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 (¹³¹I). In the present study, serum concentrations of thyroxine (T₄), triiodothyronine (T₃), thyroglobulin (Tg), thyrotropin-binding inhibitory immunoglobulin (TBII), thyroglobulin antibody (TgAb), and thyroid-peroxidase antibody (TPOAb) were measured serially for I year in patients with Graves' disease after ¹³¹I 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 T₄ and T₃ both decreased to normal within 3 months after ¹³¹I treatment in both groups. Serum Tg concentrations in group I showed significant transient increases (about four times the basal value) I month after ¹³¹I administration. Titers of TBII, TgAb, and TPOAb in group 1 also increased transiently after ¹³¹I treatment, with the maximum increase at 3 months. Antithyroid drugs significantly lessened ¹³¹I-induced increases in serum concentrations of Tg and all thyroid autoantibodies tested. One year after ¹³¹I 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 ¹³¹I treatment reduced ¹³¹I-induced damage to the thyroid and reduced therapeutic efficacy of ¹³¹I in hyperthyroidism. Drug treatment also inhibited release of Tg and blunted ¹³¹I-induced increases in titers of thyroid autoantibodies.