Hypehyroidism in pregnancy Methimazole crosses the placenta to a greater degree than propylthiouracil and has been associated with fetal aplasia cutis. However, propylthiouracil can be associated with liver failure. Some expes recommend propylthiouracil in the first trimester and methimazole thereafter. Radioiodine should not be used during pregnancy, either for scanning or for treatment, because of effects on the fetal thyroid. In emergent circumstances, additional treatment with beta blockers may be necessary. Hypehyroidism is most difficult to control in the first trimester of pregnancy and easiest to control in the third trimester. In women with high-titer thyroid stimulating antibodies, the newborn may be born with neonatal Graves' disease. Ref - harrisons internal medicine 20e p3443 For many years breastfeeding was strongly discouraged if treatment with antithyroid drugs was required.12 Both propylthiouracil and methimazole can be detected in milk; however, studies have shown that propylthiouracil crosses into milk only in minute amounts, leading to a milk-plasma ratio of approximately 0.1 Ref -