A 29-year-old woman presents with a 6-month history of episodic palpitations, tremor, anxiety, weight loss despite increased appetite, and heat intolerance. Her TSH is <0.01 mIU/L (normal 0.4-4.0), free T4 is 35 pmol/L (normal 10-20), and she has a diffuse goitre with a bruit. What is the most appropriate initial pharmacologic therapy?