A 29-year-old woman presents with a 3-month history of anxiety, palpitations, weight loss despite increased appetite, and heat intolerance. On examination, she has a fine tremor, warm peripheries, and a diffusely enlarged, non-tender thyroid gland. Thyroid function tests show: TSH <0.01 mU/L (suppressed), Free T4 35 pmol/L (elevated), Free T3 12 pmol/L (elevated). What is the most likely diagnosis?