Reference ranges for newer thyroid function tests in premature infants.

Abstract

To establish reference ranges for recently developed assays of thyroid function in premature infants. We measured serum free thyroxine (T4) 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. The free T4 level correlated positively with gestational age (p < 0.0001; r2 = 0.09) and differed significantly between adjacent gestational age groups (p < 0.05). Free T4 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, 2.8 +/- 0.8 ng/dl), respectively. Two reference ranges for free T4 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; r2 = 0.08); one reference range of 0.5 to 29 mU/L was determined for thyrotropin. This study extends information on thyroid function of preterm infants and establishes reference ranges for this population.