Methimazole for Cats: Mechanism of Action, Use, Side Effects

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What is Methimazole?

Methimazole is an antithyroid drug used to treat and manage hyperthyroidism in cats. Hyperthyroidism is a disease where the thyroid is overactive producing too much thyroxine. Thyroxine causes unwanted clinical signs and can lead to secondary diseases. By using this drug these clinical signs can be managed and secondary diseases avoided. 

Methimazole mechanism of action

To understand methimazole’s mechanism of action we have to understand what thyroxine does to the body. When a cat’s body has an overproduction of thyroxine it causes weight loss, increased appetite, polydipsia (increased thirst), hyperactivity (being unusually or abnormally active), tachycardia (increased heart rate), unkempt hair coat, and in extreme conditions vomiting/diarrhea

Thyroxine is what controls your cat’s metabolism. An influx of it will cause your cat’s body to go into overdrive leading to the unwanted signs mentioned above. 

Your cat’s body uses the element iodine to produce the thyroid hormone. Iodine is obtained from the food your cat eats. Methimazole interferes with iodine forming into tyrosyl. Tyrosol is a side group to the amino acid Tyrosine. 

Thyroxine chemical formula, Methimazole for Cats - I Love Veterinary

How is Methimazole given to cats?

Methimazole comes in a few compositions. Depending on your cat, your veterinarian will decide which option is best. You can give methimazole orally by tablet, liquid, or by syringe filled with intradermal medication. 

Methimazole can be given orally to a cat on a full or empty stomach. If your cat seems to have any gastrointestinal upset then it is best to give it with food. The intradermal medication must be applied to a part of the body with none or minimal hair. 

This is to ensure the medication makes contact with the skin where it will be absorbed. Make sure to always wear gloves when handling medication, especially ones that can be soaked into your own skin. 

As with all medications keep them in a high, locked cabinet away from pets and children. Make sure to always follow your veterinarian’s advice on the frequency of the drug, and how to monitor your pet for any signs of toxicity or overdose. 

Studies have shown that the drug in tablet form may have the best results when controlling hyperthyroidism. That being said if the only way you can get the medication into your cat is through liquid or intradermal then that is the obvious choice. 

Methimazole tablets, Methimazole for Cats - I Love Veterinary

Side effects of Methimazole for cats

Your cat should be monitored when given Methimazole. Blood work should be taken initially after the medication has been given for two weeks, followed by one month. This is to monitor your cat’s thyroid levels and to confirm that the dose given is appropriate. 

After this blood test, your cats’ thyroid level should be monitored every 3-6 months, between 4-6 hours after Methimazole is given. Adjustments can be made by your veterinarian at any point if the dose is too high or too low. 

If your cat develops any concurrent diseases while taking this medication, you and your veterinarian should discuss the quality of life. If your cat is worse without the medication you can monitor closer while giving it, under the direction of your veterinarian. 

Methimazole should be avoided in cats with liver, kidney or autoimmune diseases. It should also be avoided in any cats who have had an allergic reaction to it. If your cat develops any yellowing of the skin or eyes contact your veterinarian immediately. 

Cats can develop vomiting or diarrhea when taking this medication. An increased amount of scratching and self-mutilation may occur. Cats can also develop blood disorders. This is another reason why it is so important to monitor your cat’s metabolic function. 

Your cat will need blood work done in order to do this monitoring. It will also allow your veterinarian to decide if the dose needs an adjustment. 

Cat and owner, Methimazole for Cats - I Love Veterinary

Are there any risk factors for this medication?

Methimazole should be avoided in cats with liver, kidney or autoimmune disease. This is because Methimazole is metabolized in the liver and excreted by the urine. Both functions would be further compromising these organs if Methimazole was given. 

Also because this medication could possibly cause autoimmune disease, you would avoid giving it to any cat currently suffering. 

Are there any drug interactions I should be aware of?

Always fill your veterinarian in on anything and everything your cat is ingesting or being treated for. This includes any vitamins, herbs, nutritional diet, and of course any type of medication. This is to avoid any contraindications occurring. Methimazole should not be given with the following drugs: 

  • Panacur
  • Digoxin
  • Anti-coagulants
  • Bupropion
  • Phenobarbital
  • Theophylline 

Vet consults cat owner, Methimazole for Cats - I Love Veterinary


When giving this medication to your cat it is important to understand a few key points. These points highlight the main safety precautions to take when giving your cat Methimazole. 

  1. This is a veterinary formula. There are antithyroid medications that are intended for human use, but Methimazole is not one of them. 
  2. If human consumption occurs there are many negative side effects including: vomiting, fever, joint pain, leukopenia (low white blood cells), anemia (low red blood cells), Thrombocytopenia (low platelets). The side effects range from mild to severe. Having a deficiency in any blood cell will lead to many other issues.    
  3. Keep the medication in a safe, locked cabinet away from other pets or children. 
  4. Never break the tablets – this is especially important in any medication. If you are not instructed to break or crush the tablet you can risk giving your cat the wrong dose. You can also cause yourself to breathe it in the medication. You could also accidentally ingest it if you do not properly clean the table you crushed the medication on.
  5. To further ensure safety when handling Methimazole (or any medication for that matter) use gloves. This is to protect yourself from absorbing it. Because this medication is excreted mainly through your cat’s urine it is also important to also wear gloves when handling the medicated cat’s litter. 
  6. Even if you wear gloves you are at risk of exposure. Therefore, wash your hands with soap and water. 
  7. Wash your hands after handling your cat’s litter as well. (Please do this even if your cat is not on any medication.)
  8. Pregnant cats should not take Methimazole as it will cross the placenta and be absorbed by the fetus. Also, queens who are nursing should avoid Methimazole because it can be transferred to the kittens.
  9. Any woman who is pregnant, planning on becoming pregnant or nursing should wear gloves when handling Methimazole or the litter of cats taking the medication. 
  10. Any accidental ingestion or overdose should be overseen by a veterinarian. This is to ensure the proper precautions and procedures take place. 


Methimazole can be a life saving and altering drug for cats in need of it. When given properly, and following your veterinarian’s direction it can be used safely for life.

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Jaclyn is a Licensed Veterinary Technician (LVT) who has a bachelors degree in journalism. Combining her two interests of writing, and veterinary medicine is a true passion. Jaclyn has already created her own blog called The Four Legged Nurse. She is blessed with two children, a wonderful husband, and four devoted fur babies. In her free time she loves spending time with her family, reading, and riding horses.