Cord serum thyroid-stimulating hormone and thyroglobulin levels decline with increasing birth weight in newborns

Abstract

Cord serum thyroglobulin (Tg) and TSH levels were related to birth weight in 3 groups of newborn infants composed of 101 infants. Serum free T3 index, free T4 index, and/or T4 also were determined. Group I consisted of normal term newborns (20 females and 19 males), whose mean ± SD gestational ages (40.1 ± 0.7 vs. 40.1 ± 0.5 weeks) did not differ, but whose mean birth weights (3299 ± 282 vs. 3757 ± 447 g) differed significantly (P < 0.005). In female infants, serum Tg levels (r = −0.401; P < 0.05) and the log of TSH levels (r = −0.576; P < 0.005) correlated negatively with birth weight, while Tg levels correlated positively with the log of TSH levels (r = 0.401; P < 0.05). In contrast, none of these correlations was significant for male infants. However, T4 levels and birth weight correlated positively (r = 0.499; P < 0.025) in male infants, but not in female infants. Group II consisted of newborns whose birth weights were less than 2500 g (19 females and 19 males). Mean birth weights of female (2032 ± 301 g) and male (1850 ± 413 g) infants did not differ significantly (P > 0.05). Both the Tg levels and the log of the TSH levels correlated negatively with birth weight in female (Tg, r = −0.891 and P < 0.005; log TSH, r = 0.600 and P < 0.005) and male (Tg, r = −0.849 and P < 0.005; log TSH, r = −0.660; P < 0.005) infants. Also, Tg levels correlated positively with the log of the TSH levels in female (r = 0.554; P < 0.01) and male (r = 0.412; P < 0.05) infants. Free T4 index levels correlated positively with free T3 index levels in female (r = 0.443; P < 0.05) and male (r = 0.570; P < 0.01) infants. Group III consisted of 12 normal female term newborns whose mean birth weight (3685 ± 623 g) was not significantly (P > 0.2) different from that of the males of group I, and 12 normal male term newborns whose mean birth weight (4104 ± 248 g) was significantly (P < 0.005) greater than that of the males of group I. Unlike in lower weight female or male infants, serum Tg levels did not correlate with birth weight or the log of TSH levels. Although the log of TSH levels did not correlate with birth weight in female infants, male infants had a significant positive correlation (r = 0.539; P < 0.05). A positive correlation also was found between T4 and free T4 index levels and birth weight in both female and male infants. Also, free T4 index levels correlated positively with free T3 index levels in female and male infants. We conclude that the sex difference in cord serum thyroid indices relates principally to changes in body composition accompanying increasing birth weight rather than to sexually determined phenomena. © 1984 by The Endocrine Society.