Maternal–Fetal Transfer of Thyroxine

To the Editor: Vulsma et al. provide convincing evidence that thyroxine (T 4 ) 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 T 4 transferred to the fetuses were not sufficient to produce normal serum T 4 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 . . . No extract is available for articles shorter than 400 words.