The thyroid gland is substantially challenged during pregnancy. Total T(3) and T(4) levels increase by 50% during pregnancy owing to a 50% increase in thyroxine-binding globulin levels. Serum TSH levels decrease in the first trimester and increase in the second and third trimesters; however, not to prepregnancy levels. Hypothyroidism is present in up to 3% of all pregnant women. Subclinical hypothyroidism during pregnancy is associated with an increased rate of miscarriage and preterm delivery, and a decrease in the IQ of the child. Overt hyperthyroidism is present in less than 1% of pregnant women but is linked to increased rates of miscarriage, preterm delivery and maternal congestive heart failure. In women who are euthyroid, thyroid autoantibodies are associated with an increased risk of spontaneous miscarriage and preterm delivery. Postpartum thyroiditis occurs in 5.4% of all women following pregnancy; moreover, 50% of women who are euthyroid in the first trimester of pregnancy but test positive for thyroid autoantibodies will develop postpartum thyroiditis. The need for the essential nutrient iodine increases during pregnancy and in women who are breastfeeding, and the effect of treatment of mild iodine deficiency on maternal and fetal outcomes is consequently being evaluated in a prospective study. The debate regarding the pros and cons of universal screening for thyroid disease during pregnancy is ongoing.