Symptoms of Too Much Methimazole In Cats

According to the FDA, Washington State Veterinary Medicine, the most frequent side effects of methimazole medication overmedication are diarrhea, nausea and vomiting, blood abnormalities and haircoat and skin abnormalities.

Felimazole Coated tablets (methimazole), an FDA-approved drug, are used to treat feline hyperthyroidism. These tablets have methimazole as their active ingredient. They are safer for cats than methimazole products made for humans. Methimazole, an anti-thyroid agent, inhibits the formation thyroid hormones.

Methimazole is almost as effective as the older medication Propylthiouracil (PTU). It has fewer side effects and is easier to use. People in Australia and the UK both use carbimazole which is converted into methimazole by the body.

What is Hyperthyroidism?

Hyperthyroidism is a condition in which the thyroid gland produces too many thyroid hormones. This is the most common endocrine disorder in cats over 8 years old. An endocrine disorder is a disease that affects the hormone-secreting organs.

Nearly 99 percent of cases of hyperthyroidism are caused by benign (noncancerous) thyroid gland tumors. The neck houses the butterfly-shaped thyroid gland, which has one lobe per side. It is responsible for regulating the body’s metabolic rate or “engine”, which plays an important role. The cat’s “engine”, or metabolic rate, can be affected if the thyroid gland produces too many thyroid hormones. The cat’s metabolic rate is a negative factor that affects nearly all organs.

What is Methimazole?

Methimazole, an antithyroid drug that is used to manage hyperthyroidism among cats, can be used. Hyperthyroidism refers to a condition where the thyroid produces too much thyroxine. Thyroxine can cause unwanted clinical signs, and may lead to secondary illnesses. These clinical signs can easily be controlled and secondary diseases prevented by using Thyroxine.

Methimazole Mechanism Of Action

Understanding the effects of thyroxine on the body is necessary to fully understand methimazole’s mechanism. A cat with an excessive amount of thyroxine in their body can cause weight loss, hyperactivity (being unusually active), polydipsia (increased appetite), increased thirst, tachycardia, an increase in heart rate, unkempt hair, and even vomiting/diarrhea.

Thyroxine controls your cat’s metabolism. It can cause your cat to become hypersensitive to it, leading to the undesirable signs described above.

The thyroid hormone is produced by your cat’s body using the element iodine. Iodine comes from the cat’s food. Methimazole can interfere with the formation of iodine into tyrosyl. Tyrosol is an additive to Tyrosine.

What is Methimazole Allergy and How Does It Work?

The seriousness of the condition (hyperthyroidism) means that side effects (methimazole), must be considered in conjunction with the benefits. Some side effects can disappear within weeks, while others may require treatment with another medication. If your cat experiences any side effects that affect the liver, such as yellowing of the skin and eyes, vomiting or skin lesions or swelling, you should have this medication evaluated. Before you give the medication to your cat, it is essential that your veterinarian performs a blood test. Regular blood tests should also be performed to monitor liver enzymes and thyroid hormones and other abnormalities.

Symptoms Of Too Much Methimazole In Cats

Change in your appetite

Methimazole treatment can cause changes in the appetite of cats. Some cats may have a greater appetite than others, while some cats will be less interested in food. You should reduce the medication to a level that does not affect your cat’s appetite.

Itchy Skin

Itchy skin is another common side effect of excessive methimazole treatment in cats. This includes the neck, head, and face. Itchy skin is common in cats within the first few weeks after treatment. Some cats may scratch their heads, neck and faces for a long time. Excessive scratching can cause skin lesions.

Facial itching is a common problem in cats, but less than 4% experience it. This can be treated with anti-itch medication and discontinuing methimazole. If the anti-itch or temporary discontinuation from methimazole does not solve the skin itching, an alternative medication should be considered.

Blood Counts are changing

Temporary changes in blood count occur in 15% of cats receiving methimazole medication. This happens within the first two-months of treatment. Only 4% of cats experience serious changes to their blood and bone marrow.

About 50% of cats who receive methimazole medication for more than six month have other blood abnormalities.

Digestive Upsets

In cats, upset stomach is the most common side effect. About 20% of cats experience nausea, vomiting, or diarrhea. Most cats will feel better within a few days.

Transdermal gel is less irritating for cats than the oral medication. However, if your cat experiences persistent digestive problems, it is best to reduce the dosage of methimazole or consult your veterinarian.

Bleeding Disorders

Methimazole may cause bleeding disorders in cats, according to studies. These bleeding disorders can be characterized by prolonged PIVKA (proteins inducible by vitamin K absence) or a prolonged prothrombin times.

Myasthenia Gravis

This neuromuscular condition causes skeletal muscle weakness in varying degrees. Methimazole is rarely prescribed for cats. However, this condition can affect the eyes, swallowing, and face muscles. Myasthenia Gravis can cause trouble walking, double vision, difficulty seeing, drooping and eyelids problems, as well as trouble vocalizing.

Weight Loss

After 2-3 months of constant medication, weight loss can be a sign of too much methimazole. This could be due to a decreased appetite or other health problems such as diarrhea and vomiting.

Worsening of an existing kidney problem

High blood pressure and heart disease can often be associated with hyperthyroidism in cats. This makes the kidneys appear more efficient while concealing a kidney problem. Once methimazole is administered, the kidney disease can be unmasked or worsened. The flow of blood through the kidneys will return to normal.

You can find a happy middle ground where you can simultaneously treat hyperthyroidism and the kidneys. To avoid abrupt changes in kidney blood flow, it is best to start with a lower dose.

Liver Failure

Serious liver disease in cats can occur, although it is rare. About 2% of cats who take methimazole will experience liver disease. This can be treated by stopping the medication. If the cat is unable to tolerate methimazole, an alternative thyroid therapy may be required.

Haircut and Skin Abnormalities

Your cat’s coat and skin condition are indicators of her health. According to the VCA Animal Hospital, a cat’s hair should be shiny and smooth. The skin of a healthy cat should be smooth and shiny, and not rough, flaky or greasy.

You should consult your veterinarian immediately if you notice any changes in your cat’s hair while grooming. It could be a sign of hypothyroidism, but it may also be a sign of methimazole excess.

These are common skin and hair abnormalities:

  • Excessive hair fall and dander
  • Uncontrollable tangles
  • Greasiness
  • Balding or thinning of certain areas
  • Under the skin: Lumps
  • Dry and flaky skin

Vocalization Abnormalities

Your cat might start to vocalize excessively during methimazole treatment. This could be a sign that she is experiencing extreme stress, discomfort or pain. This could be due to a chronic condition or excessive methimazole.

Methimazole overdose may also be indicated by cats who yelp when touched.

Try to get rid of any stressors that could trigger your cat for the first few days. You should not leave your cat in a noisy area. You should ensure that she gets enough water and food every day.

Continue with the prescribed methimazole dose if her meowing is normal after the first week. If your cat’s meowing becomes urgent or louder, you should consult a veterinarian immediately.

If the vet determines that the cause of her distress is methimazole, the vet can lower the dose, change the transdermal form to oral, or suggest other treatments.

Side effects of Methimazole for Cats

First, the vet must be notified if the cat is experiencing discomfort or fever. It is important to perform a blood test to determine if there are any underlying medical conditions. The vet will recommend the correct medication to treat the problem if it is discovered.

However, there have been some reports of adverse reactions to methimazole administered to cats. These effects are usually mild and disappear quickly without stopping the treatment. If these effects are severe, the medication should be stopped and an alternate treatment sought. Negative reactions are rare, however. The following are some examples:

  • Vomiting
  • Appetite loss or decrease
  • Lethargy
  • Itching sensations
  • Recurrent irritations in the neck and head.
  • Eruptions
  • Jaundice is a yellowing of the skin or mucous membranes. It can also be associated with liver problems.
  • Hematologic abnormalities
  • Hypothyroidism can occur if a cat is given a high dosage of medication. However, it is uncommon.

Contraindications to Methimazole for Cats

Methimazole is not recommended for cats with certain conditions. These cats are those who are in the following situations.

  • Systemic diseases like diabetes and liver failure
  • Autoimmune diseases
  • White blood cell changes
  • Clotting problems or platelet alterations
  • Gestation and lactation
  • To ensure no interaction between medications, the veterinarian must know if the cat is on any other medication. Vaccines count as well.
  • It is not recommended for cats allergic to the active ingredient.

Keep in mind, however, that methimazole is not approved for use in cats with kidney disease. Your vet will weigh the pros and cons and recommend a different method depending on your cat’s needs. Methimazole can reduce glomerular filtration so if you do decide to medicate your cat, make sure to monitor its kidney function.

Take care

This and other drugs should be kept out of the reach of children. Methimazole can be prescribed by your veterinarian. It should only be given to the animal it was prescribed for. This medication should not be given to anyone.

Methimazole should not be used in cats with liver disease, autoimmune diseases or pre-existing blood abnormalities.

Methimazole can cause individual reactions. To avoid drug-induced hypothyroidism, it is important to monitor your thyroid hormone levels regularly.

Low thyroid levels may result in kittens born to cats who have been given this drug. Your veterinarian might recommend a milk substitute for your kitten.


As with any prescription medication, excessive methimazole can cause side effects in cats. These side effects include diarrhea, blood abnormalities and changes in appetite. You should immediately take your cat to the vet if she reacts negatively to the drug.

Exposure to long-term high levels of methimazole can prove fatal. To avoid major changes in your cat’s health, always follow your vet’s recommendations.

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