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Difficulty: MediumDiagnostic Workup and Test Sequencing

A 42-year-old female patient is evaluated in the outpatient clinic for progressive weight gain, proximal muscle weakness, and easy bruising developing over the past 14 months. Physical examination reveals central adiposity, facial plethora, and wide violaceous abdominal striae. Her blood pressure is 158/96 mmHg. A diagnosis of endogenous hypercortisolism (Cushing syndrome) is suspected. Arrange the following diagnostic steps in the most appropriate sequential order to establish the diagnosis and identify its underlying etiology.

  1. 1Measurement of 24-hour urine free cortisol or late-night salivary cortisol
  2. 2Measurement of plasma adrenocorticotropic hormone (ACTH) concentration
  3. 3Magnetic resonance imaging (MRI) of the pituitary gland
  4. 4Bilateral inferior petrosal sinus sampling (IPSS)

Answer

The correct diagnostic sequence begins with confirming hypercortisolism, followed by measuring ACTH to determine dependence, then obtaining a pituitary MRI, and finally performing inferior petrosal sinus sampling if the MRI is inconclusive.
The diagnostic algorithm for Cushing syndrome requires a tiered, stepwise approach to prevent diagnostic errors caused by incidental findings on imaging. The sequence mandates establishing the diagnosis of hypercortisolism first (via urine free cortisol or salivary cortisol). Only after hypercortisolism is confirmed should a plasma ACTH level be evaluated to differentiate ACTH-dependent from ACTH-independent causes. If the condition is ACTH-dependent, a pituitary MRI is indicated to localize an adenoma. If the MRI is negative or equivocal, inferior petrosal sinus sampling (IPSS) is the definitive test to distinguish Cushing disease from ectopic ACTH syndrome.

Step-by-Step Solution

1
Confirm the clinical suspicion of endogenous hypercortisolism.
Perform screening with a 24-hour urine free cortisol, late-night salivary cortisol, or a low-dose dexamethasone suppression test.
Clinical features of Cushing syndrome can overlap with common conditions like obesity and metabolic syndrome; biochemical confirmation of excess cortisol is mandatory before proceeding.
2
Determine the source of excess cortisol production (ACTH-dependent vs. ACTH-independent).
Measure plasma ACTH concentration.
A suppressed ACTH level indicates an adrenal source (requiring adrenal imaging), while a normal or elevated ACTH level indicates a pituitary or ectopic source.
3
Localize the ACTH-dependent source.
Obtain an MRI of the pituitary gland.
The most common cause of ACTH-dependent Cushing syndrome is a pituitary adenoma (Cushing disease). MRI is the preferred non-invasive modality to identify pituitary lesions.
4
Differentiate pituitary from ectopic ACTH production if imaging is unclear.
Perform bilateral inferior petrosal sinus sampling (IPSS).
Pituitary incidentalomas are common, and small ACTH-secreting adenomas may not be visible on MRI. IPSS confirms the pituitary as the source by comparing central to peripheral ACTH levels before surgery.

Key Concept

The diagnostic algorithm for Cushing syndrome requires a strict sequence: confirm hypercortisolism, establish ACTH dependence, and finally localize the source.
Estimated Time:1m 30s
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Topics

Diagnostic Workup and Test Sequencing

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