A 45-year-old woman presents to her GP complaining of palpitations, weight loss despite eating more, feeling hot and sweaty, and poor sleep over the past few months. On examination, her pulse is regular at 110 bpm, she has a fine tremor of her outstretched hands, warm peripheries, and mild bilateral lid lag. Thyroid function tests show TSH <0.01 mU/L (suppressed) and Free T4 35 pmol/L (elevated). What is the single most common underlying cause for this presentation in the UK?