Serum free thyroxine concentration is not reduced in premature infants with respiratory distress syndrome

Objective: We used improved methods of assay to determine whether pituitary-thyroid function is altered in premature infants with respiratory distress syndrome (RDS) during the first week of postnatal life. Methods: Serum free thyroxine (T₄) was measured by direct equilibrium dialysis, total thyroxine (TT₄) by radioimmunoassay, and thyrotropin by a sensitive immunometric assay in 90 premature infants (45 healthy control subjects and 45 with RDS) during their first week of life after 25 to 30 weeks of gestation. Infants in the RDS group received exogenous surfactant therapy. Results: Free T₄ and thyrotropin concentrations of infants were not significantly different between RDS and control groups. As expected, infants with RDS had significantly lower serum total T₄ concentrations compared with control infants (p < 0.001). This difference was present even after stratification for gestational age (25- to 27-week group, p = 0.012; 28- to 30-week, group, p = 0.002). Lower total T₄ concentrations were attributable to lower T₄ binding to serum proteins among infants with RDS compared with control subjects, especially in the 25- to 27-week gestation group (p = 0.0075). Conclusion: These data indicate that pituitary-thyroid function is not altered in premature infants with RDS. The low total T₄ state in these premature infants is attributable solely to reduced serum T₄ binding, as is often seen in acute nonthyroidal illnesses.