How to initiate methimazole (antithyroid medication) therapy?

From the Guidelines

Methimazole should be initiated at a dose of 10 to 30 mg daily, typically divided into two or three doses, with a starting dose of 15 mg twice daily for moderate to severe hyperthyroidism, or 5-10 mg twice daily for mild cases, as this approach has been shown to effectively manage hyperthyroidism while minimizing the risk of adverse effects 1. When initiating methimazole therapy, it is essential to consider the following key points:

• The medication should be taken orally with or without food.
• The dose should be adjusted based on thyroid function tests every 4-6 weeks until the patient is euthyroid, then gradually taper to the lowest effective dose.
• Before starting, obtain baseline complete blood count and liver function tests, and monitor these periodically during treatment, especially in the first three months.
• Inform patients about potential side effects, particularly agranulocytosis, and advise them to report any signs of infection, jaundice, or unusual bleeding or bruising.
• Methimazole works by inhibiting thyroid hormone synthesis, so symptom improvement may take several weeks.
• Ensure adequate iodine intake during treatment, as methimazole can exacerbate iodine deficiency. It is also important to note that the goal of treatment is to maintain FT4 or FTI in the high-normal range using the lowest possible thioamide dosage, and measuring the FT4 or FTI every two to four weeks can be helpful in achieving this goal 1. Additionally, a beta blocker (e.g., propranolol) can be used to reduce symptoms until thioamide therapy reduces thyroid hormone levels.

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.

To initiate methimazole therapy:

• Adults: Start with 15 mg daily for mild hyperthyroidism, 30-40 mg daily for moderately severe hyperthyroidism, and 60 mg daily for severe hyperthyroidism, divided into 3 doses at 8-hour intervals.
• Pediatrics: Initiate with 0.4 mg/kg of body weight daily, divided into 3 doses at 8-hour intervals. 2

From the Research

Initiating Methimazole Therapy

To initiate methimazole therapy, the following points should be considered:

• The initial dose of methimazole can vary, but it is usually started at 30 mg/day for severe Graves' disease hyperthyroidism 3.
• However, a lower dose of 15 mg/day can be effective, especially when combined with inorganic iodine, and may have fewer adverse effects 3.
• A single daily dose of methimazole can be as effective as divided doses, with one study showing that 15 mg once daily was comparable to 10 mg three times daily 4, 5.
• The time to achieve euthyroidism can vary, but most patients can expect to become euthyroid within 8 weeks of starting treatment 4, 5.
• Factors such as TSH receptor antibody concentration can influence the risk of relapse, but even patients with normal concentrations can experience relapse 6.

Dosage Regimens

Different dosage regimens have been studied, including:

• 30 mg/day as a single dose or divided into multiple doses 3, 7
• 15 mg/day as a single dose, with or without inorganic iodine 3, 4, 5
• Low-dose methimazole (1.25-2.5 mg/day) for long-term maintenance therapy 6

Adverse Effects

Adverse effects can occur more frequently with higher doses of methimazole, such as 30 mg/day, compared to lower doses like 15 mg/day 3, 5.