Abstract
Data in a number of species suggest that placental transport of thyronine is limited in extent. Data regarding placental passage of TSH is indirect but suggests little or no transfer. The present studies were conducted to quantify T4, T3 and TSH transfer directly in bothe the maternal-fetal and fetal-maternal directions. Indwelling exteriorized fetal catheters and maternal jugular vein catheters were placed in pregnant sheep. simultaneous tracer doses of 131-I-T4 and 125-I-T4 were injected into the fetus and mother, respectively, in one study (6 animals); 131-I-T3 and 125-I-T3 in another (6 animals); and 131-I-BTSH and 125-I-BTSH in a third (5 animals). Serial blood samples were collected for periods up to 96 hours. Thses were extracted with butanol (for T3 and T4) or ppt with specific BTSH antibody (for TSH), and counted in a double isotope counting system to assess placental transfer of labeled hormones. There was no significant transfer of labeled T4 or TSH. However significant transfer of labeled T3 occured in both the fetal-maternal and maternal -fetal directions. By assessing maternal and fetal T3 kinetic data and assuming exchange between two compartments it was poosible to quantify this transfer. Fractional transfer rates were 0.045 and 0.0023 hr.-1 in the fetal-maternal and maternal-fetal directions respectively. Since hormone pools and turnovers were measured it could be estimated that about 2 μg T3 were transferred daily in both directions. This approximated 0.5% and 2.0%, respectively, of total daily maternal and fetal thyronine turnover. These data further support the veiew that the fetal pituitary-thyroid axis functions autonomously of the maternal system.
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Dussault, J., Distefano, J. & Fisher, D. Placental transfer of thyronines and thyrotropin in sheep. Pediatr Res 5, 399 (1971). https://doi.org/10.1203/00006450-197108000-00118
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DOI: https://doi.org/10.1203/00006450-197108000-00118