TSH has been incriminated in Graves' disease for increasing the production of antibodies against TSH receptor (TRAb). It has been, therefore, suggested that T4 administration after successful antithyroid drug (ATD) treatment may indirectly decrease the production of TRAb and, therefore, the frequency of recurrence of hyperthyroidism. To study the role of T4 and T3 on the recurrence rate of Graves' disease 108 patients with Graves' disease (22 males, age: 49.8 +/- 14.3 yr, mean +/- SD, and 86 females, age: 41.7 +/- 12 yr) were followed-up for 24 months after successful treatment with ATD (carbimazole). During the follow-up period, patients daily received either 100 microg T4 or 25 microg T3 or placebo after random and double-blinded assignment into three groups. They were evaluated trimonthly up to 12 months and at 24 months. Plasma TRAb levels were measured at the beginning and at 12 months. At 12 months of the follow-up period, 14 out of 33 (42.4%), 6 out of 38 (15.8%), and 9 out of 37 (24.3%) patients receiving T4, T3 and placebo, respectively, recurred. Recurrence rate of T4-treated patients was statistically higher than that of the T3-treated patients or controls (p < 0.05). At the beginning of the follow-up period patients who were going to recur had significantly higher TRAb levels and goiter weight than patients who were not (p < 0.05). At 24 months of the follow-up period, from the patients who did not drop out of the study, none out of 11 (0%), 2 out of 19 (10.5%) and 1 out of 12 (8.3%) receiving T4, T3 and placebo, respectively, recurred. We conclude that T4 administration after successful ATD treatment of Graves' disease is associated with increased recurrence of hyperthyroidism as compared to the T3 or placebo administration. High TRAb levels and goiter weight at the end of ATD treatment may hint at recurrence.