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What is it used for?

Methimazole is a prescribed medication used for the treatment and ongoing management of hyperthyroidism in cats. 

Hyperthyroidism is not only a metabolic disorder that affects humans but also pets, including felines. Hyperthyroidism is a condition that is classified when the thyroid gland is over-producing thyroid hormones, disrupting the endocrine system. In felines, hyperthyroidism is one of the most well-known endocrine disorders to date. 

With hyperthyroidism, the overproduction of thyroid hormones (which include both T3 as well as thyroxine, T4) can cause significant and long-term damage to the body without proper treatment. The treatment works to interfere with traditional interactions triggered by a combination of iodine and peroxidase with thyroglobulin. This treatment helps to minimize and/or reduce the overall formation of excess thyroid hormones, helping to balance and normalize thyroid function in felines. 


Side Effects

There are multiple side effects to keep in mind whenever using this medication on your cat. Many of these symptoms that occur when felines are treated occur within the first one to three months of treatment.

Common side effects that are most likely to occur within the first one to three months of treatment:

Hair loss

Drowsiness/increased tiredness




Muscle, nerve, or joint pain

The most common side effect likely to be experienced by a cat using this treatment for hyperthyroidism is nausea and/or vomiting. Up to approximately 20% of all felines utilizing this treatment will likely experience digestive upset or an issue with their digestive treatment throughout the course of treatment.

In order to reduce the risk of digestive upset, it is also possible to consider an alternative application, such as a transdermal gel. With a transdermal gel, it is still possible for your cat to find it itchy or uncomfortable, so it is important to monitor your cat throughout the duration of the treatment. 


More serious side effects that require an immediate call to the veterinary office may include:

Blood in stool and/or urine


Swelling of the tongue, lips, and in some cases, even the entire face

Excessing and continuous vomiting

Difficult breathing (if your feline is flaring up its nostrils, opening its mouth, panting excessively, or sticking its neck out further in an attempt to breathe)

Loss of appetite, depression

Insomnia along with a change in sleeping and/or eating patterns

Autoimmune side effects such as developing issues such as jaundice

If you notice your cat is having difficulty breathing, is no longer eating, or has blood in its urine or stool, contact your emergency veterinarian ASAP. If your cat has a fever with swelling anywhere on its face, seek immediate medical attention from your vet.

Precautions with this medication


This treatment should be avoided in felines with the following conditions, ailments, or diseases:

Kidney disorders or kidney disease

Liver disease and/or disorders





In addition to the conditions not recommended for this treatment, it is also not recommended for nursing and pregnant felines to utilize this medication. In some cases, this treatment may contribute to birth defects in kittens, and/or it may also be toxic to the embryo while it is still in the womb. 

This medication should be used with caution on animals who:

Have liver disease or kidney disease, even if mild

Have autoimmune disease(s) and/or pre-existing abnormalities that show up throughout their blood

Possible drug and medication interactions

With most medications, it is important to know what potential drug interactions may worsen a condition or cause further issues that require medical attention. When working with your vet to determine whether or not this medication is the right treatment for you, inform your practicing veterinarian of any of the following conditions if you are thinking of using this medicine as your cat’s hyperthyroidism treatment: 

Digoxin (Lanoxin)

Metoprolol (Lopressor, or other heart medications and/or beta-blockers such as atenolol, or Tenormin)

Propranolol (Inderal)

Theophylline (Theo-Dur)

Warfarin (Coumadin, or other blood-thinning medications)

How long does it take to work?

With this treatment, studies have shown that a single dose may help a hyperthyroid cat in just approximately 2.3 hours. For cats who are not classified as hyperthyroid, the medication may work in approximately 4.7 hours on average. However, just one dose of the treatment has the ability to protect a feline with hyperthyroidism for up to 24 hours in total. 



Whenever you are thinking of obtaining a new medication to help treat feline hyperthyroidism for your cat, it is imperative to speak directly with your vet to determine the best course of action based on your feline’s latest labs and current health. If you are provided a capsule administer with or without food, whichever is easier for your feline. 

It is always advisable to speak with a veterinarian prior to crushing up a medication, especially for a condition such as feline hyperthyroidism, due to the capability of timed releases within the tablet itself. It is important to provide your feline with plenty of available drinking water whenever you are giving them their medication(s) to prevent choking or gagging as they attempt to swallow. 

If you are given an oral solution, you may consider a flavored solution (if available) or placing the oral droplets in wet canned food and/or treats for your cat. Because the solution is typically considered bitter and undesirable, mixing is best for optimal results.

When you are given a cream or a transdermal gel to help treat your feline, it is essential to wear proper gloves and protective equipment to prevent getting the medication onto your own skin, which can lead to potentially dangerous side effects. 

Measure the dose as accurately as possible on your own finger once you have your gloves on prior to completing the application. Apply the cream or gel to a site on your cat that has been wiped, moistened, and thoroughly cleaned. 

Always check with your cat’s veterinarian to determine the best location to place the gel or cream as well as the number of dosages and/or applications your cat requires each day to maintain a healthy thyroid. 


Dosage for Felines

Never determine the dosage of thyroid medication for felines or for individuals, as this must be done via bloodwork in a medical lab. Your veterinarian will provide you with the proper dosage of this medication for your cat based on his/her age, weight, as well as their current thyroid levels. 

Typically, the initial dosage for felines is a 2.5 mg, given every 8 to 12 hours daily. Once the initial dose is taken for one to three months, your veterinarian will likely run additional bloodwork to determine the overall effectiveness of the dosage and whether or not your cat requires an increase. Blood tests are typically repeated every 28 days to ensure optimal dosage measurements. 

Because the thyroid gland is extremely sensitive and can require ongoing maintenance, it is highly recommended to schedule regular bloodwork for your feline who has hyperthyroidism, especially if they experience thyroid issues chronically. 

The maximum dose that is permitted is typically 2.5 for traditional hyperthyroid cases in felines, 5 mg for felines with moderate hyperthyroidism, and between 10 to 15 mg for those with chronic and/or severe conditions that are debilitating and life-threatening. 


What if I miss a dose?

It can feel confusing, stressful, and scary to forget your pet’s medication. However, if you realize you have missed a scheduled dose, simply give the dose as soon as possible. If, however, you have missed a dose for an entire day, do not give two doses simultaneously. Never give your feline a double dose of medication if you have missed an entire day. 

If you are concerned about your cat’s health or if they are showing severe reactions due to a chronic condition and/or autoimmune disease, contact your vet immediately. 


Signs and Symptoms of an Overdose

The thyroid is one of the most important and fundamental organs throughout both the human and feline bodies. Ensuring your cat is safe and taking the proper dosage of his/her feline hyperthyroidism medication is essential at all times. Knowing what to look for with a possible overdose can help you identify when it is time for you to call your vet or take your feline in. Common signs of an overdose with this medication include:

  • Swelling
  • Excessive and/or abnormal bleeding and bruising
  • Itching or intensified scratching
  • Headaches (your cat may hide its face with its paws, push its head towards the ground, or become increasingly lethargic. They may also shy away from being pet or touched on the head and surrounding neck areas.)
  • Diarrhea, loss of appetite, nausea, and even vomiting may all indicate a potential overdose. Monitor any digestive changes and/or abnormal behaviors after giving this medication.
  • Fever (loss of appetite, lack of activity, heavy breathing, shivering, and even a loss of interest in drinking water may signal a fever in felines.)


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