A 35-year-old pregnant woman visited our department and had been treated with 100 µg of daily oral levothyroxine for hypothyroidism. An ultrasonography screening was performed at 25 weeks gestation and revealed a fetal goiter and an increased amniotic fluid volume. Fetal hypothyroidism was confirmed by cordocentesis and amniotic hormone levels at 26 weeks gestation. We treated the mother with 200 µg of daily oral levothyroxine to optimize the transplacental transfer. A total of four intra-amniotic injections of levothyroxine were administered, resulting in progressive reduction in the fetal thyroid volume of goiter as measured by 3D ultrasonography and increased amniotic fluid volume. Following birth, neonatal serum thyroid stimulating hormone level was within the normal range, but free T4 was reduced. Based on this case, we suggest that monitoring amniotic fluid thyroid hormone concentration and intra-amniotic levothyroxine injection can be used to reduce the thyroid volume of goiters and to prevent polyhydramnios.