The objective of this chapter is to introduce the thyroid disorders during pregnancy. Thyroid diseases affect up to 5% of all pregnancies. Pregnancy can be considered as a stress test of maternal thyroid function where women with limited thyroid reserve may develop hypothyroidism. Production of thyroxine (T4) and triiodothyronine (T3) increases by 50%, along with a 50% increase in the daily iodine requirement. 10% to 20% of all pregnant women in the first trimester of pregnancy are thyroid peroxidase (TPO) or thyroglobulin (Tg) antibody positive and euthyroid. Diagnosing thyroid diseases during pregnancy can be difficult as the clinical signs and symptoms mimic those of pregnancy. This chapter covers the thyroid dysfunction, causes of hypothyroidism in pregnancy, complications of hypothyroidism in pregnancy, hyperthyroidism, caveats and goals of antithyroid drug treatment, and autoimmune thyroiditis and postpartum thyroiditis.