Reevaluation of the effects of methylmercaptoimidazole and propylthiouracil in patients with Graves' hyperthyroidism.

The effects of methylmercaptoimidazole (MMI) and propylthiouracil (PTU) were compared in patients with Graves' hyperthyroidism. Firstly, the duration of action of the drugs was studied by the perchlorate discharge test, which was performed 2, 12, or 24 h after administering a single dose of 15 mg MMI or 300 mg PTU. After 2 h, the 9 MMI-treated patients who were tested had marked discharge (mean +/- SD, 65.0 +/- 15.8%), as did the 6 patients treated with PTU (57.6 +/- 26.6%). The mean values for the percent discharge 12 and 24 h after drug administration were 34.9 +/- 31.9% (4 patients) and 36.5 +/- 26.9% (69 patients), respectively, in the MMI group and 19.1 +/- 11.7% (11 patients) and 8.6 +/- 10.5% (7 patients) in the PTU group, indicating that the effect of MMI lasted longer. Secondly, the clinical effects of long term administration of the drugs were compared in a different group of patients with Graves' hyperthyroidism. Within 5 weeks after the onset of treatment, 34 (52%) of 66 patients treated with MMI (10 mg, 3 times daily) were euthyroid, while only 1 of 17 patients treated with PTU (100 mg, 3 times daily) was euthyroid. The average time required to achieve euthyroidism, namely normal serum T3 and T4 levels, was significantly shorter in the MMI group [6.7 +/- 4.6 (+/-SD) weeks] than in the PTU group (16.8 +/- 13.7). In spite of the well known effect of PTU on the extrathyroidal conversion of iodothyronines, the serum T3 level normalized much faster with MMI than with PTU. These results indicate that in our patient population 15 mg MMI had a longer inhibitory effect on the organification of iodide than did 300 mg PTU, and that MMI was more rapidly effective in the treatment of Graves' hyperthyroidism.