A 19-year-old woman presents with increased anxiety, irritability, weight loss despite increased appetite, and palpitations over the past 2 months. Examination reveals a pulse rate of 110 bpm, fine tremor, warm peripheries, and mild bilateral lid lag. TSH is <0.01 mU/L (ref 0.4-4.0), free T4 is 35 pmol/L (ref 11-22). What is the most likely underlying cause?