The recognition and treatment of fetal hypothyroidism are believed to be important to optimize growth and intellectual development in affected fetuses. We present a case of fetal goiter diagnosed by ultrasonography in the second trimester of pregnancy. Cordocentesis performed at 28 weeks confirmed the presence of fetal hypothyroidism. Fetal therapy was performed with weekly intra-amniotic injections of thyroxine from 29 to 36 weeks. A repeat cordocentesis at 35 weeks showed normalization of fetal thyroid function. The fetal goiter decreased rapidly in size following fetal treatment. Amniotic fluid levels of thyroid stimulating hormone (TSH) and free thyroxine were obtained with each amniocentesis. Sulfated iodothyronine concentrations in maternal blood were obtained before and after fetal thyroxine treatment. This report discusses the role of amniotic fluid levels of TSH and free thyroxine and maternal levels of sulfated iodothyronine in the diagnosis and management of fetal hypothyroidism. A review of the English literature is presented.