Female, 17 years old. Palpitations and hyperhidrosis for 6 months. Polydipsia and polyuria for 1 month. Physical exam: significant weight loss, mild exophthalmos, thyroid gland enlarged to degree II, vascular bruit audible at both upper poles. Lab: FT3 33.5 pmol/L, FT4 40 pmol/L, TSH 0.01 mU/L (0.01 μIU/mL). Which of the following tests is LEAST appropriate for comprehensive diagnosis?