A 29-year-old woman with a 3-year history of hyperthyroidism had regular treatment for 2 years, with symptom resolution. She discontinued medication 1 year ago. Currently at 32 weeks of pregnancy, she reports palpitations, hyperhidrosis, increased appetite, and weight loss since conception, without taking any antithyroid drugs. Symptoms worsened over the past week. Lab tests show markedly elevated FT<sub>3</sub> and FT<sub>4</sub>. Which drug should be initiated first?