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Difficulty: MediumDiagnostic Workup and Test Sequencing

A 38-year-old female presents to the clinic for evaluation of progressive weight gain, irregular menstrual periods, and worsening facial hair growth over the past year. She also reports new-onset proximal muscle weakness, noting difficulty rising from a seated position. Physical examination reveals a blood pressure of 155/95 mmHg155/95 \text{ mmHg}, a body mass index of 32 kg/m232 \text{ kg/m}^2, central adiposity, facial plethora, and wide purple abdominal striae.

Initial laboratory screening for hypercortisolism is performed. A 24-hour urinary free cortisol excretion is significantly elevated. An overnight 1 mg1 \text{ mg} dexamethasone suppression test reveals a morning serum cortisol level of 14 \mug/dL14 \text{ \mu g/dL} (reference range: <1.8 \mug/dL< 1.8 \text{ \mu g/dL}).

Which of the following is the most appropriate next step in this patient's diagnostic workup?

  1. A
    High-dose (8 mg8 \text{ mg}) dexamethasone suppression test
  2. B
    Magnetic resonance imaging (MRI) of the sella turcica
  3. Measurement of plasma adrenocorticotropic hormone (ACTH) concentrationAnswer
  4. D
    Computed tomography (CT) of the abdomen and pelvis
  5. E
    Bilateral inferior petrosal sinus sampling

Answer

Measurement of plasma adrenocorticotropic hormone (ACTH) concentration
This patient presents with classic signs of Cushing syndrome, and the diagnosis of endogenous hypercortisolism has been definitively established through two abnormal first-line screening tests (elevated 24-hour urinary free cortisol and a failed 1 mg1 \text{ mg} overnight dexamethasone suppression test). The next mandatory step in the diagnostic sequence is to determine whether the disease is ACTH-dependent or ACTH-independent. Measurement of plasma ACTH concentration achieves this. A suppressed ACTH level indicates an ACTH-independent adrenal source, guiding the clinician to order adrenal imaging. A normal or elevated ACTH level indicates an ACTH-dependent source, prompting pituitary imaging and further biochemical differentiation (such as a high-dose dexamethasone suppression test).

Step-by-Step Solution

1
Interpret the initial diagnostic test results in the clinical context.
The patient has clinical features of hypercortisolism and two positive screening tests (elevated 24-hour urine cortisol and lack of suppression on the low-dose dexamethasone test).
Before investigating the etiology, the diagnosis of Cushing syndrome (endogenous hypercortisolism) must be definitively established.
2
Determine the next sequence in the established diagnostic algorithm.
Measure plasma ACTH concentration.
ACTH levels differentiate the hypercortisolism into ACTH-dependent categories (pituitary adenomas, ectopic ACTH secretion) or ACTH-independent categories (adrenal adenomas, adrenal carcinomas).
3
Evaluate and reject premature imaging options.
Avoid ordering an MRI of the pituitary or CT of the adrenal glands at this stage.
Because nonfunctioning incidentalomas are highly prevalent in both the pituitary and adrenal glands, ordering imaging before biochemical localization risks targeting the wrong organ for surgical intervention.

Key Concept

Diagnostic sequencing for Cushing syndrome requires confirming endogenous hypercortisolism first, followed immediately by measuring plasma ACTH to localize the source, strictly prior to any imaging.
Estimated Time:1m 15s
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Topics

Diagnostic Workup and Test Sequencing

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