Methimazole and Propylthiouracil are the cornerstones in the management of Graves' disease. Their primary effect is to inhibit thyroid hormone synthesis at different steps, i.e. in thyroid gland and in peripheral tissues. Antithyroid drugs can be used as the primary treatment for hyperthyroidism (long term therapy: 1-2 years) or as preparative therapy before radioiodine treatment or surgery (short term therapy: weeks or months). Generally, the starting dose of methimazole is 10-30 mg, as single daily dose, while that of PTU is 100-300 mg every 6 hours. Methimazole is the drug of choice, because major side effects are less common, it can be used as single dose, it's less expensive and more available. As far as the treatment of hyperthyroidism in pregnancy, MMI and PTU have same therapeutic efficacy and are both safe for the fetus, having similar placental transfer kinetics. The use of methimazole can be associated with aplasia cutis and choanal/esophageal atresia, while there are no data supporting the association between congenital anomalies and PTU. For this reason the latter is the drug of choice in the treatment of hyperthryroidism in pregnancy. Both thionamides are present in breast milk, but there are no controindications for their use during lactation.