Background

Hyperthyroidism is a condition in which the body makes too much (hyper means “too much“) of the thyroid hormone thyroxine (also called T4). Methimazole stops the production of thyroid hormones. 

Methimazole, available under various brand names depending on location, is the most common medication prescribed to treat feline hyperthyroidism. Carbimazole is closely related to methimazole and is available in some countries but not in the U.S. Carbimazole is converted into methimazole by the body, so the same information applies to both. Only the name methimazole will be used below.

Other options for treating hyperthyroidism exist – radioactive iodine, surgical removal of part or all of the thyroid, and dietary. Which one is best depends on you and your cat. 

While radioactive iodine and surgery can potentially cure hyperthyroidism, methimazole is not a cure. With lifelong therapy, the hyperthyroidism can be controlled in most cats.

Hyperthyroidism in cats is usually caused by a benign thyroid tumor. It is important to realize that methimazole only stops the tumor from making thyroid hormones and does not kill the tumor. In fact, the tumor will continue to grow, and, after a few years or so of treatment, it can become much larger. It may also become cancerous.

Hyperthyroidism in cats is usually caused by a benign thyroid tumor. It is important to realize that methimazole only stops the tumor from making thyroid hormones and does not kill the tumor. In fact, the tumor will continue to grow, and, after a few years or so of treatment, it can become much larger. It may also become cancerous.

How This Medication is Used

Methimazole can be given by mouth or applied to the cat’s skin (called transdermal), usually the hairless part inside the ear. It is typically given twice daily; uncommonly, once daily will be enough. The same dose does not work for all cats, so your veterinarian will need to do repeated blood testing at first and may need to adjust the dose. Once a correct dose is found, testing will be less often. 

Over time, as the tumor grows, an increasing dose of methimazole may be needed. In some cats, the methimazole will no longer control hyperthyroidism. Regular testing is recommended to make sure the dose is still correct and the hyperthyroidism is controlled without the thyroid hormones going too low.

It may take longer for transdermal methimazole to control the hyperthyroidism compared to giving it by mouth. If your cat has severe signs of hyperthyroidism, it may be better to start by giving it by mouth. Transdermal therapy can be considered once the hyperthyroidism is under control and your cat is doing well.

Side Effects

The most common side effects of methimazole when given by mouth, seen in about one in five cats, are stomach or intestinal, such as decreased appetite, vomiting, and/or diarrhea. If these happen, methimazole should be stopped and not restarted until your cat has returned to normal; it should take no more than a few days. The medication should be restarted at a lower dose and gradually increased. Most, but not all, cats will be able to take methimazole. Stomach or intestinal side effects are very uncommon when giving transdermal methimazole. If your cat can’t take methimazole due to stomach or intestinal upset, the transdermal route can be tried.

If the dose of methimazole is too high, thyroid hormones can be too low in the blood. That’s called hypothyroidism (hypo means low or “not enough”), which can cause its own set of problems, and the condition should be avoided. Hypothyroidism is easily fixed by lowering the methimazole dose.

Severe side effects may occur in rare instances. These can develop whether your cat is taking methimazole by mouth or on the skin. Usually, only one side effect is seen in cats, not a combination. If any of these happen, the methimazole must be stopped and never used again by any route. All these side effects will go away when the medication is stopped. Other medications may be needed to help your cat during recovery.

The severe side effects are:

• Severe itchiness of the face. Your cat will constantly scratch their face, which can cause sores.
• Low white blood cell counts. Specifically, a type of white blood cell called neutrophils is involved and disappears from the blood. Neutrophils help fight infection in the body. With no neutrophils, your cat will be much more likely to get sick from things like bacteria or viruses.
• Low platelet counts. Platelets help blood to clot. Without platelets, bleeding can start on its own. Your cat may bleed from the gums, bruise easily, or have nosebleeds. A lot of blood can be lost.
• Liver failure. Liver failure will make a cat very sick. Typically, low activity and appetite, vomiting, and other problems accompany liver failure.

Low neutrophils, low platelets, and liver failure can all be deadly if not dealt with immediately. The severe side effects almost always happen in the first 3 months of therapy, if they occur at all.

Interactions With Other Drugs

Hyperthyroidism can potentially be treated with a specific diet from Hill’s called y/d. Methimazole should be used with caution in cats, also on a y/d diet, as the combination can make the thyroid hormones go too low and cause hypothyroidism.

Concerns and Cautions

Hyperthyroidism can mask kidney disease or kidney failure. In other words, the kidneys are not functioning normally, but hyperthyroidism helps them function better.

Treatment of hyperthyroidism with any method can help your veterinarian detect kidney disease. This occurs in about 20% of cats treated for hyperthyroidism, and the kidney disease identified is usually mild. Although some treatments for the kidney disease may be recommended, in the long term, allowing hyperthyroidism to support the kidneys will be bad for your cat’s health.

It is much better to treat the hyperthyroidism and mild kidney disease together.

If kidney disease is already diagnosed when the hyperthyroidism is found, treatment of the hyperthyroidism is more difficult. The starting dose of methimazole should be lower than usual, and the kidney function should be watched very closely. The methimazole dose may be able to be increased over time.

Handling Cautions

Avoid getting methimazole on your skin. This is a bigger concern with the transdermal form, and it is recommended to wear gloves while handling transdermal methimazole. When giving your cat methimazole by mouth, you should wear gloves, and if not, wash your hands immediately afterward. Don't swallow any of the medication. Keep out of the reach of children and other pets.