What test will confirm the primary cause of this patient's condition, characterized by confusion, hypotension, tachycardia, hyperpigmentation, hyponatremia, hyperkalemia, and metabolic acidosis?

ACTH Stimulation Test is Most Appropriate for Confirming Primary Adrenal Insufficiency

The ACTH stimulation test (F) is most likely to confirm the primary cause of this patient's condition, which is adrenal crisis due to primary adrenal insufficiency (Addison's disease). 1

Clinical Presentation Analysis

This 26-year-old woman presents with a classic constellation of findings strongly suggestive of primary adrenal insufficiency in acute crisis:

• Marked confusion and lethargy
• Hypotension (80/40 mmHg) and tachycardia (140/min)
• Generalized hyperpigmentation, especially in palmar creases
• Hyponatremia (124 mEq/L)
• Hyperkalemia (6.4 mEq/L)
• Metabolic acidosis (HCO3- 16 mEq/L)
• Eosinophilia (20%)
• Recent history of autoimmune thyroiditis

Diagnostic Approach

According to current guidelines, the diagnosis of primary adrenal insufficiency (PAI) requires a two-step approach:

1. First, confirm adrenal cortex dysfunction
2. Then, establish the etiology 1

The ACTH stimulation test is the gold standard diagnostic test for confirming PAI. In this test, synthetic ACTH (tetracosactide/synacthen) is administered, and cortisol levels are measured. A peak serum cortisol <500 nmol/L is diagnostic of PAI. 1

Why ACTH Stimulation Test is Superior to Other Options

• Measurement of serum TSH level (E): While the patient has autoimmune thyroiditis, her TSH was recently normal (3 μU/mL), and thyroid dysfunction doesn't explain her current presentation with hyperpigmentation, hypotension, hyponatremia, and hyperkalemia.

• Measurement of antithyroglobulin antibody level (C): This would only confirm autoimmune thyroiditis, which is already established and doesn't explain the current crisis.

• Blood cultures (H): While infection could exacerbate adrenal insufficiency, the hyperpigmentation and electrolyte abnormalities point to a primary adrenal problem rather than sepsis as the primary cause.

• Hemodynamic measurements (A, B) and Echocardiography (I): These would assess cardiovascular status but wouldn't identify the underlying endocrine disorder.

• Measurement of serum LDH (D): This is nonspecific and not indicated for diagnosing adrenal insufficiency.

• Dexamethasone suppression test (G): This is used to diagnose hypercortisolism (Cushing syndrome), not hypocortisolism.

Clinical Pearls and Pitfalls

• In suspected acute adrenal crisis, treatment should never be delayed for diagnostic testing. Immediate administration of hydrocortisone, fluid resuscitation, and glucose if hypoglycemic is essential. 1

• The combination of hyponatremia and hyperkalemia is not always present in PAI. While hyponatremia is present in 90% of newly presenting cases, hyperkalemia is only seen in approximately half of patients. 1

• Patients with PAI may have mildly elevated TSH (4-10 IU/L) due to the lack of cortisol's inhibitory effect on TSH production. 1

• Autoimmune adrenalitis is the most common cause of PAI in developed countries, and often occurs with other autoimmune conditions (autoimmune polyglandular syndrome), such as the thyroiditis seen in this patient. 1

• After confirming PAI with the ACTH stimulation test, measuring 21-hydroxylase (anti-adrenal) autoantibodies should be the next step to establish the etiology. 1

The clinical picture of hypotension, hyponatremia, hyperkalemia, metabolic acidosis, and hyperpigmentation in a young woman with known autoimmune disease is highly characteristic of primary adrenal insufficiency, making the ACTH stimulation test the most appropriate diagnostic choice to confirm the diagnosis.