A thyroid hormone binding inhibitor (THBI) which affects the ability of thyroxin binding globulin (TBG) to bind T4 was first described by Chopra et al as a possible explanation for the low T4 levels observed in some adults patients with non-thyroidal illness. Because low T4 levels are likewise a common clinical feature of prematurity, we examined THBI activity in 30 neonates (GA 26-43 weeks, BW 930-4360 grams) using a competitive ligand binding assay in order to determine whether increased THBI levels might be responsible for the hypothyroxinemia of prematurity. Serum concentrations of T4 and TBG were measured by RIA and the free T4 index calculated. THBI activity was expressed as the % CPM T4 1125 displaced from serum binding protein in the absence of THBI. A THBI index was then calculated, normalizing patients' samples relative to a baseline (control value of 1.0). Significant positive correlations between THBI activity and birthweight (r=0.76, p< 0.0001) and gestational age (r=0.74, p< 0.0002) were noted in infants born between 26 and 39 weeks of gestation, but not in more mature infants. No significant correlations were observed between THBI activity and TBG, T4 and free T4 index in these infants. The results of this study indicate that: 1) increased THBI activity is not a likely etiology for the hypothyroxinemia of prematurity; 2) the activity of THBI increases with fetal maturation until term gestation, and then it appears to decline; 3) the relationship between THBI activity, birthweight, gestational age, and systemic illness needs to be further investigated.