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 recommended dose of methimazole for treating hyperthyroidism 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
- Maintenance dose: 5 mg to 15 mg daily 1
From the Research
The recommended initial dose of methimazole for treating hyperthyroidism is 15 mg daily, as it has been shown to be effective in achieving euthyroidism while minimizing adverse effects 2.
Key Considerations
- The dose of methimazole may need to be adjusted based on the severity of the condition, with mild hyperthyroidism requiring lower doses (5-15 mg daily) and moderate to severe cases requiring higher doses (20-40 mg daily) 3, 4.
- The total daily dose is usually divided into three equal doses during the first month of treatment, then can often be given once daily as maintenance therapy.
- Once thyroid function normalizes (usually within 6-12 weeks), the dose can be gradually reduced to a maintenance level of 5-15 mg daily.
- Patients should have thyroid function tests (TSH, free T4, free T3) monitored every 4-6 weeks initially, then every 2-3 months once stabilized.
Long-Term Management
- Long-term continuation of low-dose methimazole therapy (2.5-5 mg daily) has been shown to be effective and safe in preventing recurrent hyperthyroidism in patients with Graves' hyperthyroidism who have completed a standard course of methimazole therapy without adverse effects 5.
- This approach can help maintain euthyroid status and reduce the risk of recurrent hyperthyroidism.
Adverse Effects
- Common side effects of methimazole include rash, itching, and rarely, agranulocytosis (severe decrease in white blood cells) 3, 4.
- Patients should be advised to report symptoms like fever, sore throat, or mouth ulcers immediately.
Mechanism of Action
- Methimazole works by blocking thyroid hormone production by inhibiting the enzyme thyroid peroxidase, which is necessary for incorporating iodine into thyroid hormones.