Your cat has been your best pal for years. You make sure your cat is pampered and feels loved. It hurts you that your furry friend no longer wants to play and constantly feels fatigued with hyperthyroidism. Your veterinarian may prescribe the only antithyroid medication known as methimazole. What is methimazole and will it work? What do I do if my cat will not take the tablets? Compounded medications can save the day and help your cat get back to long days of playtime and meowing. Purr-fect! 

What is hyperthyroidism?1-3

When the thyroid gland, a butterfly-shaped gland in the neck, is enlarged and over produces thyroid hormone, this is known as hyperthyroidism. Thyroid hormone is responsible for the body’s metabolism, growth, and development, so it has a huge effect all over the body. It is one of the most common diseases in cats that typically occurs during their middle ages. No breeds are at a greater risk, but Siamese, Persian, and Himalayan cats have less of a risk. 

Symptoms include weight loss, increased appetite, frequent urination, increased thirst, weakness, greasy hair, restlessness, aggressive behavior, depression, difficulty breathing, and stomach problems. The symptoms may be mild at first, then they become more severe over time. If you notice these signs in your cat, you may want to check with your veterinarian about performing a thyroid test.

If your cat is diagnosed with hyperthyroidism, your veterinarian may prescribe methimazole. The prognosis for cats being treated with hyperthyroidism is promising. Your cat will have a better quality of life and can continue to be your snuggle buddy, so it is worth getting treatment! 

What is methimazole?2,4

Methimazole is an antithyroid medication, so it works by suppressing your cat’s thyroid to release a normal amount of thyroid hormone. Since the thyroid works all over the body, this medication helps decrease the risk of your cat getting high blood pressure and heart disease. Thyroid hormones increase your cat’s heart rate, so more blood is pumped faster around the body which can damage the eyes, liver, and other organs. This would require more medications and even surgery. Methimazole helps your cat continue to live a healthy and long life by managing the thyroid, but it is not a cure. This medication is given for the rest of your cat’s life. 

Josefs Pharmacy offers the generic version, methimazole, that is less expensive than the brand name, Felimazole, and will work with your pet’s insurance to give you an affordable price.

Does methimazole work?5

In a 3-year study with 60 cats with hyperthyroidism, methimazole compounded transdermal gel was shown to have less side effects and lower thyroid levels within a normal range with regular monitoring. The pet owners found the gel to be easier to give than the manufactured oral tablets. The tablets were also associated with more side effects like stomach issues. However, this medication is given lifelong and dose adjustments may be needed after getting an established dose. 

How do I give methimazole to my cat?4,6

Administer methimazole just like your veterinarian told you. Always wash your hands before and after giving this to your pet. The oral tablet and oral compounded liquid can be given to your pet on an empty stomach or with food. Giving it with food helps your pet feel less sick. Never crush or break the oral tablets. 

This medication also comes in a compounded transdermal gel that can be applied on hairless skin like the inside of the ears. Rubber gloves should be worn when applying the gel. It is important you measure the liquid and gel doses properly to give your cat the correct dose. Giving your cat too much or too little can cause the medication to fail or be toxic. 

It takes a couple of weeks for this medication to work, so you will not notice improvement in signs immediately. If you miss a dose, give it to your cat as soon as you remember unless it is close to the next dose. Then, skip the missed dose and return to the normal dosing schedule. Never double up on doses. 

What are the side effects?4,5

Your cat may experience tiredness, vomiting, and weight loss within the first 3 months of treatment. Your veterinarian may shortly stop the medication and prescribe a lower dose. Less common side effects include upset stomach, itchiness, and low blood cell counts. In cats with liver or kidney disease, this medication may cause more side effects. Let your veterinarian know if your pet experiences any of these effects. 

Is there any special monitoring with this medication?5,6

Your veterinarian will check your cat’s thyroid levels before starting treatment, then every 2 weeks for the first 3 months of therapy. This is to make sure the medication is working right without limiting side effects. Once the appropriate dose is determined, thyroid levels are checked every 3 to 6 months. Even after the dose is established, it may need to be adjusted as your cat continues long-term treatment. It is crucial your cat is regularly monitored to make sure the medication is working and your cat is not at risk of toxic effects. 

What is the difference between the manufactured and compounded methimazole?5,6

The compounded forms, liquid solution and transdermal gel, work just like the compounded oral tablet. The tablet has a bitter taste, so it may be difficult giving this to your cat. Your cat may not like the taste, have trouble chewing, or have trouble swallowing, so a liquid or gel may be better. The gel is applied inside the ears, so it may be easier for both you and your pet. That is the beauty of compounded medications, they make it easier for your fur baby!  

 A pharmacist at Josefs Pharmacy can help you make this switch to a formulation that is both easier for you to give and your cat to take. Compounded medications can be more expensive, but Josef’s Pharmacy will help you get the best treatment in a form your pet will enjoy with the best price.

Can I share my thyroid medications with my cat?6

If you have a medication that is for treating your thyroid or for your child, do not share them with your cat. We may think of our pets as our children, but they do not get the same medications as us or respond to them like us. Human medications are different from vet medications and they can be toxic to your cat. It is only legal to use medications in the people that are prescribed them. You may take your daily thyroid medication and think you can split the tablet with your cat, but it actually could be too much for your cat since thyroid medications are highly personalized.

Methimazole Safety Tips4,5

If you have questions about treating your cat’s hyperthyroidism with compounded or manufactured medications, talk with your veterinarian or pharmacist today.


  1. Cat hyperthyroidism (overactive thyroid). WebMD website. Accessed November 14, 2021.
  2. Hyperthyroidism in cats. Cornell University website. Accessed November 14, 2021.
  3. Caring for an ill senior cat and the lessons I learned along the way. Ethos website. Accessed November 14, 2021.
  4. Methimazole. VCA hospitals website. Accessed November 14, 2021.
  5. Methimazole: Management of feline hyperthyroidism. Today’s veterinary practice website. Accessed November 14, 2021.
  6. Methimazole transdermal gel and cream for cats. Avriorx website. Accessed November 14, 2021.
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