Hyperthyroidism in Cats—Two FDA-Approved Drugs Available to Treat It
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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 methimazole work?
Do cats on methimazole need to be monitored?
What are the side effects of methimazole?
Are there any warnings for people who handle methimazole?
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.
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 due to 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.
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 blood tests that measure the level of your cat's thyroid hormones.
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 doing another treatment option, such as radioactive iodine therapy that uses radiation to destroy the abnormal thyroid tissue or surgery to remove the overactive thyroid gland.
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 label. 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 labels for the human methimazole products include information about how to use the drug safely and effectively in people, not in cats.
Two drugs, Felimazole oral tablets and Felanorm oral solution, are FDA-approved to treat hyperthyroidism in cats. Both drugs contain methimazole as the active ingredient. Unlike the methimazole products for people, Felimazole and Felanorm have been shown to be safe and effective in cats when used according to the directions on the label. In addition, the drugs' labels provide dosing and safety information specific to cats. Both Felimazole and Felanorm are only available with a veterinarian’s prescription.
How does methimazole work?
Methimazole, the active ingredient in Felimazole and Felanorm, is an antithyroid drug that works by blocking the production of thyroid hormones.
Do cats on methimazole need to be monitored?
Yes. Before starting methimazole, 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 tests to measure the level of thyroid hormones. Blood tests should be repeated 3 weeks and 6 weeks after starting methimazole 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 methimazole (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 methimazole. Older cats often have both hyperthyroidism and kidney disease, and treating these cats is a delicate balancing act. In cats with hyperthyroidism, the high level of thyroid hormones increases the blood flow to the kidneys, which helps their kidneys and may even hide kidney disease. Once the excessive level of thyroid hormones is lowered by methimazole (or any treatment for hyperthyroidism, such as radiation therapy or surgery), 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.
What are the side effects of methimazole?
The most common side effects of methimazole reported to FDA’s Center for Veterinary Medicine are:
- decreased appetite;
- vomiting;
- decreased activity level;
- skin reactions, such as itchy skin (especially on the face, head, and neck), hair loss, and other skin lesions;
- weight loss;
- elevated liver values;
- decreased red blood cell level (anemia);
- behavioral changes (for example, vocalizing or hiding);
- elevated kidney values; and
- diarrhea or loose stool.
FDA encourages you to call your veterinarian if you think your cat is having a side effect from Felimazole or Felanorm. A side effect associated with a drug is called an adverse event. Adverse events also include a lack of effectiveness (the drug doesn’t do what it’s supposed to do) and reactions in people who handle the drug. Call your healthcare provider if you have a reaction to Felimazole or Felanorm.
FDA also encourages you to work with your veterinarian to report any adverse event - in either cats or people - associated with Felimazole or Felanorm and any product defect such as the bottle has a broken seal or is leaking or the drug is off-color. See Report a Problem.
Less frequently reported, but serious side effects of methimazole 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. When a cat has low platelets, the blood can’t clot as well, which may lead to bleeding problems; 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 methimazole for signs of illness, including fever, enlarged lymph nodes, and anemia. If your cat seems sick, your veterinarian may recommend stopping the methimazole and doing appropriate tests.
Are there any warnings for people who handle methimazole?
Wash your hands with soap and water after giving your cat methimazole to avoid accidental exposure to the drug. Trace amounts of methimazole can be found in a treated cat’s feces (stool) and bodily fluids, so 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 Felimazole or Felanorm, including when you give the drug to your cat.
Methimazole 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 use extra caution when handling or administering Felimazole or Felanorm and when coming into contract with a treated cat's litter or bodily fluids.
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 methimazole, call your healthcare provider immediately and show the drug’s label to the provider.
Resources for You
See Animal Drugs @ FDA for more information about Felimazole and Felanorm.
Contact FDA’s Center for Veterinary Medicine at either AskCVM@fda.hhs.gov or 240-402-7002.