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Difficulty: HardDiagnostic Workup and Test Sequencing

A 42-year-old woman is evaluated for progressive weight gain, easy bruising, and proximal muscle weakness over the past 8 months. Blood pressure is 158/96 mmHg. Physical examination reveals central adiposity, facial plethora, and broad violaceous striae on her abdomen. Initial laboratory testing confirms endogenous hypercortisolism with a 24-hour urine free cortisol of 145 mcg/24h (normal: 4-50 mcg/24h) and a 1-mg overnight dexamethasone suppression test demonstrating a morning serum cortisol of 12 mcg/dL (normal: <1.8 mcg/dL). Which of the following is the most appropriate next step in the diagnostic workup?

  1. Measurement of plasma adrenocorticotropic hormoneAnswer
  2. B
    Magnetic resonance imaging of the pituitary gland
  3. C
    Computed tomography of the adrenal glands
  4. D
    High-dose dexamethasone suppression test
  5. E
    Bilateral inferior petrosal sinus sampling

Answer

Measurement of plasma adrenocorticotropic hormone
The correct sequence for diagnosing Cushing syndrome involves three distinct phases: (1) confirming hypercortisolism, (2) determining the biochemical source via ACTH dependency, and (3) anatomically localizing the lesion. This patient has passed phase one with two abnormal screening tests (24-hour urine free cortisol and 1-mg DST). The absolute best next step is to enter phase two by measuring a plasma adrenocorticotropic hormone (ACTH) level. This measurement dictates the rest of the algorithm: if ACTH is suppressed (< 5 pg/mL), it points to an ACTH-independent adrenal source; if ACTH is normal or elevated (> 20 pg/mL), it points to an ACTH-dependent pituitary or ectopic source.

Step-by-Step Solution

1
Analyze the initial diagnostic test results provided in the clinical vignette.
The patient has elevated 24-hour urine free cortisol and fails the 1-mg overnight dexamethasone suppression test.
Two abnormal first-line screening tests confirm the diagnosis of endogenous hypercortisolism (Cushing syndrome).
2
Determine the next phase in the diagnostic algorithm for Cushing syndrome.
The next required clinical objective is to identify the source of the excess cortisol by determining if it is ACTH-dependent or ACTH-independent.
Establishing the biochemical dependency dictates which anatomic region (pituitary/ectopic vs. adrenal) requires advanced imaging.
3
Select the appropriate test to differentiate the source.
Measurement of plasma adrenocorticotropic hormone (ACTH) is selected.
A suppressed ACTH level indicates an adrenal source (requiring adrenal CT), whereas a normal or elevated ACTH level indicates a pituitary or ectopic source (requiring pituitary MRI and further dynamic testing).

Key Concept

The diagnostic sequence for Cushing syndrome requires biochemical confirmation of hypercortisolism followed immediately by measurement of plasma ACTH to establish ACTH dependency, which must be done prior to ordering any anatomic imaging.
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Diagnostic Workup and Test Sequencing

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