Evaluation of the pituitary-thyroidal axis in newborns undergoing exchange transfusion.

To evaluate whether the hypothyroxinaemia, previously noted in hyperbilirubinaemic newborns immediately after exchange transfusion for Rh or AB0 incompatibility, was due to impairment in the secretion of thyroid stimulating hormone (TSH) by the pituitary, we studied the thyroid hormone response to thyrotropin releasing hormone (TRH) and compared this response to that seen in a control population of healthy neonates. All infants studied responded with a brisk TSH increase; 30 min after TRH injection the mean TSH concentration of the hyperbilirubinaemic patients was 37 microU/ml, ten times their basal level, which was not different from the value noted in the control population. No significant change in total thyroxine (T4), 3,5,3' triiodothyronine (T3), free thyroxine (FT4) or 3,3',5' triiodothyronine (rT3), (FT4) or (rT3) was noted after TRH administration in either group of neonates. In addition the effect of exchange transfusion on the thyroid axis of hyperbilirubinaemic newborns was evaluated. Before the exchange transfusion TSH, T4, rT3, T3 and FT4 levels were higher in the hyperbilirubinaemic newborns than in donor blood; immediately post-exchange transfusion TSH and T4 concentrations of the hyperbilirubinaemic neonates decreased significantly and remained significantly below pre-exchange values 30 h later. Newborns undergoing an exchange transfusion respond appropriately to TRH stimulation and seem to have an intact pituitary-thyroidal axis.