Maternal-Fetal Transfer of Thyroxine

To the Editor: Vulsma et al. provide convincing evidence that thyroxine (T₄) is transferred from mother to fetus in selected cases of congenital hypothyroidism (July 6 issue).¹ However, we believe that the placenta must still be considered a clinically relevant barrier to the treatment of the fetus with hypothyroidism. The “substantial amounts” of T₄ transferred to the fetuses were not sufficient to produce normal serum T₄ levels in the infants studied at birth, despite maternal hormone concentrations estimated to be threefold greater than those in the fetuses.¹ This observation supports our belief that the placental transfer.