Methimazole for cats: how it treats hyperthyroidism and side effects

What is methimazole for cats used for?

Methimazole for cats treats hyperthyroidism, a common hormone disease of older cats. In hyperthyroid cats, the thyroid gland in the neck makes too much thyroid hormone. That speeds up the whole body. Typical signs are weight loss despite a big appetite, drinking and peeing more, restlessness, vomiting and a scruffy coat. Read more in our guide to hyperthyroidism in cats.

Methimazole blocks the thyroid from making hormone. The FDA has approved two methimazole products for cats in the US: Felimazole tablets and Felanorm oral solution. Carbimazole, used in many other countries, turns into methimazole in the body, so it works the same way.

The key point for owners is that methimazole controls hyperthyroidism. It doesn’t cure it. Cornell Feline Health Center and International Cat Care both make this clear. If you stop giving it, the thyroid goes back to overproducing hormone. Used on its own, it’s a lifelong medicine.

How is methimazole given to cats?

Your vet will set the dose for your cat and tell you how often to give it. They will adjust it over time based on blood tests. There are three main forms.

If your cat vomits after an oral dose given on an empty stomach, VCA suggests giving the next one with food or a small treat. If pilling is a struggle, see our guide on how to give a cat a pill.

How does transdermal methimazole work?

Transdermal methimazole is absorbed through the thin skin inside the ear. It helps cats that fight tablets. The 2016 AAFP guidelines say stomach upset may be less common with the gel. If the skin gets red or irritated, your vet may switch your cat back to tablets.

A few rules make the gel safer for your household:

• Wear gloves every time you apply it
• Rub it in until it’s fully absorbed
• VCA advises keeping children and other animals from touching the ear for two hours afterward
• If your cats groom each other, the gel may not suit your home, because the other cat can swallow it

Why does my cat need blood tests on methimazole?

Blood tests are part of the treatment, not an extra. They do three jobs:

1. Check the thyroid level. Too little medicine leaves your cat hyperthyroid. Too much can make the thyroid underactive.
2. Catch side effects early. Methimazole can rarely affect blood cells and the liver, often before you’d notice anything at home.
3. Watch the kidneys. More on this below.

The FDA advises repeating blood tests 3 weeks and 6 weeks after starting methimazole, then every 3 months. Merck Veterinary Manual notes that most side effects develop during the first 3 months, which is why early checks are closer together. Your vet may use a different schedule for your cat, especially if the dose changes or your cat has other health problems.

How will I know methimazole is working?

Changes can be slow. VCA notes it can take days to weeks before you see an effect, and sometimes the effects aren’t obvious at home. The AAFP guidelines say cats can be expected to improve once their thyroid level is back in the normal range.

Things worth noting down between visits:

• Weight, if you can weigh your cat at home or at the clinic
• Appetite, and whether your cat still seems ravenous
• How much they drink and pee
• Vomiting, restlessness or extra yowling

Bring these notes to each recheck. Together with the blood results, they help your vet decide whether to change the dose.

If you miss a dose, VCA advises giving it when you remember, unless the next one is nearly due. Never give two doses at once to catch up. If doses are often missed, tell your vet, since a different form may be easier to give.

Can methimazole unmask kidney disease?

Yes. This is one of the most important things to understand about treating hyperthyroid cats. An overactive thyroid increases blood flow through the kidneys. That can make kidney blood tests look better than the kidneys really are.

When methimazole brings the thyroid back to normal, that extra blood flow drops. The Felimazole label warns that this can unmask kidney disease that was already there. Merck notes this can happen even in cats whose kidney tests looked normal before treatment. It happens with any successful thyroid treatment, not only methimazole.

This doesn’t mean you should leave the thyroid untreated. The 2016 AAFP guidelines recommend treating hyperthyroidism even in cats with kidney disease, and warn that keeping a cat “a little bit” hyperthyroid to protect the kidneys does more harm than good. Your vet will manage both conditions together.

Because methimazole’s effect is reversible, the guidelines also note that a methimazole trial before radioactive iodine or surgery can show how the kidneys will cope once the thyroid is controlled.

What are the side effects of methimazole for cats?

Most cats take methimazole without problems. International Cat Care describes side effects as uncommon, and mild stomach upset often settles in the first few weeks. The serious reactions are rare, but they’re the reason for regular blood tests.

The FDA lists low platelets, immune-related anemia and a severely low level of the white blood cells that fight infection among the serious side effects. The AAFP guidelines note that side effects haven’t been shown to depend on the dose. So a serious reaction usually means stopping methimazole, not just lowering it. Your vet will then discuss other treatments.

If your cat keeps vomiting or stops eating, call your vet rather than skipping doses on your own. They may suggest giving it with food, switching to the gel or checking bloodwork.

Is methimazole safe for people to handle?

Methimazole is also used in human medicine, and the cat labels warn owners to limit their own exposure. The Felimazole label and the FDA advise:

• Wash your hands with soap and water after every dose
• Don’t break or crush tablets
• Wear gloves to scoop litter or clean up urine, feces or vomit from a treated cat
• Pregnant women, women who may become pregnant and nursing mothers should wear gloves when handling tablets, litter or body fluids from treated cats
• Keep it out of reach of children, and store it somewhere secure

Wear gloves for the ear gel, and keep the treated ear away from children for the time your vet advises.

What are the alternatives to methimazole?

Methimazole is often the first treatment because it’s reversible. There are three main alternatives. Our hyperthyroidism guide covers them in more detail.

• Radioactive iodine. A treatment that destroys the overactive thyroid tissue. Merck calls it the curative treatment of choice. It’s done at specialist centers and needs a hospital stay.
• Iodine-restricted prescription diet. The thyroid can’t make hormone without iodine. The cat must eat only this food for life, with no treats, other food or hunting. Merck notes it works better in cats with milder disease.
• Surgery. Removing the affected thyroid tissue under anesthesia.

Some cats with other illnesses aren’t good candidates for every option. Your vet will help you weigh them. For more on other medicines, browse our cat medications guides. If your cat seems unwell after hours, see our cat emergency guide.

Written by the CatsMed Team from the sources above. Reviewed by Dr. Jarod Grantland, DVM. It isn’t a substitute for seeing a vet. See how we research.