A 25-year-old woman presents with a 3-month history of palpitations, weight loss despite increased appetite, and anxiety. On examination, she has a fine tremor, warm peripheries, and a resting heart rate of 110 bpm. Thyroid function tests show TSH <0.01 mU/L (0.35-4.94), Free T4 35 pmol/L (9.0-19.0), and Free T3 12 pmol/L (2.6-5.7). What is the most appropriate initial treatment to manage her symptoms of hyperthyroidism while awaiting further investigations (e.g., autoantibodies, uptake scan)?