Indwelling, exteriorized femoral artery catheters were placed in the fetuses and jugular vein catheters in ewes of 100–135 days of gestation. Postoperatively, tracer doses of 125I-T4 and 131I-T4 were injected into the mother and fetus, respectively, and serial blood specimens were drawn from each for a period of 96 hr during maternal perchlorate administration. Alkali- washed butanol extracts were prepared of each specimen for dual isotope counting; from these data the volume of distribution and plasma disappearance of isotope were determined in the mother and fetus. Measurements of total T4 were determined by the method of Murphy and Pattee. No placental passage of fetal or maternal isotopes as T4 was detected. The mean t½, values for T4 in the mother and fetus were 1.1 and 0.7 days and mean T4 secretion rates 367 and 103 μg/day or 5.5 and 41 μg /kg /day. The data indicate a) an autonomous fetal pituitarythyroid system and b) a high rate of fetal T4 secretion during the last trimester of gestation. (Endocrinology88: 47, 1971)
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Dussault et al. (1971) studied this question.