Thyroid function in pregnancy - maternal-fetal relationship in health and disease.

The article reviews the changes in maternal and fetal thyroid function during pregnancy. During the first trimester the fetus is dependant on maternal thyroxine and later on the direct supply of iodine from the mother for its own thyroxine production. The placental deiodinase metabolism provides the fetus with additional iodine and protects the fetus from excessive iodothyronine transfer and enables the development of its own hypothalamic pituitary thyroid axis. Maternal and fetal hypothyroxinemia can lead to irreversible CNS damage. In autoimmune thyroid diseases immunoglobulins and goitrogenic drugs can cross the placenta and affect the fetal thyroid. Therefore, careful monitoring of maternal iodine supply, even in areas with mild to moderate iodine deficiency as well as iodine FT4 before and during pregnancy, and avoidance of unnecessary goitrogenic drugs is mandatory for optimal fetal growth and development. In cases where fetal hypothyroidism is diagnosed, intraamniotic T4 treatment is suggested.