Exchange transfusion in premature newborns: effect of maturity on thyrotropin and thyroxine responses

The effects of exchange transfusion on plasma T4 and TSH were studied in two groups of premature newborns to evaluate the effect of maturation on the reactivity of the pituitary-thyroid axis. In newborns with a birth weight between 1900-2500 g (gestational age 35.4 +/- 0.45 weeks) the responses of both hormones were essentially similar to those reported previously for mature newborns (a profound decrease during the procedure and an increase 24 h later). In a group of newborns with a birth weight below 1900 g (average 1673 +/- 55, gestational age 32.7 +/- 0.72 weeks), however, the secondary increase in plasma T4 at 24 h after the procedure was absent. At this time both T4 and TSH levels were significantly lower than in those in heavier newborns. It is concluded that the ability to respond to exchange transfusion by an increase in plasma thyroxine at 24 h after the procedure matures at the gestational age between 32 and 35 weeks.