Methimazole for cats is a medication used to treat hyperthyroidism in cats. It is an antithyroid thioamide that inhibits the formation of thyroid hormones. Methimazole for cats can have several potential side effects, however, it remains a popular choice for treating cat hyperthyroidism due to its efficacy and low cost. Continue reading to learn more about using methimazole in cats.

What is methimazole for cats and how does it work?

Methimazole for cats is an antithyroid thioamide that inhibits the formation of thyroid hormone and is dose-dependent. It works by reducing the amount of the hormone thyroxine (T4) produced by the thyroid gland, thus treating conditions such as hyperthyroidism in cats and can be used long-term.

It takes the average hyperthyroid cat 2-4 weeks to reach a normal T4 level after beginning methimazole therapy.

What are the benefits of using methimazole in cats?

Methimazole for cats is an effective and inexpensive way to treat hyperthyroidism. It can be used long-term with few side effects and is particularly useful in older cats and other treatments such as radioactive iodine therapy are not feasible.

What are the side effects of methimazole for cats?

The most common clinical side effects of methimazole in cats are vomiting, inappetence/anorexia, weight loss, lethargy and severe itchiness (pruritus) and excoriations of the head and neck. These side effects resolve within 7 days but up to 45 days in some cases after discontinuing methimazole.

Your veterinarian may see side effects in blood tests such as elevated liver enzymes, increased kidney levels from reduced filtration, and decreased red and white blood cell counts. These side effects usually resolve after methimazole is discontinued.

Therefore, in the initial three months of methimazole treatment, it is essential to perform basic blood tests 2-3 times. If your cat suffers from gastrointestinal adverse effects (lack of appetite, vomiting) with oral methimazole, changing to transdermal methimazole should resolve the issue.

Abrupt cessation of methimazole is necessary if adverse reactions such as neutropenia, thrombocytopenia, hepatopathy/jaundice, facial itching resulting in excoriations, hemolytic anemia, myasthenia gravis, or generalized lymph node enlargement occur.

When methimazole should not be used in 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 cats with autoimmune diseases.

Can you use methimazole in cats with chronic renal disease?

Yes, methimazole can be used in cats with chronic renal disease but more frequent monitoring of other clinical parameters is recommended due to methimazole's effects on kidney function.

Furthermore, if your cat has renal disease, it's not a reason to avoid methimazole. In most cases, the renal disease will only progress one IRIS stage following treatment, and if the renal disease is managed well, these cats will survive for years. Although it's crucial to check renal values when your cat is on methimazole, your veterinarian shouldn't try to control the hyperthyroidism too much in order to make the renal values look better; under-controlled hyperthyroidism has been shown to reduce kidney function and survival.

How to give methimazole to your cat?

Methimazole for cats is available as an oral tablet or compounded into a liquid or transdermal gel. Treatment with methimazole for cats yields greater results when administered every 12 hours as opposed to every 24 hours. Taking methimazole orally has been shown to be more successful than topical transdermal applications, although the convenience of the topical route should be factored in when considering its decreased efficacy compared to oral dosage.

It is important to always follow your veterinarian’s instructions for administering methimazole and have regular blood tests performed as recommended by your veterinary team.

Alternatives to methimazole for treating hyperthyroidism in cats

An alternative to methimazole for treating hyperthyroidism in cats is radioactive iodine therapy. This therapy involves injecting the cat with a small amount of radioactive iodine and allowing it to be absorbed by the thyroid gland. The radiation destroys the overactive cells, normalizing the production of hormones and reversing symptoms associated with hyperthyroidism.

There is also a diet (Hill's® Prescription Diet® y/d®) that has been proven to help cats with hyperthyroidism. The diet is heavily restricted in iodine. It is important to note that this should only be done under veterinary supervision.

Frequently asked questions

How long does it take for methimazole to work?

Methimazole typically takes between 1-4 weeks to take effect, but this depends on the cat’s individual response and metabolism. Some cats may not respond at all while some will feel a difference within days. It is important to be patient during this time as methimazole begins to work and your veterinarian can adjust the dose as needed and required.

What should I do if my cat misses a dose of methimazole? 

It is important to give your cat methimazole as prescribed by the veterinarian, so if a dose is missed, it needs to be given as soon as possible. If it has been more than 12 hours since the missed dose was due, then the next dose should be skipped and the regular dosing schedule resumed. It is important not to double up on methimazole doses. If you're ever unsure of what to do, contact your veterinarian for advice.

Can I give my cat other medications while on methimazole? 

Fortunately, there are few drug interactions to be aware of when using methimazole for cats. The concurrent use of phenobarbital however has been shown to reduce methimazole's effectiveness.
When methimazole is prescribed for your cat, it's important to discuss other medications and supplements your cat may be taking with your veterinarian. Some drugs and supplements can interact with methimazole and cause serious side effects.

In conclusion, methimazole for cats is an effective treatment option for hypothyroidism as long as it is given following your veterinarian's instructions. Regular blood tests should be performed to monitor for potential side effects and methimazole should not be used in cats with pre-existing liver or hematological diseases, pregnant or nursing cats, or cats with autoimmune diseases. Adverse effects of methimazole in cats may include vomiting, lack of appetite, weight loss, tiredness, itching, liver disease, weakness, and reduced kidney function. Routes of administration include oral tablets, liquid, and transdermal gel; oral methimazole is more successful than topical transdermal applications. If you have any questions or concerns about methimazole for cats, please consult your veterinarian.