Serum thymidine activity and insulin-like growth factors in the neonatal period.

Growth-promoting activity measured as [3H] thymidine incorporation into lectin-activated lymphocytes was determined simultaneously with radioimmunoassayable IGF I and IGF II in cord and capillary blood collected from human neonates 30 min and 24 h after birth. All the parameters studied in cord blood were lower than in normal adults. During the early postnatal period, IGF I decreased and IGF II remained unchanged, but thymidine activity increased above the normal adult level. The differences between the values found in cord blood and in capillary blood collected within the first 1/2 h after birth agree with a production of growth factors by the infant. All these growth factors may play a role in foetal growth, as suggested by their correlation with birth weight. Finally, factors other than IGFs contributing to thymidine activity may play a role in neonatal growth since they are higher in newborns than in adults.