Thyroid Function in the Premature Infant

Our understanding of thyroid function and its maturation in the fetus and newborn has been advanced greatly during the past decade. We have characterized the normal pattern of transient hyperiodothyroninemia in the healthy term infant and have developed sensitive radioimmunoassay procedures and normal ranges of values for serum thyroidstimulating hormone (TSH), thyroxine (T₄), triiodothyronine (T₃), and reverse triiodothyronine (rT₃) concentrations during the neonatal period.^1-6 We have come to recognize that the placenta is essentially impermeable to TSH and to the iodothyronines and that the fetal hypothalamic-pituitary-thyroid system functions autonomously of the maternal system in utero.^7-9 These methods and insights set the stage for the ongoing exciting experiments in mass screening for congenital thyroid disorders.^10,11 Approaches to intrauterine evaluation and treatment of fetal thyroid dysfunction also are being developed, including measurement of thyroid hormones in amniotic fluid^12-14 and fetal or intra-amniotic injection of thyroid