The thyroid hormone and retinol binding protein transthyretin (TTR) is synthesised by human trophoblasts. Polarised JEG-3 choriocarcinoma cells grown in bicameral chambers secrete TTR predominantly apically but also basally and these cells and human trophoblasts also take up TTR suggesting that there may be a placental TTR shuttle that participates in materno-fetal transfer of thyroid hormones and retinol.Our objective was to investigate TTR secretion into the maternal and fetal circuits of the ex vivo dually perfused placental lobule to confirm that placenta secretes TTR into the fetal circulation. We also investigated translocation of Alexa Fluor-594 labelled TTR from incubation medium into the fetal placental capillaries in early (14-15 weeks) and term placental villus explants.The perfused placental lobule secretes TTR into the maternal and fetal circuits. Secretion in both circuits is linear with time and is predominantly into the maternal circuit (mean maternal/fetal ratio 99.4 ± 25.6). The mean data fitted well to a three compartment mathematical model (maternal circuit, placenta and fetal circuit, constant secretion of TTR and return of maternal circuit TTR to the placental compartment). Explants from early (14-15 weeks) and late (38-40 weeks) placentas translocated fluorescently labelled TTR from medium to villus (fetal) capillaries.Our results confirm that human placenta secretes TTR into maternal and fetal circulations and supports the hypothesis that placental TTR secreted into the maternal placental circulation can be taken up by trophoblasts and translocated to the fetal circulation, forming a TTR shuttle system. This may have important implications for materno-fetal transfer of thyroid hormones, retinol/retinol binding protein and xenobiotics (such as polychlorinated biphenyls) all of which bind to TTR.