Cats Who Are Oversensitive to Thyroid Medication

Am I still radioactive?
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Most cats do well on methimazole, a drug used to treat hyperthryoidism. If your cat isn't one of them, don't despair. There's more than way to treat a cat, or even cure him. Medication can't offer a cure, but other options do. Ask your vet about what's best for Kitty.


Your cat has two thyroid glands in his neck. Hyperthyroidism, or too much of the thyroid hormone circulating in his body, usually occurs because of a benign tumor growing on the gland. Mostly affecting older felines, hyperthyroidism causes symptoms including weight loss, constant hunger, unkempt appearance and lots of drinking and urinating. Signs you can't see include high blood pressure. If you opt for drug treatment for your cat, your vet prescribes methimazole, marketed under the brand names Felimazole and Tapazole. If your cat is sensitive to the drug, it's obvious within a month of beginning treatment.

Methimazole Side Effects

Approximately a sixth of cats receiving methimazole suffer from side effects, according to Veterinary Partner. Your sensitive cat might throw up, become lethargic and stop eating. Take your cat off the medication and consult your vet. She might decrease Kitty's dosage, gradually upping it to the necessary amount needed to control his condition. Most sensitive cats tolerate methimazole when it's given the second time in incremental doses. Cats receive methimazole pills twice daily for the rest of their lives.

Facial Itching

If your cat's sensitivity to methimazole expresses itself in facial itching, that's another story. Your vet can prescribe anti-itching medication to stop the scratching, but the problem will remain for as long as your cat's on methimazole. At that point, you need other options for Kitty.


If your cat is in relatively good health otherwise, surgery to remove the thyroid glands, called a thyroidectomy, is an option. Your vet could determine that your cat is too fragile to go undergo anesthesia. If she gives the thumbs-up, the surgery can cure Kitty. However, before surgery his thyroid levels must be brought down to normal levels, so if he's extremely sensitive to methimazole surgery could be out of the question.

Radioactive Iodine Therapy

Radioactive iodine therapy has become the treatment of choice for hyperthyroidism, although only certain veterinary facilities offer it. It consists of an injection of radioactive iodine into the cat, much like any shot. The downside is that Kitty stays radioactive for several days, so he must remain at the facility—and away from you—until his radiation levels go down. Since caretakers can provide only minimal care during this period, it's not a suitable treatment for cats who require daily medication for other health problems.


Dietary therapy is another option, especially for a cat who can't take methimazole and isn't a candidate for surgery. Your vet prescribes an iodine-deficient food for Kitty. It lowers the amount of thyroid circulating in his body, but he can't eat anything but that particular food. No treats, and no people food or any other kind of cat food. If you have other cats in your household, this approach requires careful oversight.

Always check with your veterinarian before changing your pet’s diet, medication, or physical activity routines. This information is not a substitute for a vet’s opinion.

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