Maternal, fetal, and amniotic fluid transport of thyroxine, triiodothyronine, and iodide in sheep: a kinetic model.

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.