A 53-year-old male presents with low-grade fever for 1 week, accompanied by anxiety, irritability, palpitations, and hyperhidrosis. Examination reveals: T 37.6°C, P 100/min, palpable thyroid gland with a hard, tender nodule on the right side, no thrill or bruit, fine tremor of tongue and hands (+), ESR 78 mm at the end of the first hour. If the diagnosis remains uncertain after these findings, which confirmatory test should be chosen?