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Difficulty: MediumDiagnostic Workup and Test Sequencing

A 3838-year-old woman presents to the clinic with progressive weight gain, facial fullness, and proximal muscle weakness over the past 88 months. She also reports irregular menstrual periods and easy bruising. Blood pressure is 155/95155/95 mmHg, and BMI is 3232 kg/m 2^2. Physical examination reveals facial plethora, supraclavicular fat pads, and wide violaceous striae on her abdomen. There is trace peripheral edema. The physician suspects an endogenous endocrine etiology for her symptoms.

Arrange the following diagnostic steps in the most appropriate sequential order for the comprehensive workup of this patient's suspected condition, from initial biochemical screening to final definitive localization.

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

Answer

The correct sequence is: 1) Measurement of 24-hour urinary free cortisol or late-night salivary cortisol, 2) Measurement of plasma ACTH, 3) MRI of the pituitary gland, 4) Bilateral inferior petrosal sinus sampling.
The correct diagnostic sequence for Cushing syndrome begins with establishing the presence of hypercortisolism using biochemical screening (e.g., 24-hour urinary free cortisol, late-night salivary cortisol, or low-dose dexamethasone suppression test). Once hypercortisolism is confirmed, a plasma ACTH level is drawn to determine if the condition is ACTH-dependent or ACTH-independent. If ACTH is elevated or inappropriately normal, it indicates an ACTH-dependent source, prompting a pituitary MRI as the initial localizing study. If the MRI is unequivocally positive for a macroadenoma (>10>10 mm), the diagnosis of Cushing disease is confirmed. However, if the MRI is negative, equivocal, or shows a microadenoma (<6<6 mm), bilateral inferior petrosal sinus sampling (IPSS) is required to definitively differentiate a pituitary source from an ectopic ACTH-secreting tumor.

Step-by-Step Solution

1
Establish hypercortisolism
Confirms the clinical suspicion of Cushing syndrome
Biochemical confirmation is strictly required before any etiology-specific testing to avoid false-positive diagnoses.
2
Determine ACTH dependence
Differentiates ACTH-dependent versus ACTH-independent causes
This step guides whether to look at the adrenal glands (independent) or the pituitary/ectopic sources (dependent).
3
Perform localizing imaging
Evaluates for a pituitary adenoma
Pituitary MRI is the first-line imaging study for ACTH-dependent Cushing syndrome (Cushing disease).
4
Perform invasive localization
Definitively differentiates pituitary from ectopic ACTH production
IPSS is indicated when MRI is equivocal or shows a lesion <6<6 mm, as non-functioning pituitary incidentalomas are common in the general population.

Key Concept

The diagnostic workup for Cushing syndrome proceeds in a strict sequence: confirm hypercortisolism, determine ACTH dependence, and then perform anatomical/invasive localization.
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Diagnostic Workup and Test Sequencing

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