From the FDA Drug Label
DOSAGE AND ADMINISTRATION Methimazole tablets, USP are administered orally. The total daily dosage is usually given in 3 divided doses at approximately 8-hour intervals. Adult The initial daily dosage is 15 mg for mild hyperthyroidism, 30 mg to 40 mg for moderately severe hyperthyroidism and 60 mg for severe hyperthyroidism, divided into 3 doses at 8-hour intervals. The maintenance dosage is 5 mg to 15 mg daily. Pediatric Initially, the daily dosage is 0.4 mg/kg of body weight divided into 3 doses and given at 8-hour intervals. The maintenance dosage is approximately 1/2 of the initial dose.
The appropriate dose of Methimazole (MMI) for treatment is:
- Initial dose:
- 15 mg daily for mild hyperthyroidism
- 30 mg to 40 mg daily for moderately severe hyperthyroidism
- 60 mg daily for severe hyperthyroidism
- 0.4 mg/kg of body weight daily for pediatric patients
- Maintenance dose:
- 5 mg to 15 mg daily for adults
- approximately 1/2 of the initial dose for pediatric patients 1
From the Research
Methimazole should be started at a dose of 15 mg daily for adults with hyperthyroidism, as this dose has been shown to be effective in inducing euthyroidism while minimizing adverse effects. This recommendation is based on the most recent and highest quality study available, which compared the efficacy of once daily versus divided daily administration of low daily dosage (15 mg/day) of methimazole in inducing euthyroidism in Graves' hyperthyroidism 2. The study found that treatment with methimazole at a low daily dosage of 15 mg/day once daily was as effective as divided daily doses in reducing serum thyroid hormone levels and inducing euthyroidism, with a lower risk of hypothyroidism.
Key points to consider when initiating methimazole therapy include:
- Starting dose: 15 mg daily for adults with hyperthyroidism
- Dose adjustment: Gradually reduce the dose to a maintenance dose of 5-15 mg daily after achieving euthyroidism
- Monitoring: Regular monitoring of thyroid function tests (TSH, free T4, free T3) every 4-6 weeks initially and then every 3-6 months during maintenance therapy
- Adverse effects: Common side effects include rash, itching, and gastrointestinal upset, while rare but serious side effects include agranulocytosis and liver injury
- Patient instruction: Patients should be instructed to take the medication consistently at the same time each day and report any unusual symptoms like sore throat, fever, or unusual bleeding, which could indicate serious adverse effects.
It is also important to note that children require weight-based dosing, typically 0.5-0.7 mg/kg/day divided into three doses, and elderly patients or those with liver dysfunction may need lower doses 3, 4, 5, 6. However, the study by 2 provides the most recent and highest quality evidence for the recommended starting dose of 15 mg daily.