A case of fetal hyperthyroidism treated with maternal administration of methimazole.

Prenatal ultrasonography of a pregnant woman with a past history of total thyroidectomy for Graves' disease detected fetal tachycardia, fetal growth restriction and oligohydramnios at 30 weeks gestation. Because a high titer of thyroid-stimulating hormone receptor antibody was noted in maternal serum and the fetal goiter was detected on ultrasonography, fetal hyperthyroidism was strongly suspected and subsequently confirmed with cordocentesis at 31 weeks gestation. After treatment of fetal hyperthyroidism through oral maternal administration of methimazole (MMI) starting at 33 weeks gestation, fetal heart rate and amniotic fluid volume returned to normal ranges. Complete resolution of the fetal goiter was observed at 35 weeks gestation. A male infant was born at 35 weeks 6 days gestation via cesarean section in the absence of thyrotoxic findings; however, cord blood chemical analysis at birth indicated iatrogenic fetal hypothyroidism. In the present report, maternal therapy using MMI to resolve symptoms of fetal thyrotoxicosis, including fetal tachycardia and oligohydramnios, was successfully conducted.