A 32-year-old woman presents with a 6-week history of palpitations and anxiety. She has also noticed weight loss despite an increased appetite and complains of feeling hot all the time. On examination, she has a fine tremor of her outstretched hands, a resting heart rate of 110 bpm, and warm, moist skin. Her thyroid gland is diffusely enlarged and non-tender. A bruit is audible over the thyroid. What is the most likely diagnosis?