Hypothyroidism during pregnancy is associated with adverse outcomes that can be ameliorated or prevented by adequate therapy with thyroxine. Currently, there are no guidelines for universal screening for thyroid dysfunction in pregnant women or in women of reproductive age. Therefore, it is important to recognize those groups of women who may be at higher risk for development of hypothyroidism so that serum TSH testing may be performed with appropriate initiation of thyroxine therapy. In addition, the thyroxine therapy of women with established hypothyroidism should be optimized prior to conception and during pregnancy when the thyroxine dosage requirement generally increases early in gestation. The diverse etiologies of maternal hypothyroidism may require different increments in thyroxine dose during pregnancy, and generally the postpartum dosage requirement returns to pre-pregnancy levels.