How Well Do Cats Tolerate Hyperthyroid Medication?

I always get a reward for taking my pills.

I always get a reward for taking my pills.

If your cat is diagnosed with hyperthyroidism, you have several treatment options, depending on his overall health. If Fluffy's not a pill about being pilled, you can give him a twice-daily medication called methimazole for cats, marketed under the brand names Tapazole or Felimazole.


Your cat's thyroid glands regulate his metabolism. In cats diagnosed with hyperthyroidism, these glands produce too much thyroid hormone, affecting many of his organs. The condition usually results from a benign tumor on one or both glands. This condition generally affects older felines. Symptoms include weight loss although he's constantly hungry; vomiting and diarrhea; high blood pressure; excessive drinking and urinating; funky coat, and hyperactivity. You might be able to feel enlarged thyroid glands in Fluffy's neck.


Methimazole works by inhibiting excess thyroid hormone. It's generally well tolerated. You'd probably have to pill your cat with it twice daily for the rest of his life. A cat on methimazole requires regular trips to the vet for monitoring dosage and thyroid level. On the plus side, this medication is inexpensive. If pilling your cat proves too daunting, methimazole is available in a transdermal gel that you place in Fluffy's ear. That might be easier on you but it's less effective than the oral medication. The medication treats hyperthyroidism, but it doesn't cure it.


Another hyperthyroid medication, propylthiouracil, marketed as PTU, is used less often than methimazole because of a lower tolerance level and a higher chance of side effects. These include vomiting, appetite loss, anemia and decrease in blood platelets.

Methimazole Side Effects and Contraindications

If your cat has a bad reaction to methimazole, it usually appears within the first three months. Some cat have temporary gastrointestinal upset, which doesn't necessarily warrant stopping the medication. Taking the cat off the drug for a short time until symptoms clear up, and then starting again at a lower dosage, usually does the trick. However, a small number of cats on methimazole exhibit severe facial scratching and self-mutilation. Taking them off methimazole stops the problem, but their hyperthyroidism must then be controlled in another way. More serious side effects include bone marrow suppression Cats with liver, kidney or blood diseases shouldn't receive methimazole.


If your cat doesn't tolerate methimazole or propylthiouracil, there are alternatives. The treatment of choice these days is radioactive iodine, offered only at certain facilities. Fluffy receives an injection of radioactive iodine, which cures his disease. However, because he's radioactive, he must stay at the veterinary facility for a several days or a week until his radiation levels recede. You wouldn't be able to visit him during this time. Another option is surgery to remove the thyroid gland, or a thyroidectomy. This might not be suitable for older, fragile cats, and there's always a risk with any surgery. A prescription veterinary diet restricted to food low in iodine can control the disease in an old or delicate cat.

About the Author

Jane Meggitt has been a writer for more than 20 years. In addition to reporting for a major newspaper chain, she has been published in "Horse News," "Suburban Classic," "Hoof Beats," "Equine Journal" and other publications. She has a Bachelor of Arts in English from New York University and an Associate of Arts from the American Academy of Dramatics Arts, New York City.

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