Thiourea drugs have been postulated to possess radioprotective property. We studied the effect of adjunctive antithyroid drugs (ATD) and L-thyroxine (L-T4) on the result of radioiodine (RAI) 131I therapy and determined the incidence of hypothyroidism and relapse of hyperthyroidism. One hundred and fifty-nine patients with Graves' disease were randomized prospectively to receive either RAI alone or adjunctive ATD in a form of block-replacement regimen of methimazole (MMI) plus L-T4 for 6 months. The patients were observed for a mean period of 4.6 (range 2-10) years. The incidence of permanent hypothyroidism was studied and the effect of ATD on iodine kinetics was analyzed. The cumulative incidence of hypothyroidism in the ATD group was significantly lower than the RAI group (p = 0.0009), and the difference is accounted by a reduction of early hypothyroidism within 12 months from 20.2 to 3.7% (p = 0.003). The incidence of late hypothyroidism was similar between the two groups. Treatment with ATD did not affect the one dose cure rate with RAI (61.2 vs 55.5%, p = NS), but the time to achieve euthyroidism was significantly earlier with adjunctive ATD (2 vs 8 weeks, p < 0.02). The incidence of relapse within the first year after one dose was also similar between the two groups (38.7 vs 44.5%, p = NS). Comparing the kinetics of the therapeutic dose with a tracer dose, patients receiving MMI were found to be underdosed by 22% (p = 0.003) and the biological half-life was significantly shortened. We conclude that ATD rendered euthyroidism earlier without compromising the one dose cure rate of RAI.(ABSTRACT TRUNCATED AT 250 WORDS)