The Action of Methimazole and L-Thyroxine in Radioiodine Therapy: A Prospective Study on the Incidence of Hypothyroidism

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 aff...