A 38-year-old woman is evaluated for significant weight gain, worsening fatigue, and new-onset facial hair growth over the past year. She has not seen a physician in five years and takes no over-the-counter or prescription medications. Vital signs show a blood pressure of 158/96 mmHg. Physical examination demonstrates central adiposity, facial plethora, prominent supraclavicular fat pads, and wide violaceous abdominal striae. Proximal muscle weakness is noted when she attempts to rise from a chair. The physician suspects an endocrine disorder and plans a diagnostic workup. Arrange the following steps in the most appropriate chronological sequence for the diagnostic evaluation of this patient's suspected condition.
- 1Confirm the presence of pathological hypercortisolism using a 24-hour urine free cortisol or late-night salivary cortisol test.
- 2Measure a baseline plasma adrenocorticotropic hormone (ACTH) level to classify the condition as ACTH-dependent or ACTH-independent.
- 3Obtain a gadolinium-enhanced magnetic resonance imaging (MRI) scan of the pituitary gland to visualize a potential adenoma.
- 4Perform bilateral inferior petrosal sinus sampling (IPSS) with corticotropin-releasing hormone (CRH) stimulation.
Answer
The correct sequence for evaluating suspected Cushing syndrome is to first confirm hypercortisolism biochemically, then measure ACTH to determine dependency, followed by a pituitary MRI if ACTH is normal or elevated, and finally perform inferior petrosal sinus sampling if the MRI is inconclusive.
The diagnostic workup for Cushing syndrome is strictly algorithmic to avoid false positives and unnecessary invasive procedures. The sequence begins with confirming endogenous hypercortisolism using a late-night salivary cortisol, 24-hour urine free cortisol, or low-dose dexamethasone suppression test. Once confirmed, a plasma ACTH level must be measured to distinguish ACTH-independent (adrenal source) from ACTH-dependent (pituitary or ectopic source) hypercortisolism. If ACTH is normal or elevated, a pituitary MRI is indicated to look for an adenoma. Because small pituitary incidentalomas are common, if the MRI is negative or reveals a lesion smaller than 6 mm, inferior petrosal sinus sampling (IPSS) is the gold standard required to definitively differentiate a pituitary source (Cushing disease) from an ectopic ACTH-secreting tumor.
Step-by-Step Solution
Key Concept
The diagnostic algorithm for Cushing syndrome requires biochemical confirmation of hypercortisolism prior to localization, followed by ACTH measurement to guide appropriate imaging and invasive testing.