Both methimazole and propylthiouracil (PTU) can present with risk to the fetus. Both methimazole and PTU given in the first trimester of pregnancy present an unacceptable risk for congenital malformations. With methimazole there is approximately a 10% rate of birth defects. This can include urinary system malformation and choanal atresia which is a narrowing of the back of the nose which results in difficulty breathing. Esophageal atresia or malformation of the esophagus can be present. Omphalocele is also a potential risk of methimazole.
PTU and Birth Defects
Propylthiouracil presents birth defects in approximately 8% of patients. This includes malformations in the face and neck region. Typically, propylthiouracil is recommended before conception and it is recommended that this be switched to methimazole after the first trimester of pregnancy if antithyroid medications are required.