A 45-yeаr-оld wоmаn presents with а several-mоnth history of progressive fatigue, muscle weakness, unintentional weight loss of 15 pounds, and episodes of dizziness upon standing. On examination, you note diffuse hyperpigmentation of her skin, particularly in the palmar creases and buccal mucosa, and a blood pressure of 90/60 mmHg. Primary adrenal insufficiency is suspected. Which of the following laboratory panels would be the most appropriate initial investigation to order?