The Effects of Thyroid Medications on Cats

The Effects of Thyroid Medications on Cats
The Effects of Thyroid Medications on Cats (Image: Marktilley |

Uncontrolled hyperthyroidism can take a toll on every organ in a cat's body. But to a cat struggling with the side effects of thyroid medication, the cure must seem almost as bad as the disease. Although uncommon, side effects do occur. If your cat has an adverse reaction to methimazole (brand names Tapazole or Felimazole), ask your veterinarian for an alternative treatment plan.

Facial Itching

If your cat is scratching around the face and ears, calendula cream will relieve the itchiness until your veterinarian can start the cat on a medication other than methimazole. Health food stores sell this herbal remedy.

Loss of Appetite and Vomiting

If your cat vomits often or stops eating, discontinue the methimazole until the stomach upset is gone. Then ask your veterinarian to prescribe a lower dose and increase it gradually until the cat is getting the correct amount of medication.


Lethargy, too, can be a side effect of methimazole when the starting dose is too high. Discontinue the medicine until your cat's energy level is back to normal. Then start again at a low dose and work your way up.

Bone Marrow Changes

If blood tests reveal changes in your cat's white blood cell patterns, your veterinarian will stop the methimazole immediately and decide on another course of treatment.

Liver Toxicity

Liver toxicity usually resolves when the cat stops taking methimazole.


Most of the side effects of methimazole occur during the first three months of therapy. Minimize or avoid side effects by asking your veterinarian to start your cat on a very low dose and gradually work up to a therapeutic amount. Careful monitoring and frequent blood tests will tell your vet when the cat is getting the correct dosage.

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