A 45-year-old woman presents with palpitations and anxiety. Heart rate is 110 beats/min, blood pressure 126/78 mm Hg, and she has a fine tremor. Laboratory tests show TSH <0.01 mIU/L and free T4 38 pmol/L (elevated). According to Canadian Thyroid Association guidance, what is the most appropriate initial pharmacologic therapy while awaiting definitive treatment?