REFERENCE RANGES FOR NEWER THYROID-FUNCTION TESTS IN PREMATURE-INFANTS

Objective: To establish reference ranges for recently developed assays of thyroid function in premature infants. Methods: We measured serum free thyroxine (T-4) by direct equilibrium dialysis and serum thyrotropin by a sensitive immunometric method in 104 preterm infants (25 to 36 weeks of gestational age) during the first week of life, Results: The free T-4 level correlated positively with gestational age (p <0.0001; r(2) = 0.09) and differed significantly between adjacent gestational age groups (p <0.05). Free T-4 concentrations (mean +/- SD) for the 25- to 27-, 28- to 30-, 31- to 33-, and 34- to 36-week groups were 18.0 +/- 5.2, 25.7 +/- 9.0, 30.9 +/- 9.0, 36.0 +/- 10.3 pmol/L (1.4 +/- 0.4, 2.0 +/- 0.7, 2.4 +/- 0.7, and 2.8 +/- 0.8 ng/dl), respectively, Two reference ranges for free T-4 were determined, one for 25 to 30 weeks (6.4 to 42.5 pmol/L (0.5 to 3.3 ng/dl)) and one for 31 to 36 weeks (16.7 to 60.5 pmol/L (1.3 to 4.7 ng/dl)). The logarithm of the value for thyrotropin correlated positively with gestational age (p <0.001; r(2) = 0.08); one reference range of 0.5 to 29 mU/L was determined for thyrotropin. Conclusion: This study extends information on thyroid function of preterm infants and establishes reference ranges for this population,