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 suddenly developed high fever, palpitations. Physical examination: T 40°C, heart rate 160 bpm, atrial fibrillation, tachypnea, profuse sweating, restlessness. Lab tests: elevated WBC, neutrophils, FT<sub>3</sub>, FT<sub>4</sub>; decreased TSH. The drug of choice is: