MRCP2-1661

A 36-year-old woman complains of fatigue, palpitations, and weight loss. As part of her diagnostic workup, thyroid function tests were conducted:

TSH <0.01mU/L 0.4-4.0mU/L
T4 15.3pmol/L 9.0-25.0pmol/L

What is the next most suitable test to perform for this patient?