A middle-aged woman with hyperthyroidism received irregular drug therapy for 2 years. One week after switching to radioactive <sup>131</sup>I therapy, she developed high fever and palpitations. Physical examination: T 40°C, heart rate 160/min, atrial fibrillation, tachypnea, profuse sweating, and restlessness. Lab tests: elevated WBC, neutrophils, FT<sub>3</sub>, and FT<sub>4</sub>; decreased TSH. The most likely diagnosis is: