Hyperthyroidism in pregnancy is a rare, serious condition which can increase the risks of miscarriage, stillbirth, preterm birth, and intrauterine growth restriction. Pregnant women who are hyperthyroid may also develop severe pre-eclampsia or placental abruption. Most of these women have Graves' disease, an autoimmune disease most common in women aged 20 to 40 years. Most pregnant women with hyperthyroidism are diagnosed with thyroid disease prior to conception and will have previously received treatment for the condition. Generally only drug therapy is considered for treating pregnant women with hyperthyroidism. Radioiodine treatment is not used in pregnancy because it destroys the fetal thyroid gland, resulting in permanent hypothyroidism in the newborn.
The main antithyroid drugs used are the thionamides, propylthiouracil (PTU), methimazole and carbimazole. PTU is currently the favoured drug for use in pregnancy, as it is associated with fewer teratogenic effects (scalp lesions) than methimazole. However, since there have been reports of liver damage in people taking PTU, it may be reasonable for pregnant hyperthyroid women to be treated with PTU in the first trimester (to reduce any teratogenic effects of methimazole) and then to change to methimazole.
We could not identify any randomised trials to help inform women and their doctors about which antithyroid drugs are most effective, with the lowest potential for harm.