OBJECTIVE: Serum 3, 3',5-triiodothyronine (T3) remains low in near-term fetus to prevent the growing fetus from undue exposure to its active catabolic effect in mammals. The present study was undertaken to gain insight in the role of placenta in T3 metabolism, fetal to maternal transfer of T3, and its metabolites by in situ placenta perfusion with outer-ring labeled [(125)I]-T3 in pregnant guinea pig, a species showing increased sulfated 3, 3'-diiodothyronine (T2S) levels in maternal serum in late pregnancy (term = 65 days), similarly to humans in pregnancy.

MATERIALS AND METHODS: One-pass placenta perfusions performed on pregnant guinea pigs were studied between 58 - 65 days of gestation. In two separate experiments, the umbilical artery of the guinea pig placenta was perfused in situ at 37 degrees C with outer-ring labeled [(125)I]-T3. Maternal sera and umbilical effluents were obtained for analysis at the end of a 60-minute perfusion, when the steady-state levels of radioactivity were reached in the placenta effluent after 30-minute.

RESULTS: Sulfated [(125)I]-T2S was readily detected in the maternal serum as the major metabolite of T3 following the perfusion of placenta with [(125)I]-T3, suggesting that placental inner-ring deiodinase and sulfotransferase may play an important role in fetal T3 homeostasis and in the fetal to maternal transfer of sulfated iodothyronine metabolites.

CONCLUSIONS: The expression of type 3 deiodinase (D3) and thyroid hormone sulfotransferase activity in placenta may play an important role to protect developing organs against undue exposure to active thyroid hormone in late gestation in the fetus. The combined activities of D3 and sulfotransferase promoted a placental transfer of T2S into maternal circulation. The maternal circulation of T2S is fetal T3 in origin and its role as a fetal thyroid function biomarker deserves further evaluations and studies.