A 65-year-old man is found collapsed at home. In the emergency department he is drowsy, confused and hypotensive. Family report he has been on long-term Prednisolone for polymyalgia rheumatica which was stopped abruptly 1 week ago. Blood pressure is 80/50 mmHg, sodium 128 mmol per litre, potassium 5.8 mmol per litre and glucose 3.0 mmol per litre. What is the most appropriate immediate treatment?