Feline Hyperthyroidism Methimazole Starting Dose Calculator
Methimazole starting dose for feline hyperthyroidism, with target T4 and monitoring schedule.
Initial T4 elevation
Methimazole starting dose
5 mg/day
q12h (2.5 mg per dose)
First recheck: 2-3 weeks - T4 + CBC + chem + UA.
Target T4: Lower half of normal range (1.0-2.5 μg/dL).
Dose adjustment: By 1.25-2.5 mg/day at a time. Re-check 2-3 weeks after each change.
Monitor for: Vomiting / inappetence (first 2 weeks), elevated BUN/creat (unmasked CKD), neutropenia/thrombocytopenia, facial pruritus.
Once stable: Recheck T4 every 3-6 months indefinitely.
Frequently asked questions
What is the target T4 range on methimazole?
Lower half of the normal range - typically 1.0-2.5 μg/dL. Suppressing T4 below normal increases the risk of unmasking concurrent CKD.
When does methimazole therapy unmask CKD?
Hyperthyroidism increases GFR; treating it reveals true renal function. Check renal values at 2-3 weeks and 6 weeks. If creatinine rises significantly, allow T4 to drift toward the high-normal range rather than fully suppressing.
When should I consider I-131 instead of methimazole?
I-131 is curative, doesn't require lifelong medication, and is preferred for compliant owners with healthy cats. Methimazole is reasonable for cats with concurrent CKD (you can stop / adjust), older cats with multiple co-morbidities, or where I-131 isn't locally available.