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Maternal, Fetal, and Amniotic Fluid Transport of Thyroxine, Triiodothyronine, and Iodide in Sheep: A Kinetic Model^*

Laboratory of Theoretical Biology, National Cancer Institute, National Institutes of Health Bethesda, Maryland 20014

Address requests for reprints to: Dr. Robert A. Mc-Guire, General Medical Research Service, Veterans Administration Hospital, Washington, DC 20422.

Abstract

A mathematical model of iodine kinetics in maternal and fetal sheep has been developed by combining separate iodide, T₃, and T₄ subsystems. The individual subsystem models were developed from literature studies of maternal-fetal exchange under thyroid-blocked and unblocked conditions. Rates of exchange, concentrations, and spaces of distribution were calculated by the SAAM computer program. The models for each of the subsystems required exchange compartments within the mother and fetus, exchanges between maternal and fetal circulations, and between the fetus and amniotic fluid. The fetal-amniotic fluid exchange was observed directly for iodide and indirectly for T₃ and T₄. No exchange between mother and amniotic fluid was required. It is possible that the amniotic fluid acts as a reservoir for these and other substances. Maternal-fetal kinetics suggest that low fetal T₃ levels are maintained by an active transport of T₃ from fetus to mother, a decreased transport from mother to fetus, and a low fetal T₃ production. The model also requires that all fetal T₃ loss occur via transport to the maternal system rather than via fetal utilization. In contrast, the fetal T₄ system is largely autonomous, the small maternal exchange not significantly contributing to the fetal T₄ economy. Fetal iodide seems to be supplied by a facilitated bidirectional exchange with the mother.

Footnotes

^* Portions of this work were presented at the Annual Meeting of the Endocrine Society in June, 1975, New York, New York.

Received October 13, 1977.