Question

In a suspected case of adrenal insufficiency with borderline serum and salivary cortisol, what is the next step?

• A. ACTH stimulation test
• B. Low-dose dexamethasone suppression test
• C. High-dose dexamethasone suppression test
• D. Adrenal vein sampling

Hint:

Endocrine Testing Axiom:
- If you suspect a hormone deficiency (e.g., Addison's), you must perform a Stimulation test.
- If you suspect a hormone excess (e.g., Cushing's), you must perform a Suppression test.
A peak cortisol $>$ 18 mcg/dL after ACTH rules out primary adrenal insufficiency.

Answer 1

The Correct Option is A

Concept:
The diagnostic algorithm for suspected adrenal insufficiency relies on baseline hormone levels followed by dynamic functional testing to assess the reserve capacity of the adrenal cortex.
Explanation:
• The initial screening for adrenal insufficiency involves measuring early morning (8 AM) serum cortisol levels, as this is when physiological cortisol secretion peaks.

• A severely low morning cortisol (e.g., $<$ 3 mcg/dL) strongly suggests adrenal insufficiency, while a robustly high level (e.g., $>$ 15-18 mcg/dL) effectively rules it out.

• When the baseline serum and salivary cortisol levels fall into an equivocal or borderline range (typically between 3 and 15 mcg/dL), a definitive diagnosis cannot be made on static numbers alone.

• In such indeterminate cases, dynamic testing is required. The gold standard test to evaluate adrenal reserve is the Short Synacthen Test, also known as the ACTH stimulation test (Option A).

• During this test, a synthetic form of ACTH (Cosyntropin, 250 mcg) is administered. Serum cortisol is then measured at 30 and 60 minutes. A healthy adrenal cortex will respond by synthesizing and releasing cortisol, achieving a peak level of $>$ 18 mcg/dL. A failure to reach this peak confirms the diagnosis of adrenal insufficiency.

• Dexamethasone suppression tests (Options B and C) are utilized for the exact opposite clinical scenario: they are used to diagnose and differentiate the causes of hypercortisolism (Cushing's syndrome).

• Adrenal vein sampling (Option D) is an invasive radiological procedure used to lateralize aldosterone secretion in patients with primary hyperaldosteronism (Conn's syndrome).
Final Answer:
When baseline cortisol levels are borderline, an ACTH stimulation test is the mandatory next step to dynamically assess adrenal reserve and confirm insufficiency.