Cord blood reverse T3 in congenital hypothyroidism.

Reverse triiodothyronine (rT3) was measured in cord serum from 5 infants with congenital hypothyroidism and compared with normal values in 70 euthyroid control infants. The mean (and SEM) value in the affected infants (135 +/- 12 ng/dl) was significantly lower than that in the control population (270 +/- 9 ng/dl). However, the large overlap in range of concentrations in affected and control infants indicates that newborn screening based on the determination of rT3 in cord blood specimens offers no advantage over present screening methods.