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Difficulty: MediumEndocrinology and Metabolism

A 34-year-old woman presents to the clinic with progressive weight gain, easy bruising, and new-onset facial hair growth over the past year. Examination reveals facial plethora, central adiposity, proximal muscle weakness, and wide purple striae on her abdomen. Her blood pressure is 155/95 mmHg. Arrange the following diagnostic steps in the correct sequential order for the evaluation of this patient's suspected hypercortisolism.

  1. 1Perform a 24-hour urine free cortisol, late-night salivary cortisol, or low-dose dexamethasone suppression test.
  2. 2Measure serum adrenocorticotropic hormone (ACTH) level.
  3. 3Perform a high-dose dexamethasone suppression test and/or pituitary magnetic resonance imaging (MRI).
  4. 4Perform inferior petrosal sinus sampling (IPSS).

Answer

The correct diagnostic sequence is: 1) Initial screening to confirm hypercortisolism, 2) Measurement of serum ACTH to determine dependence, 3) High-dose dexamethasone suppression test and/or MRI to localize the source, and 4) Inferior petrosal sinus sampling (IPSS) if prior tests are discordant.
The evaluation of Cushing syndrome must strictly follow a stepwise approach to prevent diagnostic errors caused by physiologic variations and incidental imaging findings. The sequence begins with proving endogenous cortisol overproduction (24-hour urine free cortisol, late-night salivary cortisol, or low-dose dexamethasone suppression). Only after hypercortisolism is established should the provider measure ACTH to distinguish adrenal (ACTH-independent) from pituitary/ectopic (ACTH-dependent) etiologies. Once ACTH-dependence is confirmed, high-dose dexamethasone suppression and pituitary MRI are utilized to differentiate a pituitary adenoma from ectopic ACTH secretion. IPSS is reserved as the final, invasive step when non-invasive localization results are ambiguous.

Step-by-Step Solution

1
Confirm the presence of hypercortisolism.
Establishes the biochemical diagnosis of Cushing syndrome.
Clinical features alone overlap with common conditions like obesity and polycystic ovary syndrome. Pathological excess of cortisol must be proven before any localization attempts.
2
Determine if the hypercortisolism is ACTH-dependent or ACTH-independent.
Categorizes the etiology into primary adrenal vs. central/ectopic sources.
A suppressed ACTH level (< 5 pg/mL) indicates an adrenal source (requiring adrenal CT). A normal or elevated ACTH level indicates a pituitary or ectopic source.
3
Attempt non-invasive localization of the ACTH source.
Differentiates a pituitary adenoma (Cushing disease) from an ectopic ACTH-secreting tumor.
Pituitary adenomas typically suppress cortisol production in response to high-dose dexamethasone, whereas ectopic tumors (e.g., small cell lung cancer) typically do not. MRI visually identifies structural pituitary lesions.
4
Perform invasive localization via IPSS.
Confirms pituitary lateralization of ACTH secretion.
IPSS is necessary when MRI is normal but biochemical testing suggests a pituitary source, as pituitary microadenomas can be too small to be detected on standard imaging.

Key Concept

The diagnostic algorithm for Cushing syndrome proceeds chronologically: confirm hypercortisolism, determine ACTH dependence, and then localize the anatomic source.
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Topics

Endocrinology and Metabolism

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