Methimazole 101: What You Need to Know if Your Cat Needs Methimazole

Methimazole is a medication commonly prescribed by veterinarians for the long-term medical management of hyperthyroidism in cats (Feline Hyperthyroidism).

Your veterinarian may prescribe methimazole under the brand names Tapazole® or Felimazole®. It can be given as an oral tablet, oral liquid, oral suspension, or applied topically as a transdermal gel, usually in the cat's ear.

Methimazole is an antithyroid medication that’s used to treat hyperthyroidism in cats. Hyperthyroidism is a condition where your cat’s thyroid gland is overactive. It is most common in older cats. If left untreated, it can cause serious health problems such as heart disease, blindness, weight loss and vomiting/diarrhea.

Methimazole is given long-term to manage thyroid hormone levels and can also be used to normalize thyroid levels before thyroid surgery. Treatment with Methimazole is a form of medical management; it does not cure hyperthyroidism.

Methimazole works by blocking the production of thyroxine (T4), a hormone produced by the thyroid gland, which helps to lower thyroid hormone levels in the blood.

Hyperthyroidism occurs when the thyroid gland produces excessive amounts of thyroid hormones. Methimazole inhibits the body's production of thyroid hormones by interfering with some of the metabolic steps. It has no effect on pre-existing circulating or stored thyroid hormones and it has no effect on supplemented thyroid hormones.

It can take 2-4 weeks before you notice a change in your cat’s symptoms.

Adverse side-effects due to methimazole use usually occur within the first three months of therapy.

• Gastrointestinal side effects are the most-common issues for cats on methimazole. Your cat may experience nausea, vomiting, loss of appetite, and just not feel well. This will usually resolve itself after a few days and most cats stop vomiting and begin to feel better without any change to medication.
• There is a lower incidence of digestive upset in cats that are receiving the transdermal gel, but it may take longer before the full effect of the transdermal treatment can be seen. Also, skin lesions and other cutaneous reactions may occur on the application site.
• Within the first few weeks of treatment, a small number of cats self-mutilate their face and neck through scratching. These animals will probably need to discontinue treatment.
• Temporary changes in blood counts usually occur within the first two months of therapy. A very small number of cats will develop very serious changes in their bone marrow, blood counts or liver problems.
• Some cats receiving methimazole for more than six months develop other blood abnormalities (positive ANA). These cats may require a dose reduction.
• Medications may interact with methimazole and should be used with caution: benzimidazole antiparasitics, beta-blockers, digoxin, phenobarbital, theophylline, and warfarin.
• Very rarely, acquired myasthenia gravis (a disease that weakens muscles) may occur.
• Be sure to tell your veterinarian about any medications, including vitamins, supplements, or herbal therapies that your pet is taking.

• Methimazole should be avoided or used with extra monitoring in cats with liver disease, autoimmune disease, or pre-existing blood abnormalities.
• Regular monitoring of thyroid hormone levels is necessary in order to avoid drug-induced hypothyroidism (under-active thyroid).
• Kittens born to cats receiving methimazole may be born with low thyroid levels. Your veterinarian may recommend a milk replacement for your kitten.

Use caution when handling Methimazole and your cat’s bodily fluids or stool. Exposure can be harmful to humans. Pregnant and nursing women, or women who may become pregnant should wear gloves when handling the medication, cat litter, or body fluids. Anyone applying the transdermal gel should wear gloves during application.

If you are exposed to Methimazole, you may experience side effects. These can include vomiting, headache, and abdominal pain. Let your healthcare provider know right away if this happens.

Wedgewood provides medication options that help ensure accurate dosing, especially for hard to medicate pets. Click below for a complete list of Wedgewood’s dosing forms and strengths.

View all Methimazole options

If you miss giving your pet a dose, give the next dose as soon as you remember or, if it is close to the next scheduled dose, return to the regular schedule. Do not double dose to catch up. If you are not sure what to do, call your veterinarian and follow their directions.

Methimazole metabolizes quickly and overdose effects should not last more than 24 hours but may last longer in pets with liver or kidney disease. Symptoms of a methimazole overdose are generally stronger incidences of the side effects listed above.

If you suspect your pet or another animal has accidentally overdosed or has eaten this medication inadvertently, immediately contact your veterinarian or the A.S.P.C.A.’s Animal Poison Control Center at 888-426-4435. Remember to take your prescription container with you when you take your pet for treatment.

If you or someone else has ingested this medication, call the National Capital Poison Center at 800-222-1222.

There are several other treatment options for hyperthyroidism in cats. Which one is best for your cat depends on the cause of its hyperthyroidism, its age, and its other health conditions.

• Radioiodine Treatment: Radioactive iodine is absorbed into the thyroid gland to kill abnormal cells. The procedure has a cure rate of over 95% and doesn’t require general anesthesia nor continued medication post treatment. There are minimal to no side effects to the treatment in most cases.
• Thyroid Surgery: Surgery to remove abnormal thyroid tissue. But surgery can be risky, especially in older cats or those with certain health conditions. It can also cause issues, like low calcium levels in the blood. In some cases, another surgery is needed to remove any remaining tissue.
• Low Iodine Diet: The thyroid gland needs Iodine to make thyroid hormones. Restricting the amount of iodine in your cat’s diet can lower thyroid hormone levels and improve symptoms. Like methimazole, this treatment option will need to be continued for the rest of your cat’s life. There are specific prescription diets available and require that the cat gets no other food or treats.

Does Methimazole Cure Hyperthyroidism in Cats?

No. Methimazole doesn’t cure the cause of feline hyperthyroidism, but it does help keep thyroid hormone levels under control. It is likely that your cat will be on methimazole for the rest of its life.

What Does Methimazole Do to Cats?

Methimazole is an antithyroid medication that's used to treat hyperthyroidism. It's given long term to manage thyroid hormone levels in cats. It can also be used to normalize levels before thyroid surgery. Methimazole works by blocking thyroid hormones from being made.

What Happens if I’m Exposed to Methimazole?

It is possible to be exposed to methimazole while handling the medication or through your cat’s vomit, urine, or stool. You should wear gloves when cleaning up vomit or emptying the litter box and when you are administering the medication, particularly in topical gel form. Don’t break or crush tablets.

If you’re exposed to methimazole, you may experience vomiting, headache, and abdominal pain. It can also cause harm to babies, so pregnant or nursing mothers should use extra caution.

Contact your healthcare provider right away if you think you are having adverse effects from methimazole.

Is There Any Monitoring That Needs to Be Done with Methimazole?

Your cat should be monitored closely for adverse effects such as tiredness, vomiting, loss of appetite, yellowing of the skin or eyes, or itchiness. If these are seen, discontinue the medication and contact your veterinarian.

Your veterinarian will most likely check your cat’s blood before starting methimazole to establish a baseline of thyroid function, and then check again every 2-3 weeks for the first 3 months to track thyroid hormone levels. Once dosing is stabilized, thyroid levels should be checked every 3-6 months.

If your cat has liver disease, your veterinarian may monitor liver enzymes to ensure it is not affecting renal function.

When Should Methimazole NOT Be Given to Cats?

Methimazole should not be used in cats that are pregnant or nursing and in those with pre-existing liver or hematological diseases such as anemia, platelet disorders, and in cats with autoimmune diseases.

How Long Does It Take for Methimazole to Work?

It depends on the cat’s metabolism. Methimazole typically takes between 2-4 weeks to take effect. During this time, it is important to observe and record your cat’s symptoms and behavior so you can report them during your next visit. Your veterinarian may need to fine tune the dosage up or down depending on your cat’s response to methimazole.

This article is meant to provide general and not medical advice. We strongly recommend that a veterinarian be consulted for the specific medical needs of your animal.

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Methimazole for Veterinary Use