A 30-year-old woman with known primary adrenal insufficiency (Addison disease) presents with vomiting, abdominal pain and profound weakness after a gastroenteritis illness. She is hypotensive at 80/50 mmHg, tachycardic and appears shocked. Sodium is 126 mmol/L and potassium 5.6 mmol/L. What is the most appropriate immediate treatment?