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What’s Wrong with Methimazole?

Nothing, if you can tolerate up to a 20% side-effect rate. My clients and your clients that come from radio-iodine therapy (131I) are angry when their cat has had an adverse reaction to methimazole. Some cats become very sick and at that point most of the owners ask me, “why didn’t my veterinarian recommend 131I treatment for my cat in the first place?”

When our referring doctors are ask the question “What’s wrong with methimazole?”, we answer with a question: “Is methimazole the treatment you would recommend for your own cat?” Most veterinarians answer – “NO!” We then proceed to tell the doctors, “if you want to make the right medical decision for your patient, look at your patient as if he was your own cat.”

Methimazole side-effects /drug reactions are a nightmare and I don’t know how to treat them except to stop the drug, treat the adverse clinical signs, give good supportive care and time.

Side-effects commonly seen:

In the first 3 months: Anorexia, Vomiting & Depression


Facial Excoriations


Liver dysfunction (elevated ALR, ALP, T.bili, AST)


Icterus (cholangiohepatitis)


Positive ANA titer


Thrombocytopenia, agranulocytosis, leukopenia, eosinophilia


Positive Coombs, hemolytic anemia


Myasthenia gravis & Cold agglutinin-like disease


>4 yrs use of methimazole induces adenocarcinomas from 

2% -20%

Paraneoplastic syndrome with poor control



Methimazole came from human medicine and is used in the treatment of hyperthyroidism when patient when patients are terminally ill with only days to live, pregnant women, or patients waiting for surgery and/or radio-iodine treatment. WHy? The number of side-effects seen in human patients and concerns of making a sick patient sicker make the drug a short-term treatment option. Methimazole does not treat the cause of the problem – the tumor.

Treating for a cure is the goal in veterinary medicine for all diseases. If you could cure diabetes, would you ever use insulin? 131I cures greater than 96% of patients with hyperthyroidism! Methimazole is a crude therapy that disconnects active thyroid hormone when the proper dose frequency is used but it does nothing to halt the tumor growth. Even the ‘well controlled methimazole patients’ having hourly T4 levels taken, show peaks and valleys in hormone levels that have internal effects that are progressive and deadly. The cost of methimazole and laboratory monitoring easily pay for 131I therapy after the first year, and the tumor continues to grow! The average life expectancy of a methimazole patient is ~3 years. The average life expectancy of& 131I patients is 4-5+ years with many going as long as 11 years in our experience. Radio-iodine is the best choice of treatment for long-term survival and improved quality of life. Senior patients usually have multiple problems so a treatment plan to treat the ‘whole’ patient needs to be formulated.