A 65-year-old man on ICU has atrial fibrillation and is receiving IV amiodarone. He also takes levothyroxine for hypothyroidism. After 3 weeks, his TSH falls to <0.01 mU/L and free T4 rises to 65 pmol/L (normal 9-25). He develops tremor, tachycardia, and a new fever. What is the most likely amiodarone-related thyroid complication?