Fig 5. Proposal model for placental deiodinases in gestational diabetes mellitus (GDM) mothers.
In normal glucose tolerant pregnancies (NGT), the thyroid hormones Free T4 (FT4) and T3 are transported from the maternal blood through the placenta, where their concentrations are regulated by deiodinase 2 (DIO2), which converts T4 to T3; and deiodinase 3 (DIO3), which converts T3 to T2, and T4 to rT3; in order to keep a steady and healthy concentration of thyroid hormones from the mother to the fetus. In GDM however, in order to prevent an influx of an excess of T3 from the mother to the fetus, the expression and activity of DIO3 increases while DIO2 decreases, causing less T3 to be transported to fetal blood by a currently unknown mechanism.