Methimazole (MMI) is presented as being the most potent antithyroid drug in humans. The antithyroid effectiveness of MMI depends primarily on its ability to inhibit hormone synthesis within the thyroid. Limited studies have suggested that MMI has a duration of action of 20-40 hours within the thyroid. A prospective randomized study was performed to compare the antithyroid effectiveness of a small single dose (15 mg once daily, N=38) and the conventional divided doses (10 mg three times daily, N=52) of MMI in restoring the euthyroid state in Graves hyperthyroid patients. Serum total and free thyroxine and triiodothyronine concentrations were determined during theraphy (2nd, 4th and 8th weeks). Decreases in serum thyroxine and triiodothyronine concentrations during treatment were similar in both groups. There was no difference between the time required to achieve the euthyroid state (6.92±1.72 vs 7.05±1.88 weeks) respectively. No change in hematological and other parameters were observed in the two groups. However only after two patients of the divided dose group had transient pruritus and a skin rash that ended after several days.
Finally we concluded that a small single and the conventional divided dose of MMI are equally effective. A single daily dose will be more convenient and may be of particular importance when long term antithyroid treatment is given.