Chооse the cоrrect аnswer: A middle-аged аdult presents to an urgent care clinic with complaints of tremors, nervousness and periodic "palpitations". The patient has experienced a 15-pound unexplained weight loss over the past 3 months. Past medical history is positive for osteoarthritis. The patient has no medication allergies. Vitals: B/P 110/62; Pulse 121 and regular; respirations 20; temp; 99.3. The patient's thyroid is tender and deep tendon reflexes are brisk. The following thyroid labs are obtained: TSH 0.02 uIU/L Total T4 14.9 uIU/L (range 5-13) T3 44 uIU/L (range 23-39) Free T4 6.5 uIU/L Anti-TPO antibodies negative Which of the following diagnostic tests should be incorporated into the clinical work-up at this point?