A 34-year-old woman from Mumbai presents with a 3-month history of palpitations, tremor, and heat intolerance. She reports recent weight loss despite good appetite. On examination, she has a diffuse goitre, exophthalmos, and pretibial myxoedema. TSH is <0.01 mIU/L, free T4 is 28 pg/mL (normal 7.5–21), and free T3 is 8.5 pg/mL (normal 2.3–4.2). TPO antibodies are positive. What is the most appropriate initial pharmacological management?