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 starting dose of methimazole (MMI) for subclinical hyperthyroidism is not explicitly stated in the drug label. However, for mild hyperthyroidism, the initial daily dosage is 15 mg. Since subclinical hyperthyroidism is a milder condition, a conservative approach would be to consider the starting dose similar to that of mild hyperthyroidism, which is 15 mg daily, divided into 3 doses at 8-hour intervals 1.
From the Research
The starting dose of methimazole for subclinical hyperthyroidism is typically 10 mg daily, as indicated by the most recent and highest quality study 2. This dose is often used as the initial treatment, and then adjusted based on the patient's response to the medication.
- For most patients with mild subclinical hyperthyroidism, beginning with 10 mg daily is appropriate,
- while those with more significant laboratory abnormalities or mild symptoms might benefit from dose adjustments made accordingly to achieve euthyroidism. Before initiating treatment, baseline complete blood count and liver function tests should be obtained.
- Patients should be monitored with thyroid function tests (TSH and free T4) every 4-6 weeks initially,
- with dose adjustments made accordingly to achieve euthyroidism. Treatment duration typically ranges from 12-18 months, after which a trial off medication can be considered if TSH normalizes. Methimazole works by inhibiting thyroid hormone synthesis by blocking the incorporation of iodine into tyrosine residues. Side effects to watch for include rash, arthralgias, and rarely agranulocytosis (which presents with fever and sore throat). Patients should be instructed to take the medication at the same time each day and report any unusual symptoms promptly. The study by 2 provides the most recent and highest quality evidence for the starting dose of methimazole, and its findings should be prioritized in clinical decision-making. Other studies, such as 3, 4, 5, and 6, provide additional information on the treatment of hyperthyroidism, but their findings are not as directly relevant to the starting dose of methimazole for subclinical hyperthyroidism.