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Difficulty: MediumDiagnostic Workup and Test Sequencing

A 42-year-old woman presents to the clinic for evaluation of progressive weight gain and easy bruising over the past 8 months. She also reports new-onset facial hair growth and difficulty climbing stairs due to muscle weakness. Her blood pressure is 155/95 mmHg. Physical examination reveals central obesity, facial plethora, prominent supraclavicular fat pads, and wide, purplish striae on her abdomen. Laboratory evaluation shows a fasting blood glucose of 128 mg/dL. The physician suspects a syndrome of cortisol excess. Assuming each subsequent test is indicated based on the results of the preceding evaluation and is necessary to reach a definitive diagnosis, arrange the following diagnostic steps in the most appropriate sequential order from initial screening to definitive confirmation of the source.

  1. 124-hour urinary free cortisol measurement
  2. 2Plasma adrenocorticotropic hormone (ACTH) measurement
  3. 3Magnetic resonance imaging (MRI) of the pituitary gland
  4. 4Inferior petrosal sinus sampling (IPSS)

Answer

The correct sequence is: 24-hour urinary free cortisol measurement, followed by Plasma ACTH measurement, then MRI of the pituitary gland, and finally Inferior petrosal sinus sampling.
The diagnostic workup for suspected Cushing syndrome follows a strict sequential algorithm. The first step is to confirm the presence of hypercortisolism using a screening test such as 24-hour urinary free cortisol, late-night salivary cortisol, or a low-dose dexamethasone suppression test. Once hypercortisolism is confirmed, the second step is to measure plasma ACTH levels. This differentiates ACTH-independent causes (primary adrenal disease, characterized by suppressed ACTH) from ACTH-dependent causes (pituitary or ectopic tumors, characterized by normal or elevated ACTH). If the condition is ACTH-dependent, the third step is an MRI of the pituitary gland, as Cushing disease (pituitary adenoma) is the most common cause. If the MRI does not show a clear adenoma (>6 mm), the definitive fourth step is inferior petrosal sinus sampling (IPSS) to measure the ACTH concentration gradient between the petrosal sinus and peripheral blood, which reliably distinguishes a pituitary source from an ectopic source.

Step-by-Step Solution

1
Confirm the presence of hypercortisolism
24-hour urinary free cortisol measurement
Before searching for a cause, the clinical suspicion of Cushing syndrome must be biochemically confirmed using a first-line screening test such as 24-hour urinary free cortisol, late-night salivary cortisol, or an overnight low-dose dexamethasone suppression test.
2
Determine if the hypercortisolism is ACTH-dependent or ACTH-independent
Plasma adrenocorticotropic hormone (ACTH) measurement
Once hypercortisolism is confirmed, checking plasma ACTH levels determines the next branch in the algorithm. Suppressed ACTH points to an adrenal source, while normal or elevated ACTH points to a pituitary or ectopic source.
3
Localize the source of ACTH production
Magnetic resonance imaging (MRI) of the pituitary gland
For ACTH-dependent Cushing syndrome, a pituitary adenoma (Cushing disease) is the most common cause. Therefore, a pituitary MRI is the initial imaging study of choice.
4
Definitively differentiate pituitary from ectopic ACTH production if imaging is inconclusive
Inferior petrosal sinus sampling (IPSS)
If the pituitary MRI is negative or shows a microadenoma <6 mm (which could be an incidentaloma), IPSS is required to confirm whether the excess ACTH is coming from the pituitary gland or an ectopic tumor.

Key Concept

The diagnostic algorithm for Cushing syndrome involves three distinct phases: (1) confirming hypercortisolism, (2) determining ACTH dependence, and (3) localizing the source of ACTH using imaging and venous sampling.
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Diagnostic Workup and Test Sequencing

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