Antithyroid drugs inhibit radioiodine-induced increases in thyroid autoantibodies in hyperthyroid Graves' disease

Methimazole (MMI) has been reported to affect prognosis in hyperthyroid Graves' disease patients treated with radioiodine (I-131). I, the present study, serum concentrations of thyroxine (T-4), triiodothyronine (T-3), thyroglobulin (Tg), thyrotropin-binding inhibitory immunoglobulin (TBII), thyroglobulin antibody (TgAb), and thyroid-peroxidase antibody (TPOAb) were measured serially for 1 year in patients with Graves' disease after I-131 treatment either given alone (group 1, 41 patients) or followed by an antithyroid drug (group 2, 19 patients). The effect of antithyroid drugs on these parameters was analyzed retrospectively. Mean serum concentrations of T-4 and T3 both decreased to normal within 3 months after I-131 treatment in both groups. Serum Tg concentrations in group 1 showed significant transient increases (about four times the basal value) 1 month after I-131 administration. Titers of TBII, TgAb, and TPOAb in group I also increased transiently after I-131 treatment, with the maximum increase at 3 months. Antithyroid drugs significantly lessened I-131-induced increases in serum concentrations of Tg and all thyroid autoantibodies tested. One year after I-131 treatment, 33 of 41 patients (80%) were euthyroid or hypothyroid in group 1; this was true for only 4 of 19 group II patients (22%). The results indicate that administering antithyroid drugs after I-131 treatment reduced 131I-induced damage to the thyroid and reduced therapeutic efficacy of I-131 in hyperthyroidism. Drug treatment also inhibited release of Tg and blunted 131I-induced increases in titers of thyroid autoantibodies.