Levothyroxine
Essential in pregnancy. Maternal thyroid hormone is critical for fetal brain development, particularly in T1 before the fetal thyroid is functional (~12 weeks). Untreated hypothyroidism causes irreversible fetal neurodevelopmental impairment. Dose requirements increase by approximately 30–50% in T1 due to increased TBG, placental T4 metabolism, and hCG-driven thyroid stimulation. TSH targets in pregnancy: T1 <2.5 mIU/L, T2 and T3 <3.0 mIU/L (or institution-specific).
