A 52-yeаr-оld wоmаn presents tо the emergency depаrtment with a 3-week history of progressive fatigue, nausea, vomiting, and a 12-pound weight loss. She reports feeling increasingly weak and dizzy when standing. Her past medical history includes hypothyroidism treated with levothyroxine. On examination, she appears ill and dehydrated. Vital signs: blood pressure 88/54 mm Hg (drops to 72/48 mm Hg when standing), heart rate 108 bpm, temperature 37.8°C. Notable hyperpigmentation is observed on her palmar creases, knuckles, and buccal mucosa. Laboratory results: Sodium: 128 mEq/L Potassium: 5.8 mEq/L Glucose: 62 mg/dL Creatinine: 1.6 mg/dL (baseline 0.8 mg/dL) Morning cortisol (8 AM): 3.2 µg/dL ACTH: 485 pg/mL (reference range 15-65 pg/mL) TSH: 8.2 mIU/L (elevated) Based on the clinical presentation and laboratory findings, which diagnosis is most likely? AND What is the most appropriate immediate management for this patient?