Triiodothyronine turnover in maternal and fetal sheep.

Abstract

Indwelling exteriorized femoral artery catheters were placed in the fetuses and jugular vein catheters in ewes of 98–125 days gestation. Postoperatively, tracer doses of 125I-T3 and 131I-T3 were injected into the mother and fetus, respectively; and serial blood specimens were drawn from each for a period of 48 hr during maternal perchlorate administration. Alkali washed butanol extracts were prepared of each blood specimen for dual isotope counting; from these data the vol of distribution and plasma disappearance of isotope were determined in the mother and fetus. Measurements of serum T3 concentration were conducted by radioimmunoassay; T4 was measured by the method of Murphy and Pattee. The mean t1/2 values for T3 in the mother and fetus, respectively, were 7 and 5.5 hr, mean vol of distribution of T3 28.8 and 5.1 1, mean serum T3 concentrations 79 and <18 ng/100 ml and mean T3 turnover rates 67 and <2.8 M-g/day (1.43 and <1.45 μg/kg/day). Significant placental transfer of T3 occurred, and chromatographic analysis of the butanol extracts indicated that the radioactivity crossed the placenta as T3. Estimated maternal to fetal and fetal to maternal fractional transfer rates were 0.004 and 0.05/hr-1. Absolute placental T3 transfer was estimated to be 2.0 and <1.2 μg/day in the maternal to fetal and fetal to maternal directions, respectively. Net transfer approximated 1 μ-g/day in the maternal to fetal direction. (Endocrinology90: 1301, 1972)