Because thyroid disease is the second most common endocrine disease affecting women of reproductive age, obstetricians often care for patients who have been previously diagnosed with alterations in thyroid gland function. In addition, both hyperthyroidism and hypothyroidism may initially manifest during pregnancy. Obstetric conditions, such as gestational trophoblastic disease or hyperemesis gravidarum, may themselves affect thyroid gland function. This document will review the thyroid-related pathophysiologic changes created by pregnancy and the maternal-fetal impact of thyroid disease.