Postnatal triiodothyronine concentrations in healthy preterm infants and in infants with respiratory distress syndrome.

Postnatal changes in triiodotyronine (T3) concentration were investigated in 12 preterm infants of 26-34 weeks of gestational age. Blood for measurement of T3 was obtained from the cord at delivery and from infants at 1 day of age and at weekly intervals for 4 weeks. Seven of the babies suffered from respiratory distress syndrome (RDS) and five were considred healthy. Gestational ages and body weight were comparable in both groups. In preterm infants with RDS, cord blood T3 concentration was significantly lower than that in cord blood of babies without RDS (22 +/- 2.6 versus 36 +/- 5 ng/dl, P less than 0.05). There was no significant rise in T3 concentration of RDS babies at 24 hr of age (22 +/- 2.6 versus 34.0 +/- 8 ng/dl, P greater than 0.05), and hypotriiodothyroninemia persisted for 3 weeks. At 4 weeks of age, T3 concentration in babies with RDS, although within the normal range (80-190 ng/dl), was significantly lower than that in the healthy preterm infants (110 +/- 10 versus 165 +/- 11 ng/dl, P less than 0.05). Postnatal T3 changes in healthy preterm infants wre characterized by the absence of the initial hypertriiodothyroninemia and by a gradual rise within the first month of life. The noted difference in the pattern of postnatal T3 changes in healthy preterm infants compared to full term infants may reflect thyroid immaturity. The machanism and the significance of the neonatal hypotriiodothyroninemia in RDS and its long term effects on the development of these babies remain to be investigated.