A 40-year-old woman presents with a 2-week history of palpitations and anxiety. She has also noticed weight loss despite a good appetite, heat intolerance, and tremor. Examination reveals a fine tremor, warm peripheries, lid lag, and a resting heart rate of 110 bpm. Thyroid function tests show: TSH <0.01 mU/L, Free T4 45 pmol/L, Free T3 15 pmol/L. What is the most appropriate initial pharmacological treatment to control her symptoms?