SERUM-FREE THYROXINE CONCENTRATION IS NOT REDUCED IN PREMATURE-INFANTS WITH RESPIRATORY-DISTRESS SYNDROME

Objective: We used improved methods of assay to determine whether pitu itary-thyroid function is altered in premature infants with respirator y distress syndrome (RDS) during the first week of postnatal life. Met hods: Serum free thyroxine (T-4) was measured by direct equilibrium di alysis, total thyroxine (TT4) by radioimmunoassay, and thyrotropin by a sensitive immunometric assay in 90 premature infants (45 healthy con trol subjects and 45 with RDS) during their first week of life after 2 5 to 30 weeks of gestation. Infants in the RDS group received exogenou s surfactant therapy. Results: Free T and thyrotropin concentrations o f infants were not significantly different between RDS and control gro ups. As expected, infants with RDS had significantly lower serum total T-4 concentrations compared with control infants (p< 0.001). This dif ference 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 T-4 concentrations were attributable to lower T-4 binding to ser um proteins among infants with RDS compared with control subjects, esp ecially in the 25- to 27-week gestation group (p = 0.0075). Conclusion : These data indicate that pituitary-thyroid function is not altered i n premature infants with RDS. The low total T state in these premature infants is attributable solely to reduced serum T-4 binding, as is of ten seen in acute nonthyroidal illnesses.