Medication7 connections · 2 sources
Methimazole
Methimazole is the most commonly prescribed medication for feline hyperthyroidism. It blocks the production of thyroid hormones T4 and T3 but does not destroy the thyroid tumor itself.
Key Facts
- Blocks thyroid hormone production; takes 3-4 weeks to show effect on blood tests
- Available as tablets, flavored liquid, or transdermal ear gel
- Carbimazole (used in UK/Australia) converts to methimazole in the body
- Side effects (15% of cats): lethargy, appetite loss, vomiting; usually resolve with dose adjustment
- Serious side effects (rare): facial itching (<4%), liver toxicity (<2%), bone marrow changes (<4%)
- Most side effects occur in first 3 months of therapy
- Hazardous to humans; gloves recommended when handling; pregnant women must avoid contact
- Does not shrink the thyroid nodule; tumor may grow over time and become resistant
- Treating hyperthyroidism may unmask underlying chronic-kidney-disease
- Transdermal gel: lower GI side effects but slower to achieve control (67% vs 82% at 4 weeks)
- radioactive-iodine-therapy is the gold standard; methimazole is often used as a bridge or when I-131 is unavailable
- Species: cats