The risk to the fetus is very low

Thyrotoxicosis affects up to 0.2% of pregnant women.1 If left untreated it is associated with increased fetal mortality and morbidity.2 Treatment is with antithyroid drugs such as propylthiouracil or carbimazole, with β blockers reserved for presurgical treatment and immediate control of severe thyrotoxic symptoms. Considerable concern exists, however, about the potential adverse fetal consequences of maternal antithyroid treatment, and sometimes conflicting or inappropriate advice is given. Women exposed to antithyroid drugs or radioiodine immediately before or in early pregnancy need accurate and timely information when deciding whether to proceed with the pregnancy.

There are two concerns about antithyroid drugs for thyrotoxicosis: that the drugs cause hypothyroidism in the fetus and that they have teratogenic effects. These drugs cross the placenta and can sometimes cause fetal hypothyroidism and goitre.3 The fetal thyroid begins to develop at 5-6 weeks' gestation, with follicles and colloid production at 10-12 weeks. Adverse effects on fetal thyroid function are thus unlikely unless treatment begins after 10 weeks' gestation.4 In two studies in which antithyroid therapy was used in moderate doses maternal and fetal outcomes were satisfactory, regardless of which antithyroid drug …