid gland produces an excessive amount of thyroid hormones, it causes the cat’s “engine” to run at an abnormally high speed. This high metabolic rate negatively affects almost all of the cat’s organs.

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What are the signs of hyperthyroidism in cats?

The most common sign of hyperthyroidism in cats is weight loss despite an increased appetite. Other common signs include vomiting, diarrhea, hyperactivity, drinking and urinating more than normal, and an unkempt hair coat. Because the disease develops gradually, signs are often easy to miss at first.

Hyperthyroidism often leads to high blood pressure (hypertension) and heart disease. The high blood pressure is a consequence of the increased pumping pressure of the heart. In some cats, the blood pressure becomes so high that the retina detaches from the back wall of the eye, resulting in sudden blindness. Heart disease develops because the heart must pump faster and more forcefully to meet the body’s increased metabolic demands (due to the cat’s revving engine). To compensate for this increased workload, the muscles of the heart thicken, causing the heart to enlarge and eventually fail. Untreated hyperthyroidism is almost 100 percent fatal.

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How is hyperthyroidism in cats diagnosed?

Your veterinarian may suspect that your cat has hyperthyroidism based on the signs you describe and by feeling the enlarged thyroid gland in your cat’s neck. Your veterinarian will likely confirm the diagnosis by doing a blood test that measures the level of one of the thyroid hormones, thyroxine (T4). This is called your cat’s total T4 (TT4) level.

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How is hyperthyroidism in cats treated?

One way to treat a cat with hyperthyroidism is with an oral medication that contains methimazole. The medication can be given life-long or to stabilize the cat before other treatment options, such as radioactive iodine therapy or surgery.

For years, there was no methimazole product approved to treat hyperthyroidism in cats in the United States. Veterinarians had to rely on methimazole products approved for people, which they prescribed in an extra-label manner in cats. (Extra-label means using an approved human or animal drug in a way that isn’t listed on the drug’s labeling. It’s sometimes called off-label because the use is “off the label.”) However, the human-approved methimazole products have not been proven to be safe and effective in cats. Also, the labeling includes information about how to use the drug safely and effectively in people, not in cats.

Only one drug, Felimazole Coated Tablets (methimazole), is FDA-approved to treat hyperthyroidism in cats. Approved by FDA in May 2009, Felimazole contains methimazole as the active ingredient. Unlike the methimazole products for people, Felimazole has been shown to be safe and effective in cats when used according to the directions on the label. The drug’s label provides dosing and safety information specific to cats.

Felimazole is available only with a veterinarian’s prescription. The drug is typically given by mouth every 12 hours. Your veterinarian will adjust the dose, as necessary, based on the results of your cat’s blood tests and response to treatment.

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How does Felimazole work?

Methimazole, the active ingredient in Felimazole, is an antithyroid drug that works by blocking the production of thyroid hormones.

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Do cats on Felimazole need to be monitored?

Yes. Before starting Felimazole, your veterinarian should run blood tests on your cat, including a complete blood count to check the levels of red and white blood cells, a chemistry panel to check liver and kidney values, and a TT4. Blood tests should be repeated 3 weeks and 6 weeks after starting Felimazole and then every 3 months for as long as your cat is on the drug. Blood tests should be done more often in cats receiving high doses of Felimazole (more than 10 milligrams per day). Also talk to your veterinarian about monitoring your cat’s blood pressure.

Veterinarians should carefully monitor cats with kidney disease that are receiving Felimazole. Older cats often have both hyperthyroidism and kidney disease, and treating these cats is a delicate balancing act. Hyperthyroidism causes increased blood flow to the kidneys, which improves kidney function and may hide kidney disease. Once the excessive levels of thyroid hormones are lowered with Felimazole (or any treatment for hyperthyroidism), the blood flow to the kidneys returns to normal and the kidney function in some cats will worsen. Regularly checking blood tests and urine samples will help monitor for signs of kidney disease.

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What are the side effects of Felimazole?

The most common side effects of Felimazole reported to FDA’s Center for Veterinary Medicine are:

  • A change in appetite (the cat’s appetite may increase or decrease),
  • decreased activity level (sluggishness),
  • vomiting,
  • diarrhea or loose stool,
  • skin lesions,
  • itchy skin, especially the face, head, and neck,
  • abnormal vocalizing,
  • weight loss,
  • haircoat abnormalities,
  • increased ALT (alanine transferase, a liver value),
  • increased BUN (blood urea nitrogen, a kidney value),
  • weakness, and
  • agitation.

FDA encourages you to call your veterinarian if you think your cat is experiencing an undesired side effect from Felimazole. An undesired side effect associated with a drug is called an adverse drug experience. A lack of effect—the drug doesn’t do what it’s expected to do—is also considered an adverse drug experience.

FDA also encourages you to work with your veterinarian to report any undesired side effect associated with Felimazole. How to report animal drug side effects and product problems.

Less frequently reported, but serious side effects of Felimazole include:

  • liver disease,
  • immune-mediated anemia—the cat’s own immune system starts attacking and destroying red blood cells, causing anemia (low level of red blood cells),
  • low platelets—platelets are found in the blood of all mammals and help with blood clotting, and
  • severely low level of neutrophils in the blood—neutrophils, a type of white blood cell, are part of the body’s immune system. They fight bacteria and other harmful substances. When there aren’t enough neutrophils in the blood, the cat is at greater risk for developing a dangerous infection.

Veterinarians should monitor cats on Felimazole for any sign of illness, including fever, enlarged lymph nodes, and signs of low red blood cells. If these signs occur, your veterinarian may recommend stopping the Felimazole and doing appropriate blood tests.

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Are there any warnings for people who handle Felimazole?

Wash your hands with soap and water after giving your cat Felimazole to avoid accidental exposure to the drug. Do not break or crush the tablets. Wear gloves when you scoop your cat’s litter or if you have contact with your cat’s feces, urine, or vomit. Also wear gloves when you handle broken or moistened Felimazole tablets.

Methimazole, the active ingredient in Felimazole, can cause birth defects in people. The drug crosses the placenta and concentrates in the thyroid gland of the fetus. The drug is also found in breast milk. Pregnant women, women who may become pregnant, and nursing mothers should wear gloves when handling Felimazole tablets, cat litter, or the bodily fluids of treated cats.

In people, methimazole may cause vomiting, stomach distress, headache, fever, joint pain, itching, and low levels of red and white blood cells. If you accidentally ingest Felimazole, call your healthcare provider immediately and show the drug’s label to the provider.

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Resources for You

More information about Felimazole

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