A 38-year-old woman presents with goiter accompanied by palpitations and hyperhidrosis for over 3 months. Physical examination: grade II goiter, firm and uneven texture, no tenderness, no vascular bruit, heart rate 100 bpm with regular rhythm, slightly moist skin, fine tremor in both hands (±). To confirm the diagnosis, in addition to measuring FT<sub>3</sub>, FT<sub>4</sub>, and TSH levels, which of the following should be selected for further differential diagnosis?