PLACENTAL THYROID STIMULATORS AND THYROID FUNCTION IN PREGNANCY ,

Abstract

This chapter discusses placental thyroid stimulators and thyroid function in pregnancy. Pregnancy influences maternal thyroid function in a complex manner. Fetal thyroid function is independent of the maternal thyroid. In addition, the placenta plays an important role in the control of thyroid function in pregnancy. This chapter addresses three questions. First, what effect does the placenta have on thyroid function in mother and fetus? Second, what are the landmarks in the development of fetal thyroid hormone secretion and its control? Third, how does thyroid function in pregnant women differ from that in the nonpregnant state? The placenta forms a barrier to the transport of many substances between mother and fetus. As iodide is the limiting substrate for the synthesis of thyroid hormone, its transport across the placenta is essential for the production of thyroid hormone by the fetus. The placenta is permeable to iodide but not permeable to the iodothyronines, even though they are relatively small molecules. The development of fetal thyroid function in humans proceeds through three stages—early organ development, the maturation of hypothalamic function, and the maturation of thyroid hormone production and its control.