Postnatal thyroid function in low birth weight infants: a cross-sectional assessment of free thyroxine and thyroid hormone binding globulin.

To investigate the significance of low serum thyroxine in premature infants, serum FT4, T4, TSH and TBG were measured in 7 infants with BW less than 1000 g, 8 infants with BW 1001 to 1350 g, 9 infants with BW 1351 to 2499 g, and 11 full-term infants. FT4 concentrations were lower in the LBW infants than in the FT infants. Percent FT4 values in the infants with BW less than 1000 g were the highest in the groups studied, so that FT4 concentrations in those infants did not fall proportionally with the marked T4 decrease. TBG concentrations were lower in the VLBW infants (less than BW 1350 g) than in the FT infants, and were positively correlated with BW and GA. This study suggests that in infants with lower birth weight there is a mechanism to prevent a decrease in FT4 concentrations associated with total T4 decrease. This fact may be a reason for euthroidism in small premature infants with low T4. Measurements of FT4 may be a useful adjunct to assessment of thyroid function in the VLBW infants with low T4.