Figure 1
Schematic view of thyroid morphogenesis and maternal serum estradiol levels during early pregnancy. (A) Schematic view of thyroid morphogenesis in human development. The thyroid gland begins to develop at 3 to 5 weeks of gestation as an endodermal thickening in the floor of the primitive pharynx. From week 5, thyroid precursor cells express a combination of transcription factors which are required for the early stages of thyroid development. After losing all connections with the pharynx, the thyroid bud migrates caudally, reaching its final position in front of the trachea at 6 weeks. Thyroid follicular cells begin their differentiation program and express thyroid-specific genes and secrete thyroid hormones from week 7. Finally, primitive follicles appear and the gland displays its final morphological organization (8 to 12 weeks) [12]. (B) Maternal serum estradiol concentrations during the first trimester of pregnancy. The mean serum levels of estradiol (E2) in the fresh embryo transfer (ET) group (n = 206) at different stages were all significantly higher than those in the frozen ET (n = 124) and natural conception (NC) (n = 200) groups during the first trimester of pregnancy. Data are presented as mean ± SEM, **P <0.01 vs. NC, ## P <0.01 vs. frozen ET. ET day, day of embryo transfer; hCG day, day of hCG administration.