Pregnancy has profound effects on the regulation of thyroid function, and on thyroidal functional disorders, that need to be recognized, carefully assessed and correctly managed. Relative hypothyroxinemia and goitrogenesis may occur in healthy women who reside in areas with restricted iodine intake, strongly suggesting that pregnancy constitutes a stimulatory challenge for the thyroid. Overt thyroid dysfunction occurs in 1-2% of pregnant women, but mild forms of dysfunction (both hyper- and hypothyroidism) are probably more prevalent and frequently remain unrecognized. Alterations of maternal thyroid function have important implications for fetal and neonatal development. In recent years, particular attention has been drawn to the potential risks for the developing fetus due to maternal hypothyroxinemia during early gestation. Concerning hyperthyroidism, the two main causes of thyrotoxicosis in the pregnant state are Graves' disease and gestational transient thyrotoxicosis (GTT). The natural history of Graves' disease is altered during pregnancy, with a tendency for exacerbation during the first trimester, and amelioration during the second and third trimesters. The natural history of the disorder must be considered when treating patients, since antithyroid drugs cross the placenta and can directly affect fetal thyroid function. Algorithms to routinely screen pregnant women for thyroid dysfunction have been proposed in recent years, but these have not yet been implemented systematically, nor have they been the subject of cost-effectiveness analyses.