Abstract
The thyroid hormones, thyroxine (T4) and triiodothyronine (T3), are essential for normal growth and development of the fetus. Their bioavailability in utero depends on development of the fetal hypothalamic-pituitary-thyroid gland axis and the abundance of thyroid hormone transporters and deiodinases that influence tissue levels of bioactive hormone. Fetal T4 and T3 concentrations are also affected by gestational age, nutritional and endocrine conditions in utero, and placental permeability to maternal thyroid hormones, which varies among species with placental morphology. Thyroid hormones are required for the general accretion of fetal mass and to trigger discrete developmental events in the fetal brain and somatic tissues from early in gestation. They also promote terminal differentiation of fetal tissues closer to term and are important in mediating the prepartum maturational effects of the glucocorticoids that ensure neonatal viability. Thyroid hormones act directly through anabolic effects on fetal metabolism and the stimulation of fetal oxygen consumption. They also act indirectly by controlling the bioavailability and effectiveness of other hormones and growth factors that influence fetal development such as the catecholamines and insulin-like growth factors (IGFs). By regulating tissue accretion and differentiation near term, fetal thyroid hormones ensure activation of physiological processes essential for survival at birth such as pulmonary gas exchange, thermogenesis, hepatic glucogenesis, and cardiac adaptations. This review examines the developmental control of fetal T4 and T3 bioavailability and discusses the role of these hormones in fetal growth and development with particular emphasis on maturation of somatic tissues critical for survival immediately at birth