PRETREATMENT WITH PROPYLTHIOURACIL BUT NOT METHIMAZOLE REDUCES THE THERAPEUTIC EFFICACY OF I-131 IN HYPERTHYROIDISM

Ninety-three hyperthyroid patients were treated with 1 dose of iodine- 131 (I-131) during the past 10 years. Thirty-three were pretreated wit h propylthiouracil (PTU), 22 with methimazole (MMI), and 38 received n o antithyroid drugs (ATD). ATD were discontinued 5-55 days before I-13 1 therapy in three fourths of the cases and more than 4 months before therapy in one fourth of the cases. The frequency of cures in the 3 gr oups, 6-8 months after radioiodine therapy, was retrospectively studie d. The cure rate among those who discontinued PTU for 5-55 days before I-131 was significantly reduced (24%), compared with those who discon tinued MMI for the same duration (61%) or those who received no ATD (6 6%). When PTU was discontinued for more than 4 months, the cure rate w as similar to those who received no ATD. It is concluded that if ATD a re used as initial therapy for hyperthyroidism, then PTU (but not MMI) may reduce the therapeutic efficacy of subsequent I-131. The reductio n in cure rate was observed even when PTU was discontinued for as long as 55 days before I-131 therapy. To our knowledge, this is the first report to compare, in one study, the effects of pretreatment with PTU and MMI on I-131 therapy.