Thyroid function inside and outside of pregnancy: what do we know and what don't we know?

A workshop entitled, "The Impact of Maternal Thyroid Diseases on the Developing Fetus: Implications for Diagnosis, Treatment, and Screening," was held in Atlanta, Georgia, January 12-13, 2004. This paper reports on the session that examined the prevalence of thyroid dysfunction in reproductive-age women and the factors associated with abnormal function. For this session the following papers were presented: "Thyroidal Economy in the Pregnant State: An Overview," "The Prevalence of Thyroid Dysfunction in Reproductive-Age Women- United States," and "Risk Factors for Thyroid Disease: Autoimmunity and Other Conditions." These presentations were formally discussed by invited respondents and by others in attendance. Salient points from this session about which there was agreement include the following: physiologic changes associated with pregnancy require an increased availability of thyroid hormones by 40% to 100% in order to meet the needs of mother and fetus during pregnancy. In the first trimester of gestation the fetus is wholly dependent on thyroxine from the mother for normal neurologic development. For the maternal thyroid gland to meet the demands of pregnancy it must be present, disease-free, and capable of responding with adequate stores of iodine. Thyroid autoimmunity is common and may contribute to miscarriages, as well as to hypothyroidism. With sufficient iodine nutrition, autoimmune thyroid disease (AITD) is the most common cause of hypothyroidism. As of 1994, iodine nutrition in the United States appeared to be adequate, but its continued monitoring is essential.