Developmental Trends in Cord and Postpartum Serum Thyroid Hormones in Preterm Infants

The purpose of this study was first to clarify postnatal trends in sera T4, free T4 (FT4), T4-binding globulin, TSH, T3, rT3, and T4 sulfate levels in cord and at 7, 14, and 28 d in groups of preterm infants at 23–27 wk (n = 101), 28–30 wk (n = 196), and 31–34 (n = 253) wk gestation, and second to compare these trends to those of term infants and also with cord sera levels of equivalent gestational ages (n = 812; 23–42 wk gestation). In all preterm groups, TSH and rT3 decrease to below, T4-binding globulin increases to within, and T3 and T4 sulfate increase to above cord levels of equivalent gestational age. Term infants are hyperthyroxinemic relative to cord and nonpregnant adult levels of T4. Postnatal T4 increases are attenuated in 31- to 34-wk infants, absent in 28- to 30-wk infants (although levels are equivalent to gestational age), and crucially reversed in 23- to 27-wk infants. This immature group is hypothyroxinemic relative to other groups and to cord levels of equivalent gestational age. Compared with term infants, postnatal FT4 increases are lower in 31- to 34-wk infants, attenuated in 28- to 30-wk infants, and absent in 23- to 27-wk infants. The 23- to 27-wk group is distinctive; they are hypothyroxinemic on T4 levels, yet FT4 levels are within the cord levels of equivalent gestational age.