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At a Glance
Effective in fighting hyperthyroidism, or Graves disease
Reduces overall thyroid hormone levels
Comes in tablet form and taken orally


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At a Glance
Effective in fighting hyperthyroidism, or Graves disease
Reduces overall thyroid hormone levels
Comes in tablet form and taken orally

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Methimazole is an anti-thyroid tablet administered for the treatment of hyperthyroidism in cats. Hyperthyroidism, often known as Graves disease, is an autoimmune disease that causes hyperactivity within the thyroid gland leading to the overproduction of the thyroid hormones thyroxine (T4) and triiodothyronine (T3). Metehimazole is the generic of Tapazole.

Hyperthyroidism is a condition that may lead to weight loss, abnormal appetite, and heart issues. In this instance, the medication works by restoring the thyroid hormone levels back to normal by interfering with iodine, which plays a major role in thyroid performance. Overall, Methimazole has been proven incredibly effective against Graves disease in cat's.

Though it is generally safe when administered under the direction of a veterinarian, methimazole can cause have side effects in some animals. Adverse effects include frequent bouts of vomiting, lack of appetite, weakness, bleeding disorders, or liver problems.

  • Thyroid Pharmacy
  • King Pharma Manufacturer
  • Oral Application
  • Cat Pet Type

What is the drug methimazole used for?

David Bruyette, a DVM in West Los Angeles notes that methimazole is a medication commonly prescribed for the treatment of hyperthyroidism in cats. Cats with hyperthyroidism face a range of health issues as a result of the thyroid gland producing too many thyroid hormones. NIH notes that methimazole works by inhibiting the production of thyroid hormones, specifically blocking the synthesis of thyroxine (T4) and triiodothyronine (T3). These hormones are essential for controlling the cat's metabolism. Their overproduction can result in symptoms such as weight loss, increased appetite, hyperactivity, and other systemic complications. Administered orally, usually in the form of tablets or a transdermal gel, methimazole helps to manage the symptoms associated with hyperthyroidism. Regular monitoring of thyroid levels is essential during treatment to ensure the medication effectively controls hormone production without causing adverse effects.

What is the best medicine for hyperthyroidism?

Hyperthyroidism in cats is commonly treated with medications that help regulate thyroid hormone levels. The primary medication is methimazole, an antithyroid drug that inhibits the production of excessive thyroid hormones. It can be applied directly to the ear or administered orally. However, some cats may not respond adequately to methimazole alone, or they may experience side effects. In such cases, a combination approach may be considered. According to Cornell University, radioactive iodine therapy is another effective option, especially for cats with well-controlled kidney function. This treatment involves a single injection of radioactive iodine, which selectively destroys overactive thyroid tissue while sparing healthy tissue. Beta-blockers, such as timolol or atenolol, may also be prescribed to manage symptoms like elevated heart rate and hypertension. These medications don't address the underlying cause but help alleviate the immediate effects of hyperthyroidism.

How does methimazole work?

  1. Methimazole is typically administered orally in pill or liquid form. According to MedicineNet, it functions by preventing the production of thyroid hormones, most especially by preventing the action of the thyroid peroxidase enzyme. Thyroid peroxidase facilitates the iodination and coupling of thyroglobulin molecules, which is a critical step in the synthesis of thyroid hormones T3 and T4. Methimazole inhibits this process by decreasing the synthesis of T3 and T4 and blocking the incorporation of iodine into thyroglobulin. By decreasing the levels of circulating thyroid hormones, methimazole helps normalize the feline thyroid function. This alleviates the symptoms associated with hyperthyroidism, such as weight loss, increased appetite, hyperactivity, and other related issues.

What to avoid while taking methimazole?

When administering methimazole to cats, it's crucial to be mindful of potential interactions and considerations. First and foremost, avoid concurrently using medications that may adversely interact with methimazole. Notably, drugs like amitriptyline and theophylline can interact with methimazole, leading to increased side effects or altered efficacy. To make sure that these prescriptions are not given at the same time, it is essential to speak with a veterinarian. It's also critical to keep an eye out for any negative side effects, such fatigue, nausea, or vomiting. This might be a sign of methimazole intolerance. To evaluate thyroid function and modify the amount of medicine as necessary, routine veterinarian examinations and blood tests are essential. Furthermore, steering clear of self-adjusting the dosage without professional guidance is crucial, as improper management can lead to thyroid imbalances. Pet owners should not discontinue or modify the treatment plan without consulting their veterinarian, as abrupt changes can result in serious health complications.

What is the most serious side effect of methimazole?

The most serious side effect of methimazole in cats is agranulocytosis. Agranulocytosis is a rare but potentially life-threatening condition characterized by a severe decrease in white blood cell count, specifically neutrophils. These white blood cells are essential for the immune system's proper functioning. R. Darren Wood, a DVM from the Ontario Veterinary College notes that this condition is called neutropenia. It leaves cats vulnerable to infections that their weakened immune system may struggle to combat. Agranulocytosis is an idiosyncratic reaction to methimazole, meaning it occurs unpredictably and is not necessarily related to the dosage. Cats undergoing methimazole treatment should be closely monitored for signs of agranulocytosis, such as lethargy, fever, and reluctance to eat. If any of these symptoms manifest, immediate veterinary attention is crucial. Regular blood tests are essential to detect early signs of hematological abnormalities and adjust the methimazole dosage accordingly. If agranulocytosis is suspected or confirmed, discontinuation of methimazole is typically recommended, and alternative treatments or therapies may be explored under veterinary guidance.


Side effects occur in less than 5% of all cases, but can be serious, such as a decreased white blood cell count (agranulocytosis); contact a veterinarian immediately if your pet display signs of a sore throat. Excessive bleeding can also occur as a result of a decrease in blood platelets. Very rarely, liver failure and its ensuing complications have been associated with Methimazole.

More common side effects are skin-related, including rash, itching, hives, abnormal hair loss, and skin pigmentation. Other side effects may include swelling, nausea, vomiting, heartburn, loss of taste, joint or muscle aches, drowsiness, dizziness, numbness, and headache. Do not use in pregnant or nursing animals.

Methimazole must be used only in cats. It should never be used on animals, which are prone to allergy to the drug. Extreme care should be taken while using this medication in animals with a known history of liver impairment, immune-system disease or blood disorders. Regular blood tests to monitor toxicity on blood cells and liver are suggested in cats, which are on methimazole for a long period.The medication should be administered life long as the symptoms might reappear once the medication is stopped. It is advisable to use methimazole for the short term management of the disease as it involves side effects. Surgical removal of the thyroid gland or radiation treatment would be more appropriate for a lasting solution. Pregnant and nursing cats should not be given this drug. Drug overdose might result in blood disorders or liver toxicity.ย 

Do not take with theophylline (Theo-Dur), warfarin (Coumadin), digoxin (Lanoxin), a beta blocker such as atenolol (Tenormin), metoprolol (Lopressor), or propranolol (Inderal). Talk to your vet before starting any serious treatment plan.

Methimazole is an anti-thyroid tablet used in the treatment of hyperthyroidism in cats, which, might result in weight loss, abnormal appetite and heart problems. When administered, methimazole restores the thyroid hormone levels back to normal by interfering with iodine, which has a major role in thyroid function.ย 

Methimazole comes as 5 mg and 10 mg tablets. The recommended primary dose of methimazole is 5 mg per cat, three times daily and the dosage should be increased, if no improvement is noticed in the situation. Once the condition stabilizes, the dose could be reduced after taking the opinion of the veterinarian. Ensure that the full course is completed to prevent relapse or the chances of developing resistance to the drug.ย 

Ask your veterinarian or consult with one of our pet care specialists at 1-800-844-1427.

This information is for informational purposes only and is not meant as a substitute for the professional advice of, or diagnosis or treatment by, your veterinarian. Always seek the advice of your veterinarian or other qualified professional with any questions you may have regarding a medical condition. Never disregard or delay seeking professional advice due to what you may have read on our website. Our medications are FDA approved and/or EPA regulated when and as required by law.

Methimazole comes as an oral tablet. Be sure to administer medication with food and water two to three times a day (or every eight hours). A blood test will be conducted by your veterinarian to determine proper dosage levels. Do not use in pregnant or nursing animals

Store methimazole at room temperature, between 68 and 77° F (20 and 25° C). Brief storage at temperatures between 59 and 86° F (15 and 30° C) is permitted. Store away from heat, moisture, and light. Do not store in the bathroom.

Methimazole is manufactured by King Pharma

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