The purpose of this study was first to clarify postnatal trends in sera T-4, free T-4 (FT4), T-4-binding globulin, TSH, T-3, rT(3), and T-4 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 rT(3) decrease to below, T-4-binding globulin increases to within, and T-3 and T-4 sulfate increase to above cord levels of equivalent gestational age. Term infants are hyperthyroxinemic relative to cord and nonpregnant adult levels of T-4. Postnatal T-4 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 T-4 levels, yet FT4 levels are within the cord levels of equivalent gestational age.