A 29-year-old woman presents with a 6-week history of palpitations, heat intolerance, and a 4kg weight loss despite increased appetite. On examination, she has a fine tremor, a pulse of 110 bpm (regular), and a diffusely enlarged, non-tender thyroid gland with an audible bruit. Thyroid function tests (TFTs) show: TSH <0.01 mU/L (normal 0.4-4.0), Free T4 35 pmol/L (normal 10-22). What is the most likely diagnosis?