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Difficulty: MediumEndocrinology and Metabolism

A 42-year-old woman comes to the clinic reporting significant weight gain, worsening fatigue, and the development of dark hair on her face and chest over the past 8 months. Her medical history is unremarkable, and she takes no medications. On physical examination, her blood pressure is 160/100 mm Hg. She has a rounded face with plethora, truncal obesity with prominent supraclavicular fat pads, and several wide, purplish striae on her abdomen. Proximal muscle weakness is noted when she attempts to stand from a seated position. The physician suspects endogenous hypercortisolism. Assuming each step's result dictates the need for the subsequent one in a classic ACTH-dependent pathway, arrange the following diagnostic steps in the most appropriate chronological order.

  1. 1Perform a 24-hour urinary free cortisol excretion test
  2. 2Measure early morning plasma ACTH level
  3. 3Obtain an MRI of the pituitary gland
  4. 4Perform inferior petrosal sinus sampling

Answer

The correct sequence for evaluating Cushing syndrome is to first confirm hypercortisolism, then determine ACTH dependence, followed by imaging of the suspected source, and finally invasive venous sampling if imaging is inconclusive.
The evaluation of Cushing syndrome must follow a strict stepwise algorithm to avoid incorrect diagnoses. The first requirement is to establish the presence of hypercortisolism using tests like 24-hour urinary free cortisol, late-night salivary cortisol, or a low-dose dexamethasone suppression test. Once confirmed, plasma ACTH is measured to distinguish between ACTH-independent causes (e.g., adrenal adenoma, where ACTH is suppressed) and ACTH-dependent causes (e.g., pituitary adenoma or ectopic ACTH syndrome, where ACTH is normal or elevated). If the condition is ACTH-dependent, a pituitary MRI is performed since Cushing disease is the most common cause. If the MRI does not reveal a definitive adenoma (typically defined as >=6 mm), inferior petrosal sinus sampling (IPSS) is required. IPSS measures ACTH in the venous drainage of the pituitary and compares it to peripheral blood; a central-to-peripheral ACTH gradient confirms a pituitary source, whereas the absence of a gradient suggests an ectopic source.

Step-by-Step Solution

1
Confirm hypercortisolism with a 24-hour urinary free cortisol test.
Establishes the diagnosis of Cushing syndrome.
It is necessary to definitively prove the patient has pathological excess cortisol before attempting to determine the underlying source.
2
Measure the early morning plasma ACTH level.
Categorizes the Cushing syndrome as ACTH-dependent or ACTH-independent.
This fundamentally branches the diagnostic algorithm. Low ACTH points to an adrenal source, while normal/high ACTH points to a pituitary or ectopic source.
3
Perform a pituitary MRI.
Visualizes the pituitary gland to identify a potential adenoma.
Since pituitary adenomas (Cushing disease) are the most common cause of ACTH-dependent Cushing syndrome, imaging this gland is the next logical non-invasive step.
4
Perform inferior petrosal sinus sampling (IPSS).
Differentiates between pituitary and ectopic ACTH secretion by measuring ACTH gradients.
Required when ACTH is elevated but the pituitary MRI does not show a definitive adenoma (typically >6 mm), preventing unnecessary pituitary surgery in patients who might actually have an ectopic neuroendocrine tumor.

Key Concept

The diagnostic algorithm for Cushing syndrome involves a stepwise approach: first establishing hypercortisolism, next determining ACTH dependence, and finally localizing the source of hormone overproduction.
Estimated Time:1m 30s
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Topics

Endocrinology and Metabolism

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