Placental Transport of I131-Labeled Thyroxine and Triiodothyronine in the Guinea Pig

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Abstract
The placental transfer in the guinea pig of I131-labeled triiodothyronine and thyroxine has been studied using an in situ perfusion technique. The umbilical cord was clamped close to the fetus and an umbilical artery and vein were cannulated. The placenta was perfused with a 1:1 guinea pig plasma: saline mixture. By administering I131-labeled substances either intravenously to the mother or in the placental perfusion fluid, mother to fetus and fetus to mother, transfers were observed. When labeled thyroxine was administered in the maternal circulation, small concentrations of radioactivity, about equally divided between thyroxine and iodide, were recovered from the perfusate outflow. Little transfer of labeled thyroxine from fetus to mother was observed. I131-labeled triiodothyronine administered to the mother did not cross the placenta as such, but radioactive iodide appeared in the perfusate outflow. Perfusion of the placenta with triiodothyronine resulted in little loss of radioactivity from the perfusion fluid, and no evidence of deiodination.