Foetal methimazole syndrome is characterised by a group of symptoms that may be observed in a foetus or newborn when the mother has taken methimazole, an antithyroid agent, during pregnancy. This drug is not marketed everywhere in Europe, but it is an active metabolite of carbimazole, another antithyroid agent that is marketed in most European countries. Various case reports of congenital abnormalities have been reported in foetuses exposed in utero to methimazole, either during the first trimester of pregnancy or throughout gestation. These abnormalities included cerebral atrophy (two cases), oesophageal atresia and tracheooesophageal fistula (three cases), choanal atresia (four cases), and ten other types of birth defects, represented each by one case only. The scalp defect aplasia cutis vertices is reported more frequently in the offspring of women taking methimazole, suggesting that this anomaly might also be associated with the drug. These case reports do not substantiate the existence of a methimazole-caused syndrome of congenital abnormalities, although some authors have described a specific association between this antithyroid agent and malformations consisting of aplasia cutis, choanal and/or oesophageal atresia, minor dysmorphic features, and developmental delay. According to a comparative study, the rate of birth defects in the offspring of untreated hyperthyroid women is higher than the rate in the offspring of hyperthyroid women treated with methimazole. When a pregnant woman requires an antithyroid medication, it may be preferable to prescribe propylthiouracil because of its reduced propensity for placental transfer. However, two comparative studies on the effects of these drugs did not confirm any advantage of propylthiouracil over methimazole.
Last update: February 2006
- Dr Elisabeth ROBERT-GNANSIA