Thyroid Medicine Allergy in Cats

Thyroid Medicine Allergy in Cats - Symptoms, Causes, Diagnosis, Treatment, Recovery, Management, Cost

What is Thyroid Medicine Allergy?

Cats affected by hyperthyroidism have an abnormally increased production of the thyroid hormone. Cats of all ages can suffer from this condition, which is caused by immunological disorders, environmental situations, and nutritional factors. Any cat can develop thyroid issues, as this condition is not due to genetics. Fortunately, cats that suffer from hyperthyroidism have medication options to control this gland’s production of hormones. The most popular medication is methimazole, as it is shown to be the gentlest medication on the gastrointestinal system. Although this drug is considered safe for all cats, some cats can still have an allergic reaction to this thyroid medication.

Methimazole is a medication given to humans for hyperthyroidism and can also be used for cats. This drug is considered safe with minimal side effects, and since there is no cure for hypothyroidism, it is the drug of choice by many medical professionals to effectively manage the condition.

Cats that begin this medication are to be closely monitored for three months, and veterinarians perform lab work consisting of complete blood count and keeping track of the levels of the thyroid hormone every few weeks. The veterinarian may also perform tests on liver function in the function of the immune system. Once the medication is in the cat’s system, the veterinarian may choose to do these tests every 3 to 6 months.

Thyroid medicine allergies in cats are due to an adverse reaction cats with hyperthyroidism have with the medication. Methimazole is the drug of choice for the management of thyroid disorders in both cats and humans, and this medication has a range of side-effects that can negatively affect cats.

Symptoms of Thyroid Medicine Allergy in Cats

If your cat is having an allergic reaction to thyroid medication, he will have a variety of symptoms. The onset of symptoms will occur within a few weeks of starting the drug. Symptoms may include:

  • Scratching at the skin
  • Biting the skin
  • Excessive licking
  • Drooling
  • Gastrointestinal upset
  • Vomiting
  • Loss of appetite


There are several types of medications which can affect the way methimazole works for your cat. Methimazole should not be given if your cat is taking the following types of medications:

  • Warfarin
  • Beta-blocker medications
  • Digoxin
  • Theophylline
  • Hydrocortisone

Causes of Thyroid Medicine Allergy in Cats

Many cats have allergic reactions to the medications that they are taking for a variety of ailments. Causes of thyroid medication allergic reactions are:

  • Incorrect dosage given
  • Medication may counteract with other medications given
  • Immune system views the medication as a foreign substance
  • Over reactive immune system will “fight” the components in the medication, causing a reaction

Diagnosis of Thyroid Medicine Allergy in Cats

If your cat is showing signs of an allergic reaction to his thyroid medication, make an appointment with your veterinarian.  Once you are at the veterinarian, he will take a closer look at his symptoms and ask you a variety of questions about the symptoms you have witnessed. These questions will help the veterinarian better understand what is going on in terms of your cat’s health.

The medical professional will conduct a series of tests to rule out any other illnesses or conditions. If this veterinarian was the one that prescribed the thyroid medication for your cat, he may already have blood work, urinalysis, and biochemistry profile results on record. He may also choose to conduct these tests once again, depending on the time period from his last visit to when your cat has been on the medication.

Your veterinarian will take a close look at your cat’s skin and ask more questions about his gastrointestinal health. If the timing of the symptoms began soon after he began the medication for his thyroid, he may make a preliminary diagnosis of a thyroid medication allergy and alert you to stop the administering of the drug. He may choose this route to see if the allergic reactions come to a halt before moving forward with any other allergy tests.

Treatment of Thyroid Medicine Allergy in Cats

If your cat is allergic to Methimazole, he will need to stop taking the medication. Fortunately, there are other types of treatment available for hyperthyroidism. Always discuss other options, especially holistic options, with your veterinarian before trying. Other treatment methods may consist of:

Dietary Change

A diet, prescribed by your veterinarian, will consist of cat food which is low in iodine. This special cat food will need to be eaten by your cat for the remainder of his lifetime in order to control the hormone released by the thyroid gland. This may not be a solution if you have multiple cats, and you will need to discuss this option with your veterinarian.


Removal of the thyroid may be an option that works; however, if your cat is elderly, this may not be suitable. While being an effective choice for younger and middle-aged cats, keep in mind that this is a surgical procedure that your cat will need to recover from. He will also be kept in the animal hospital at your veterinarian’s suggested time frame.


Radioactive iodine therapy is another type of treatment your veterinarian may recommend. An injection of radioactive material can help control the hormone release by the thyroid; however, this is a specialized procedure and you will need to transport your cat to a specialized veterinarian on a regular basis. He will need to remain at the facility until he is not radioactive before you can bring him back home.

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Recovery of Thyroid Medicine Allergy in Cats

Whichever treatment method you choose for your cat, your veterinarian will outline the proper care procedures at home. He will give you instructions on how to manage your cat’s lifestyle and environment after any type of procedure, and will want to see him for follow-up visits to be sure the solution to his allergy is working. 

If you choose to look into holistic therapy with supplements to further help your cat’s thyroid (if it was not removed), be sure to ask your veterinarian and do not give any type of supplements to your cat without consulting with him. If your cat is on a special diet, be patient with him as he adapts to his new food; at first he may balk at it and refuse to eat. Give him a few days and be sure he is drinking water, and eventually he will eat it if he gets hungry enough. If you are concerned about your cat not eating his new food after a few days, contact your veterinarian.

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© 2023 Wag Labs, Inc. All rights reserved.