The development of the human placenta involves a complex process of tightly regulated proliferation and invasion by extravillous trophoblast into the uterine decidua. Inadequate placentation is a feature of intrauterine growth restriction and other gestational pathology. There is some evidence that T(3) plays a role in the regulation of these processes and that T(3) may act synergistically with epidermal growth factor (EGF). The aim of this study was to define the expression of thyroid hormone receptors in extravillous trophoblast, elucidate the effects of T(3) on both proliferation and differentiation of human trophoblast cells of varying origins, and define the potential interaction between EGF and T(3) on these processes. Using immunohistochemistry, specific thyroid hormone receptor isoforms were localized in extravillous trophoblast in first- and second-trimester placental bed biopsies, indicating potential sensitivity to T(3). In studies of human trophoblast-derived cell lines and primary cultures of cytotrophoblast cells in vitro, T(3) and EGF exerted an antiproliferative effect on an extravillous-like cell line (SGHPL-4) but stimulated proliferation in JEG-3 choriocarcinoma cells. EGF enhanced survival of nonproliferative term primary cytotrophoblast cells and significantly enhanced invasion of fibrin gels by SGHPL-4 cells, an effect attenuated by T(3). Both T(3) and EGF also significantly enhanced SGHPL-4 motility. These results suggest that EGF and T(3) may act synergistically to regulate both proliferation and differentiated function of human trophoblast.