A 28-year-old client at 8 weeks gestation with Graves disease has been on methimazole 15 mg daily before pregnancy. Free T4 remains elevated at the prenatal visit. Which medication change should the nurse anticipate the prescriber to make?

Antithyroid choice in pregnancy is determined by trimester-specific teratogenicity. Methimazole crosses the placenta and in the first trimester is associated with embryopathy (aplasia cutis, choanal atresia, esophageal atresia, omphalocele). PTU is preferred during the first trimester because its teratogenic risk is lower, although PTU has a higher rate of severe hepatotoxicity. The standard approach: (1) switch to PTU before or as early in pregnancy as possible, ideally before conception, (2) continue PTU through the first trimester, (3) consider switching back to methimazole at the start of the second trimester to minimize hepatotoxicity exposure for the rest of pregnancy, (4) use the lowest dose that maintains free T4 in the upper normal range to avoid fetal hypothyroidism, (5) avoid radioactive iodine (absolutely contraindicated) and high-dose iodine supplementation; thyroidectomy is reserved for special cases, (6) monitor maternal TSH/free T4 every 4 weeks and consider periodic fetal thyroid ultrasound. Stopping all medication risks thyroid storm and miscarriage. Iodine load suppresses hormone briefly but escapes Wolff-Chaikoff and can worsen disease.

PTU in trimester 1, switch back to methimazole in trimester 2 — this is the textbook NCLEX algorithm for Graves in pregnancy. The teratogenicity windows drive the choice.