Serum Triiodothyronine and Thyroxine in the Neonate and the Acute Increases in These Hormones Following Delivery

J. Abuid; D. A. Stinson; P. R. Larsen

doi:10.1172/JCI107286

Serum Triiodothyronine and Thyroxine in the Neonate and the Acute Increases in These Hormones Following Delivery

J. Abuid, D. A. Stinson, and P. R. Larsen

Published May 1, 1973 - More info

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Abstract

Low triiodothyronine (T₃) and high normal thyroxine (T₄) concentrations are present in cord sera from full term infants. To examine this phenomenon further, radioimmunoassay of T₃ and T₄ was carried out in paired maternal and cord sera as well as capillary sera from neonates at different intervals after delivery. Free T₃ and free T₄ concentrations were also estiamted in cord and maternal sera by equilibrium dialysis. In 12 paired specimens, the T₃ concentration in cord sera was significantly lower than the maternal level (51±4 vs. 161±11 ng/100 ml, mean ±SE). Mean free T₃ concentration was also lower in the cord samples (0.15±0.02 vs. 0.31±0.04 ng/100 ml). whereas total and free T₄ concentrations were not significantly different. Umbilical vein and artery samples from 11 neonates did not differ significantly in their T₃ and T₄ concentrations. In seven infants the mean T₃ concentration increased from 51±3 ng/100 ml at delivery to 79±13 at 15 min and 191±16 at 90 min. In four other infants the mean T₃ concentration at 24 and 48 h was not significantly different from the 90 min value of the previous group. Less pronounced changes were observed for T₄ which increased from 12.3±2.0 μg/100 ml (mean ±SE) at delivery to 14.1±1.9 at 90 min and appeared to have reached a plateau at approximately twice the cord value by 24-48 h after delivery.

The maternal-fetal gradient observed for free T₃ is further evidence of the autonomy of the fetal thyroidpituitary axis. The time course of the abrupt increase in serum T₃ in the neonate suggests that it results from the earlier acute increase in serum TSH which occurs shortly after birth. This suggests that the neonatal thyroid contains significant quantities of T₃. Therefore, unavailability of thyroidal T₃ does not appear to explain the low total and free T₃ concentrations present in the sera of newborns.