A 28-year-old woman with a known history of Graves' disease, currently on antithyroid medication, presents with acute onset of fever (39.2°C), severe tachycardia (HR 130/min), tremor, agitation, and altered mental status. TSH is <0.01 mIU/L, free T₄ is 45 pmol/L (normal 10–20), and free T₃ is 12 pmol/L (normal 3.5–7.8). What is the most appropriate immediate next step in management?