Clinical hyperthyroidism is not uncommon in pregnancy, with a reported prevalence of 0.10.4%, and is caused most frequently by Graves disease. Careful monitoring of thyroid function is critical in preventing the many potential complications that can occur in pregnancies of mothers with hyperthyroidism. Maternal complications include hypertension, thyroid storm, heart failure preterm labour, and placental abruption. Foetal and neonatal complications include stillbirth, intrauterine growth restriction, low birth weight, heart failure, hyperthyroidism, and hypothyroidism with or without goitre. Currently, the drugs used to control maternal hyperthyroidism are propylthiouracil (PTU) and methimazole (MMI)/carbimazole (CZ). Both medications cross the human placenta with relatively similar transfer and placental clearance rates. PTU and MMI/CZ are equivalent in terms of their efficacy in the treatment of clinical hyperthyroidism. Some reports suggested an association between specific congenital malformations (MMI embryopathy) and prenatal exposure to MMI. Using prospective data from the European Teratogen Information Services, Di Gianantonio concluded that there might be a higher than expected incidence of choanal and oesophageal atresia in foetuses exposed to MMI between the third and seventh weeks of gestation. Further reports of congenital anomalies, specifically of choanal atresia, have definitely suggested that MMI may be a new teratogen. For PTU the association with possible teratogenic risk is unclear, but this difference could be an artifact due to the lack of studies on PTU. However, recent studies failed to detect a significant association between PTU exposure during the first trimester of pregnancy and congenital malformations. In view of these reports, it is generally recommended that, when available, PTU be preferred as the initial therapy for maternal hyperthyroidism during the first trimester of pregnancy. However, the prescription of PTU later in pregnancy and during lactation should take into account the reported risk of hepatotoxicity in both the mother and the child.