A 30-year-old woman with known Graves disease presents acutely confused, febrile (39.5 °C), tachycardic (140 beats/min) and agitated, with diarrhoea and marked tremor. Blood pressure is 150/70 mmHg. You suspect thyroid storm. In addition to urgent specialist referral and supportive care, which medication should be given promptly to control her adrenergic symptoms and reduce conversion of T4 to T3?