A 28-year-old woman from Mumbai, 8 weeks pregnant, presents with palpitations, anxiety, and weight loss. She has a history of Graves' disease diagnosed 2 years ago and was treated with antithyroid drugs (PTU) for 18 months, after which treatment was stopped. On examination, she has a diffuse goiter and resting heart rate of 115 bpm. TSH is <0.01 mIU/L, free T4 is 25 pg/dL, and free T3 is 7.8 pg/dL. She is concerned about the safety of treatment during pregnancy. Which is the most appropriate initial management?