Insulin-like growth factor (IGF)-I and IGF-binding protein 3 during the first year in term and preterm infants.

Plasma IGF-I and IGF-binding protein 3 (IGFBP-3) were determined by radioimmunoassay in term infants (n = 51) at 0, 2, 4, 6, and 12 mo and preterm infants (n = 51) at -3 (28.4 wk postconceptional age), -2, -1.5, -1, 0, 2, 4, 6, 9, and 12 mo from expected term. The effects of gestational age at birth (term or preterm) and study age were determined by repeated measures analysis of variance and Fisher's least squares difference. In preterm infants, IGF-I increased between -3 and 2 mo from 0.75 to 10.4 nM, decreased between 2 and 9 mo to 7.3 nM, and increased again between 9 and 12 mo to 10.1 nM (p < 0.0001), whereas IGFBP-3 increased relatively little from -3 to 0 mo (14.2 to 30.2 nM, p < 0.05) and plateaued from 2 to 12 mo (49.8 to 62.3 nM). At 0 mo, IGF-I and IGFBP-3 were the same in term and preterm infants, but preterm infants had higher IGF-I from 2 through 12 mo (p < 0.05), higher IGFBP-3 at 2 and 4 mo (p < 0.05), and lower IGFBP-3 at 12 mo (p < 0.05). IGF-I and IGFBP-3 were correlated at most ages. First year IGF-I and IGFBP-3 are influenced by study age and by gestational age, even after adjusting for early birth.