Effect of Intraamniotic Fluid Thyroxine Injection on Fetal Serum and Amniotic Fluid Iodothyronine Concentrations*

Abstract

Seven hundred micrograms of T₄ were injected into the amniotic cavity 24 h before delivery of five pregnant women scheduled for elective cesarean section at term. T₄, T₃, and rT₃ concentrations were measured by RIA in amniotic fluid obtained at the time of the injection and in amniotic fluid and cord serum samples collected at delivery. Iodothyronine concentrations also were determined on cord samples from 24 full term control infants. The geometric mean serum T₄ concentration in the experimental infants was 27.2 μg/dl, almost 3 times that of the control population (10.3 μg/dl); serum rT₃ concentrations were markedly elevated to a mean of 657 ng/dl, compared to 254 ng/dl in control infants. The mean serum T₃ concentration was slightly but significantly increased to 61.3 ng/dl (control, 48.3 ng/dl; P < 0.02). Amniotic fluid T₄, T₃, and rT₃, concentrations all increased significantly. T₄ injection into the amniotic fluid is an effective method of increasing fetal serum T₄ concentrations. The preferential pathway of monodeiodination of the injected T₄ in the human fetus is to rT₃ rather than T₃.