Postnatal Triiodothyronine Concentrations in Healthy Preterm Infants and in Infants with Respiratory Distress Syndrome

Abstract

Summary: Postnatal changes in triiodothyroninc (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 considered 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 < 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 > 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 < 0.05). Postnatal T₃ changes in healthy preterm infants were characterised 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 reflecd thyroid immaturity. The mechanism and the significance of the neonatal hypotriiodothyroninemia in RDS and its long term effects on the development of these babies remain to be investigated. Speculation: Triiodo and tetraiodo hypothyroninemia seem to have been established as a concommitant of the respiratory distress syndrome. The significance of these findings in the pathophysiology of RDS remains unclear at this point. The existing evidence of thyroid deficiency before clinical onset of RDS and the experimental data in favor of a significant role of thyroid hormones in the maturation of lung surfactant suggest a causal relationship.