The most common thyroid diseases during pregnancy are hyper- and hypothyroidism and their variants including isolated hypothyroxinemia (hypo-T4), autoimmune thyroid disease (AITD) and different types of goiter. AITD represents the main cause of hypothyroidism during pregnancy ranging in prevalence between 5 and 20% with an average of 7.8%. The incidence of isolated hypo-T4 is about 150 times higher compared to congenital hypothyroidism. Prevalence of Graves' disease (GD) ranges between 0.1% and 1% and the Transient Gestational Hyperthyroidism Syndrome between 1 and 3%. Thyroid stimulating hormone (TSH) is a sensitive marker of thyroid dysfunction during pregnancy. Normal values have been modified recently with a downward shift. Thus, the upper normal range is now considered to be 2.5 mUI/mL in the first trimester and 3.0 mUI/mL for the remainder of pregnancy. Most studies have shown that children born to women with hypothyroidism during gestation had significantly lower scores in neuropsychological tests related to intelligence, attention, language, reading ability, school performance and visual motor performance. However, some studies have not confirmed these findings. On the other hand, multiple retrospective studies have shown that the risks of maternal and fetal/neonatal complications are directly related to the duration and inadequate control of maternal thyrotoxicosis. The latter is associated with a risk of spontaneous abortion, congestive heart failure, thyrotoxic storm, preeclampsia, preterm delivery, low birth weight and stillbirth. Despite the lack of consensus among professional organizations, recent studies, which are based on sophisticated analyses, support universal screening in all pregnant women in the first trimester for thyroid diseases.