Question

An occupational health clinic evaluates a $31$-year-old hospital pharmacist for a two-month history of irritability, frequent palpitations, and heat intolerance. She reports poor sleep and an unintentional $4\text{-kg}$ ($8.8\text{-lb}$) weight loss during this period. The patient has no significant past medical history and takes no prescribed medications, although she admits to taking various over-the-counter "energy supplements" to help with long shifts. Her vital signs show a pulse of $112\text{/min}$ and a blood pressure of $138/82\text{ mm Hg}$. Physical examination reveals a fine tremor of the outstretched hands and hyperactive deep tendon reflexes. The eyes show no proptosis, lid lag, or periorbital edema. The thyroid gland is not palpable, and there is no cervical lymphadenopathy or anterior neck tenderness.

Laboratory evaluation shows:
Thyroid-stimulating hormone (TSH): $<0.01\text{ }\mu\text{U/mL}$ (normal: $0.4$-$4.0\text{ }\mu\text{U/mL}$)
Free T4: $2.9\text{ ng/dL}$ (normal: $0.8$-$1.8\text{ ng/dL}$)
Free T3: $6.2\text{ pg/mL}$ (normal: $2.3$-$4.2\text{ pg/mL}$)

A subsequent $24$-hour radioactive iodine uptake scan reveals $<1\%$ uptake (normal: $10\%$-$30\%$).

Which of the following is the most appropriate next step in the diagnostic workup?