A 28-year-old woman presents to the emergency department with severe agitation, high fever (39.5°C), tachycardia (140/min), and altered mental status. She has a known history of Graves' disease diagnosed 2 months ago but defaulted on treatment. On examination, she has a tender, enlarged thyroid and proximal muscle weakness. Free T4 is 35 pg/mL (normal 7.5–21), free T3 is 12 pg/mL (normal 2.3–4.2), and TSH is <0.01 mIU/L. What is the most appropriate immediate management?