Adverse effects of prenatal methimazole exposure

Background: A specific phenotype of methimazole (MMI) induced malformations has recently been postulated. MMI embryopathy is characterized by minor dy smorphic features, choanal atresia and/or esophageal atresia, growth retard ation, and developmental delay. Methods: We prospectively studied the outcome of pregnancy in 241 women cou nseled by 10 Teratology Information Services (TIS) of the European Network of Teratology Information Services (ENTIS) because of MMI exposure, and com pared them with those of 1,089 pregnant women referred to TIS because of ex posure to nonteratogenic drugs (control group). Information was obtained by mail or telephone interview. Results: There was no increase in the general rate of major anomalies or of spontaneous or induced abortions in the MMI-exposed group in comparison wi th the control group. Two newborns were affected with one of the major malf ormations that are part of the postulated embryopathy. Conclusions: The results of this study indicate that choanal as well as eso phageal atresia may have a higher incidence than expected in fetuses expose d to MMI between 3 and 7 gestational weeks. Until further data are availabl e, thyrotoxicosis should be treated with propylthiouracil, as it is apparen tly safer for use during the fertile period. (C) 2001 Wiley-Liss, Inc.