Question

Evaluation for chronic palpitations and unintentional weight loss brings a 25-year-old female fitness instructor to the outpatient clinic. She reports feeling excessively warm even in air-conditioned environments and describes a $5$-kg ($11$-lb) weight loss over the past 2 months despite a notably increased appetite. Vital signs reveal a pulse of $112$/min and a blood pressure of $138/78$ mmHg. Physical examination shows a fine resting tremor of the outstretched hands, hyperreflexia, and warm, moist skin. The thyroid gland is non-palpable, and there is no evidence of exophthalmos or pretibial myxedema. A urine pregnancy test is negative.

Initial laboratory investigations demonstrate the following:
Thyroid-stimulating hormone (TSH): $0.02 \mu$ U/mL (Reference: $0.4–4.0 \mu$ U/mL)
Free thyroxine ($T_4$): $0.5$ ng/dL (Reference: $0.8–1.8$ ng/dL)

Which of the following is the most appropriate next step in the evaluation of this patient?