A 32-year-old woman presents to the clinic with a 2-week history of palpitations, anxiety, and weight loss despite an increased appetite. She also reports feeling hot and experiencing increased sweating. On examination, she has a fine tremor, warm moist skin, and a diffusely enlarged thyroid gland. Her heart rate is 110 bpm. Blood tests reveal suppressed TSH and elevated free T4. What is the most appropriate initial treatment for her condition?