13 Symptoms of Too Much Methimazole in Cats

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.

Although methimazole has few side effects, its use in excessive amounts can put your cat’s health at risk. There are notable symptoms of too much methimazole in cats we feel you should know.

signs of too much methimazole in cats

Symptoms of too much methimazole in cats

Although methimazole for cats has been proven to be safe, your feline friend is likely to experience a few side effects if used excessively. Below are some of the symptoms of too much methimazole in cats.

1. 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.

Here’s a simple guide on how to apply transdermal gel medication to cats. Video credits: Royal York Animal Hospital.

2. 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.

3. 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.

4. 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.

5. 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.

6. 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.

7. 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.

Symptoms of Too Much Methimazole in Cats
Signs of too much methimazole in cats

8. 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.

9. 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.

In the event that kidney problems become significantly worse when a cat is on methimazole, the medication can be discontinued. Approximately 15% of cats on methimazole are likely to show kidney problems that were masked prior to treatment.

10. Immune-mediated disorders

Though extremely rare, cats exposed to too much methimazole can suffer from immune-mediated disorders where the immune system over-reacts and attacks the body.

11. Decreased activity level

There will be a notable change in behavior in hyperactive cats. Some cats will become sluggish and disinterested in playing with their favorite toys and instead rest on the couch for longer periods than usual.

12. Abnormal Vocalizing

A cat experiencing side effects of methimazole medication might vocalize more than usual. This could be due to itchy skin, gastrointestinal issues, or other painful conditions that force them to vocalize more.

13. Haircoat abnormalities

A cat exposed to too much methimazole is likely to experience haircoat abnormalities. Other symptoms include increased ALT (alanine transferase), increased BUN (blood urea nitrogen), and agitation.

Signs of too much methimazole in cats

Although methimazole help cats suffering from hyperthyroidism, excessive use can cause dozens of health problems. It is advisable to pause the medication if you notice any of these symptoms. Consult your veterinarian in severe cases or report the drug and side effect to FDA.

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