Fetal-to-maternal transfer of 3,3',5-triiodothyronine sulfate and its metabolite in sheep.

Earlier studies have shown that sulfoconjugation is a major pathway of thyroid hormone metabolism in fetal mammals. To assess the placental transfer of sulfoconjugates in the pregnant sheep model, we measured 3,3',5-triiodothyronine (T3) sulfate (T3S), 3,3'-diiodothyronine sulfate (T2S), and T3 concentrations in fetal serum and in maternal serum and urine after T3S infusion to the fetus ( n = 5) or the ewe ( n = 6). Maternal infusion of T3S did not increase fetal serum T2S, T3S, or T3 concentrations. In contrast, fetal infusion of T3S produced significant increases in maternal serum T2S and T3S but not T3 concentrations. Fetal T3S infusion also increased maternal urine excretion of T3S. However, the 4-h cumulative maternal urinary excretion of T2S and T3S after fetal T3S infusion was less than the excretion observed after fetal infusion of equimolar amounts of T3 in our previous study. It is concluded that fetal serum T2S and T3S can be transferred to maternal compartments. However, compared with T3, these sulfoconjugates may be less readily transferred.