Symptoms Of Too Much Methimazole In Cats

According to the FDA and the Washington State Veterinary Medicine, the most common side effects of methimazole overmedication include diarrhea, nausea, vomiting, blood abnormalities, haircoat and skin abnormalities, change in appetite, and overall weakness.

Felimazole Coated Tablets (methimazole) is an FDA-approved drug used to treat feline hyperthyroidism (hyperthyroidism in cats). The tablets contain methimazole as the active ingredient and have been proven to be safer for cats than using methimazole products for people. Methimazole is an antithyroid agent that inhibits the formation of thyroid hormones. 

The use of methimazole has virtually replaced the older medication propylthiouracil (PTU) because it has few side effects and is more effective. Australians and people in the UK use carbimazole, which gets converted to methimazole in the body.

What Is Hyperthyroidism?

Hyperthyroidism results when the thyroid gland over-produces thyroid hormones. It’s the most common endocrine disease of cats older than 8-years-old. (An endocrine disease is a disease of the organs that make and secrete hormones.)

In almost 99 percent of the cases, hyperthyroidism is caused by a benign (non-cancerous) tumor of the thyroid gland. The butterfly-shaped thyroid gland is located in the neck, with one lobe on each side. It plays an important role in regulating the body’s “engine,” or metabolic rate. When the thyroid gland produces an excessive amount of thyroid hormones, it causes the cat’s “engine” to run at an abnormally high speed. This high metabolic rate negatively affects almost all of the cat’s organs.

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.

What Is Methimazole Allergy?

Because of the seriousness of the disease (hyperthyroidism), the side effects of the drug (methimazole) have to be weighed against the benefits. Sometimes the side effects will dissipate within a few weeks or can be treated with a different medication to lessen the effects. However, if your cat has any serious side effects that have to do with the liver such as yellow gums, skin, and eyes, vomiting, lethargy, skin lesions, and swollen lymph nodes an evaluation of the use of this medication must be done. It is important that your veterinarian do a blood test to determine the lowest amount of methimazole possible before starting the medication. In addition, blood tests to keep track of liver and kidney enzymes, thyroid hormones, and other abnormalities should be done on a regular basis.

Symptoms Of Too Much Methimazole In Cats

 Change In Appetite

A cat under methimazole medication is likely to experience changes in appetite levels. Some cats will experience an increased appetite while others feel disinterested in food. If your cat does not want to eat at all while on medication, you should lower your dosage to a level that doesn’t affect your pet’s eating habits.

Itchy Skin

Another common symptom of too much methimazole in cats is itchy skin, especially on the face, neck, and head. This occurs within the first few weeks of treatment, and some cats will persistently scratch their head, neck, and face. Skin lesions might appear as a result of excessive self-scratching. 

Although less than 4% of cats experience facial itching, the problem can be resolved with anti-itch medication or discontinuing methimazole. Alternative medication for thyroid disease should be given if the anti-itch or temporary discontinuation of methimazole medication does not resolve the skin itching issues.

Blood Counts Changes

About 15% of cats on methimazole medication experience temporary changes in blood counts. This occurs within the first two months of therapy, with only about 4% of cats developing serious changes in their bone marrow and blood counts.

Approximately 50% of cats that receive methimazole medications consistently for more than six months develop other blood abnormalities.

Digestive Upsets

The most common side effect of methimazole in cats is upset of the digestive tract. Approximately 20% of cats will experience nausea, vomiting, and diarrhea. Most cats, though, will begin to feel better without the need for extra medications. 

Although cats receiving transdermal gel experience fewer digestive issues, you should reduce the methimazole dosage or consult your veterinarian if you notice these symptoms persist too long.

Bleeding Disorders

Studies have indicated that methimazole inhibits vitamin K epoxide reductase, which potentially leads to bleeding disorders in cats. The bleeding disorders are characterized by a prolonged PIVKA (proteins induced by vitamin K absence) and, though rarely, a prolonged prothrombin time

Myasthenia Gravis

This is a neuromuscular disease that leads to varying degrees of skeletal muscle weakness. Although cats on methimazole rarely suffer from this condition, the muscles that will be affected the most include those of the eyes, face, and swallowing. Myasthenia gravis can cause double vision, trouble walking, drooping, eyelids, and trouble vocalizing.

Weight Loss

Weight loss is one of the notable symptoms of too much methimazole in cats and occurs after 2-3 months of continuous medications. This might result from reduced appetite and other health issues such as persistent gastrointestinal upsets, notably diarrhea and vomiting.

Worsening Of Pre-Existing Kidney Problem

When a cat is suffering from hyperthyroidism, heart disease & high blood pressure that often go along with the condition increases blood flow through the kidneys, making the kidneys seem more effective while hiding an existing kidney problem. However, once methimazole medication is started the kidney disease is unmasked and/or made worse because the flow of blood through the kidneys returns to normal.

Luckily, you can strike a happy medium where treatments for the kidneys and hyperthyroidism can be administered concurrently. The best approach is to start with a lower dose of methimazole and gradually work your way up to avoid causing an abrupt change in kidney blood flow.

Liver Failure

Although rare, serious liver failure is one of the symptoms of too much methimazole in cats. Approximately 2% of cats taking methimazole experience liver failure which can be resolved by discontinuing the medication. An alternative thyroid therapy is needed if the cat cannot tolerate methimazole without developing liver disease.

Haircoat And Skin Abnormalities

The condition of your cat’s skin and coat are good indicators of her health status. The VCA Animal Hospital states that a cat’s coat should be smooth and shiny as opposed to coarse or brittle. Healthy skin should be supple and clear, not bumpy, flaky, or greasy.

If you’ve noticed changes in your cat’s hair coat while grooming her, you might want to visit your veterinarian at the earliest possible date. Apart from being a symptom of methimazole overdose, it could also indicate an underlying health issue that’s unrelated to hypothyroidism.

Common haircoat and skin abnormalities include: 

  • Excessive hair loss and dander
  • Uncontrollable tangles
  • Greasiness
  • Thinning or balding patches
  • Lumps under the skin
  • Flaky and/or dry skin

Vocalization Abnormalities

If your cat excessively vocalizes during the first few weeks of methimazole treatment, she might be trying to tell you that she’s in severe stress, discomfort, or pain. It might be associated with her chronic disease or it might have been brought about too much methimazole in her system.

Cats who yelp or yowl when touched might also be an indication of methimazole overdose.

For the first few days, try to eliminate any stress factors that might trigger your cat. Don’t leave her in a noisy environment and keep her litter box and food area clean. Make sure she has enough food and water to eat and drink every day.

If her meowing returns back to normal after the first week of methimazole treatment, continue with the prescribed dose. But if your cat’s meows turn urgent and loud, it’s best to visit a vet to have her checked out as soon as possible.

Again, if the veterinarian found that her distress is caused by the methimazole, your vet may either lower the dose, change it from oral to transdermal, or offer other treatment solutions.

Side Effects Of Methimazole For Cats

First of all, if we notice discomfort in the cat or fever, we must immediately notify the vet. In these cases, it is necessary to do a blood test for hematological and biochemical analysis. If a problem is diagnosed, the vet will prescribe the appropriate medication to control it.

On the other hand, adverse effects have been reported after the administration of methimazole in cats, although they are usually mild and resolve spontaneously without the need to stop the treatment. In cases where these are more serious, the medication will have to be stopped and an alternative treatment found. However, adverse reactions are considered to be rare. These include the following:

  • Vomiting
  • Decrease or loss of appetite
  • Lethargy
  • Intense itching
  • Irritations in the head and neck area.
  • Eruptions
  • Jaundice, which is the yellowing of the skin and mucous membranes. It is associated with a liver problem
  • Hematologic abnormalities
  • If a high dose is administered, in addition to these signs, the cat may suffer from hypothyroidism, although it is rare.

Contraindications Of Methimazole For Cats

There are some cats for whom methimazole is not recommended. They are the ones who are in the following situations:

  • Systemic diseases such as diabetes or liver failure
  • Autoimmune diseases
  • Alterations in white blood cells
  • Platelet alterations or clotting problem
  • Gestation and lactation
  • If the cat is taking any other medication and the veterinarian does not know, it must be reported to ensure that no interactions occur. Vaccines also count.
  • Of course, it is not suitable for cats with an allergy to this active ingredient

Also, keep in mind that some other cats, such as those with kidney disease, will only be able to use methimazole once the vet carefully weighs the pros and cons of both administering the drug and not, perhpas opting for another method in your cat’s specific case. This is due to the fact that methimazole reduces glomerular filtration, hence, if you decide to medicate, it is essential to subject the cat to strict control of its kidney function.


Keep this and all drugs out of reach of children. Methimazole is a prescription drug and should be used according to your veterinarian’s directions, and given only to the animal for which it was prescribed. Do not give this medication to a person.

Methimazole should be avoided or used with extra monitoring in cats with liver disease, autoimmune disease or pre-existing blood abnormalities.

Individual responses to methimazole may vary. Regular monitoring of thyroid hormone levels is necessary in order to avoid drug-induced hypothyroidism.

Kittens born to cats receiving this drug may be born with low thyroid levels. Your veterinarian may recommend a milk replacement for your kitten.


Like any other prescription medication, too much methimazole in cats can result in an array of side effects. This includes diarrhea, blood abnormalities, changes in appetite, and more. If your cat reacts poorly to the administered drug, take her to the vet immediately.

Long-term exposure to high amounts of methimazole is extremely dangerous and can be fatal. Always follow your vet’s dosage recommendation to avoid any major changes to your cat’s health.

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