Addisonian Crisis in a Patient with Vomiting and Fever

A 40-year-old man presents to the emergency department with a 24-hour history of vomiting and fever. He has known primary adrenal insufficiency (Addison's disease), diagnosed five years ago, and takes oral hydrocortisone 10 mg each morning, 5 mg at midday and 5 mg in the evening, with fludrocortisone 0.1 mg once daily. He has been unable to keep down his regular doses since yesterday. On examination he is drowsy but orientated, temperature 38.9°C, heart rate 118 beats per minute, blood pressure 82/56 mmHg lying, respiratory rate 22 breaths per minute, oxygen saturation 96% on air. He has dry mucous membranes and reduced skin turgor. Chest is clear, abdomen soft with mild epigastric tenderness, no peritonism. Initial investigations: sodium 126 mmol/L, potassium 5.5 mmol/L, urea 8.4 mmol/L, creatinine 1.0 mg/dL (eGFR 90 mL/min/1.73m²), glucose 5.2 mmol/L, blood cultures and urine culture sent. ECG shows sinus tachycardia. The patient has no known allergies and is not on any other regular medication. He does not have a steroid card with him. You are asked to prescribe the immediate management, including any intravenous fluids and corticosteroid therapy (if indicated). Consider the sick-day rules he will need later.