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Difficulty: MediumEndocrinology and Metabolism

Match each patient vignette describing clinical features of Cushing syndrome to their most likely corresponding diagnostic laboratory profile.

  • A 32-year-old woman with systemic lupus erythematosus on chronic oral prednisone therapy presenting with facial plethora and central obesity.Undetectable plasma ACTH; low 24-hour urine free cortisol
  • A 45-year-old woman with new-onset diabetes and hypertension found to have a 4-cm unilateral right adrenal mass.Undetectable plasma ACTH; elevated 24-hour urine free cortisol
  • A 35-year-old woman with amenorrhea, violaceous abdominal striae, hyperpigmentation, and a 6-mm pituitary microadenoma.Normal to elevated plasma ACTH; serum cortisol suppresses by >50% following high-dose dexamethasone
  • A 65-year-old man with a 50-pack-year smoking history, profound hypokalemia, hyperpigmentation, and a hilar lung mass.Significantly elevated plasma ACTH; serum cortisol does not suppress following high-dose dexamethasone

Answer

Prednisone therapy maps to low ACTH/low urine free cortisol. Adrenal mass maps to low ACTH/elevated urine free cortisol. Pituitary adenoma maps to normal/elevated ACTH with cortisol suppression on high-dose dexamethasone. Ectopic ACTH maps to elevated ACTH with no suppression.
The diagnostic localization of Cushing syndrome requires a stepwise biochemical algorithm. ACTH-independent causes (adrenal tumors and exogenous steroids) present with suppressed ACTH. They are distinguished by measuring 24-hour urine free cortisol (elevated in adrenal adenomas, low with synthetic exogenous steroids like prednisone). ACTH-dependent causes (pituitary adenomas and ectopic tumors) present with normal or elevated ACTH. The high-dose dexamethasone suppression test distinguishes them: pituitary tumors generally suppress cortisol by >50%, while ectopic tumors (e.g., small cell lung cancer) are completely autonomous and do not suppress.

Step-by-Step Solution

1
Determine if the hypercortisolism is ACTH-dependent or ACTH-independent based on plasma ACTH levels.
Exogenous steroids and adrenal tumors have suppressed ACTH (ACTH-independent). Pituitary adenomas and ectopic tumors have normal or elevated ACTH (ACTH-dependent).
Excess cortisol suppresses pituitary ACTH release through negative feedback. In autonomous adrenal production or exogenous administration, ACTH falls. In ACTH-secreting tumors, ACTH is the driver of the excess cortisol.
2
Differentiate between the two ACTH-independent causes (low ACTH).
Exogenous synthetic steroids (prednisone) yield a low urine free cortisol. Adrenal tumors produce true endogenous cortisol, yielding a high urine free cortisol.
Standard laboratory assays for urine free cortisol measure true endogenous cortisol, not synthetic derivatives like prednisone or dexamethasone.
3
Differentiate between the two ACTH-dependent causes (normal/high ACTH).
Pituitary adenomas (Cushing disease) suppress cortisol production following high-dose dexamethasone. Ectopic ACTH-secreting tumors (lung mass) do not suppress.
Pituitary adenomas retain some physiological negative feedback regulation at high glucocorticoid doses, whereas ectopic neoplastic sources are completely autonomous.

Key Concept

Algorithm for the biochemical diagnosis and localization of Cushing syndrome (ACTH-dependent vs ACTH-independent)
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Topics

Endocrinology and Metabolism

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