Methimazole for Cats: The Answer for Hyperthyroidism

Methimazole for Cats: The Answer for Hyperthyroidism

Medically Reviewed by Taylor Froiland, PharmD, RPh
Written by Adam McCown, PharmD

Methimazole is the one of the most common treatment options for hyperthyroidism in cats. Due to its effectiveness and lower instances of side effects, methimazole has largely replaced previous drugs used to treat this disease like propylthiouracil or PTU. As a treatment option for feline hyperthyroidism, this drug is safe, effective, affordable, and tends to produce results quickly.

What Is Hyperthyroidism in Cats?

In a nutshell, feline hyperthyroidism is a glandular disorder occurring when one or both of the thyroid glands in your cat’s neck begin to swell and start producing too many T3 and T4 thyroid hormones. The cause of the increased T3 and T4 production is usually a non-cancerous tumor called an adenoma.

The tumor causes your cat’s thyroid glands to become enlarged, thus triggering the abnormal overproduction of the hormones. Malignant tumors, also known as thyroid adenocarcinomas, can also account for a cat’s hyperthyroidism, but these cases are rare.

Hyperthyroidism is a common disease that is typically found in older cats. In fact, fewer than 6% of cases are found in cats under 10 years of age, with the average age being 12-13 years old.

Symptoms of Feline Hyperthyroidism

Because the thyroid systems in cats (as well as in us) affect most other systems in the body, an imbalance of thyroid hormones can have far reaching effects in many different areas of health.

Symptoms of hyperthyroidism can include:

  • weight loss
  • increased appetite
  • increased activity and restlessness
  • aggressive or "cranky" behavior
  • increased urination
  • periodic vomiting
  • increased amount of stool or diarrhea
  • occasionally difficulty breathing
  • occasionally weakness
  • occasionally depression
  • a poor hair coat
  • a fast heart rate
  • increased water drinking

Effects of Hyperthyroidism in Cats

In addition to the aforementioned symptoms of hyperthyroidism, the most serious aspect of the disease is that, if left untreated, it can lead to a heightened risk of a host of secondary health problems in your cat. Like we mentioned already, the thyroid system in cats affects nearly every other organ system. The thyroid glands send hormones throughout the body, so if the amount of hormones being distributed is larger than normal, problems will occur.

These problems can include things like high blood pressure, an enlarged heart, thickening of the left ventricle, kidney damage, brain damage, and ocular problems. These secondary health problems have the potential to be fatal, which is why it is important to treat hyperthyroidism quickly after your cat is diagnosed.


If you notice that your cat displays any of the above symptoms, then you should bring them into the vet as soon as possible. If the vet agrees that the symptoms may be caused by hyperthyroidism after performing a full physical, they will most likely order a blood-chemistry panel and a thyroid-hormone test to look for elevated levels in your cat’s bloodstream.

Additionally, because of the secondary health problems that can be caused by hyperthyroidism, a complete blood-chemistry panel and urinalysis will also most likely be performed so the veterinarian can have a clearer picture of the cat’s health.

Methimazole for Cats: Advantages

Once your cat is diagnosed with hyperthyroidism, there are usually three treatment options to choose from. These include medication, radioactive-iodine therapy, and surgery. Due to their heightened risks and high costs, many people will not opt for the surgery or radioactive-iodine treatment and go instead with anti-thyroid medication.

Methimazole for cats is the most effective and affordable treatment option out there today. In fact, it is considered by vets to be just as effective as the other two treatment options but with fewer costs and fewer risks. The way it works is simply by inhibiting the production of thyroid hormones by the glands in order to bring the levels back down to normal.

Additional advantages of methimazole for cats with hyperthyroidism include:

·         Relatively inexpensive

·         Serious side effects are uncommon

·         No hospitalization required

·         No high risk medical procedures

Methimazole for Cats: Dosage

Once they prescribe methimazole for your cat, the vet will instruct you on the proper dosage and administration of the medication. It is important to follow what the vet says to the letter, especially if your cat has any history of kidney or liver problems, as too much of methimazole could cause serious health issues.

However, typically methimazole comes in 5mg tablets and the usual dosage for cats is one tablet (5mg) every 8-12 hours. The one main disadvantage of methimazole for cats is that the medication will most likely have to be administered for life. This is because the drug is actively preventing the overproduction of the thyroid hormones. If the medication is cut off, then the thyroid will begin overproducing once again. This can be difficult if your cat is finicky about taking medicine.


Hyperthyroidism in cats can be a serious disease that can lead to far reaching secondary health problems. However, if you notice the symptoms and bring your cat in to the vet for diagnosis and treatment quickly, you have options to allow your friend to live a healthy and happy life (without even using one of their nine lives).

If you do go the medication treatment route, then methimazole for cats is a safe, effective, and affordable option. But whichever treatment you choose, the healthy and happy life of your cat is the most important thing—so get it done quickly!