The placental transfer of I131-labeled triiodothyronine has been studied in Rhesus monkeys. The majority of maternal to fetal placental transfer of radioactivity was in the form of iodide and a chemically unrecognized compound on chromatograms. Only traces of triiodothyronine or thyroxine were detected. In the fetal to maternal transfer studies, triiodothyronine was more readily transferred into the maternal circulation with small amounts of iodide and a similar unknown compound appearing. This substance may be sulfate conjugated triiodothyronine. There was a considerably higher concentration gradient used in the fetal to maternal transfers than in the reverse direction. The implications of this gradient difference are discussed.
The data from this study add further evidence to the clinical experience that maternal thyroid hormones are not readily available to meet all of the fetal needs during the latter part of pregnancy.
Effects of Dietary Caloric Restriction and Aging on Thyroid Hormones of Rhesus Monkeys