Bidirectional thyroxine exchange in pregnant sheep.

Abstract

Radioisotopic exchange studies in pregnant sheep support the concept of a small but significant bidirectional transplacental thyroxine flux between mother and fetus in the last bimester. The direction and extent of this transfer is not known. There appears to be a rapid and appreciable deiodination of the administered radioactive thyroxine, but no indication that this iodide appeared to any measurable amount as radiothyroxine. The relatively small quantities of radioactive thyroxine transferred become quite significant when compared to the extremely small concentrations of free thyroxine presented to each placental interface.