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Difficulty: MediumDiagnostic Workup and Test Sequencing

A 34-year-old woman is evaluated in the outpatient clinic for a 1-year history of progressive weight gain, easy bruising, and new-onset facial hair growth. She reports increased difficulty climbing stairs due to muscle weakness. She does not take any exogenous corticosteroids, over-the-counter supplements, or herbal medications. Vital signs show a blood pressure of 158/96 mmHg and a body mass index of 34 kg/m². Physical examination reveals central obesity, facial plethora, supraclavicular fat pads, and wide violaceous striae over the lower abdomen. There is high clinical suspicion for endogenous Cushing syndrome.

Assuming the diagnostic workup ultimately confirms Cushing disease (an adrenocorticotropic hormone-secreting pituitary adenoma), arrange the following diagnostic tests in the correct sequential order from initial screening to final anatomic localization.

  1. 124-hour urine free cortisol or 1-mg overnight dexamethasone suppression test
  2. 2Plasma adrenocorticotropic hormone (ACTH) measurement
  3. 3High-dose (8-mg) dexamethasone suppression test
  4. 4Magnetic resonance imaging (MRI) of the sella turcica

Answer

The correct chronological sequence is: 1) 24-hour urine free cortisol or 1-mg overnight dexamethasone suppression test, 2) Plasma ACTH measurement, 3) High-dose dexamethasone suppression test, 4) MRI of the sella turcica.
The diagnostic workup of Cushing syndrome must strictly follow a biochemical-to-anatomical pipeline to prevent misdiagnosis. It begins with screening tests to confirm hypercortisolism (24-hour UFC, 1-mg DST, or late-night salivary cortisol). Once confirmed, ACTH levels are drawn to distinguish ACTH-dependent from ACTH-independent causes. If ACTH is normal or elevated, a high-dose DST or CRH stimulation test is used to localize the source to either the pituitary or an ectopic site. Only after biochemical tests indicate a pituitary source should an MRI of the sella turcica be ordered.

Step-by-Step Solution

1
Confirm endogenous hypercortisolism.
Abnormal screening test (e.g., elevated urine free cortisol).
Before attempting to identify a specific anatomical source, the presence of pathological cortisol excess must be definitively established to prevent false-positive diagnostic cascades.
2
Determine ACTH dependence.
Normal or elevated plasma ACTH level.
This step cleanly separates primary adrenal causes (ACTH-independent, where ACTH is suppressed) from pituitary or ectopic causes (ACTH-dependent).
3
Differentiate the specific source of ACTH production.
Cortisol suppression on a high-dose dexamethasone suppression test.
This distinguishes a pituitary adenoma (Cushing disease), which retains some negative feedback response, from ectopic ACTH secretion (e.g., small cell lung cancer), which typically does not respond to feedback.
4
Localize the anatomical lesion.
Identification of a pituitary microadenoma on imaging.
Imaging is strictly targeted based on biochemical confirmation. Premature imaging is dangerous due to the high prevalence of non-functioning incidentalomas in the general population.

Key Concept

Diagnostic sequencing for suspected Cushing syndrome
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Topics

Diagnostic Workup and Test Sequencing

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