The thyroid during pregnancy: a physiological and pathological stress test.

Pregnancy and the postpartum are times of marked and rapid change in the thyroid gland. Normal physiological changes include enhanced thyroid hormone production, modulation of thyroid hormone metabolism by placental deiodinases, and decreasing titers of thyroid antibodies in thyroid antibody positive women. Hyperemesis gravidarum is associated with suppressed thyroid stimulating hormone levels and free T4 elevations. Graves' disease typically becomes quiescent during pregnancy, followed by a postpartum flare. Women with pre-existing hypothyroidism frequently require an increase in their levothryoxine requirement in the 1(st) trimester, and subclinical hypothyroidism early in pregnancy is linked to both miscarriage and impaired neurological development in the unborn child. Postpartum thyroiditis occurs in 7.2% of women, and euthyroid women who are thyroid antibody positive in the 1(st) trimester of pregnancy have a doubling of the miscarriage rate.