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Zorluk: OrtaEndocrinology and Metabolism

Review the following diagnostic laboratory profiles of patients presenting with cushingoid features (such as central obesity, proximal muscle weakness, and abdominal striae). Match each laboratory and testing profile with its most appropriate underlying pathophysiologic etiology.

  • Elevated 24-hour urine free cortisol; suppressed plasma ACTH levelUnilateral adrenal adenoma
  • Elevated 24-hour urine free cortisol; elevated plasma ACTH level; serum cortisol suppresses by >50% following a high-dose dexamethasone testPituitary corticotroph adenoma (Cushing disease)
  • Elevated 24-hour urine free cortisol; elevated plasma ACTH level; no significant suppression of serum cortisol following a high-dose dexamethasone testEctopic ACTH-secreting paraneoplastic syndrome
  • Low 24-hour urine free cortisol; suppressed plasma ACTH level; low morning serum cortisol levelExogenous systemic glucocorticoid use

Cevap

Profile 1 matches Adrenal adenoma. Profile 2 matches Pituitary adenoma. Profile 3 matches Ectopic ACTH secretion. Profile 4 matches Exogenous glucocorticoid use.
The standard diagnostic approach to Cushing syndrome requires step-wise biochemical evaluation. Once hypercortisolism is confirmed (via 24-hour urine free cortisol, late-night salivary cortisol, or low-dose dexamethasone suppression), plasma ACTH is measured. Suppressed ACTH (<5 pg/mL) indicates ACTH-independent disease (e.g., adrenal adenoma). If a patient has suppressed ACTH, low urine cortisol, and cushingoid features, it is diagnostic of exogenous corticosteroid use. Conversely, an elevated ACTH level indicates an ACTH-dependent state. High-dose dexamethasone (8 mg) is then utilized: pituitary adenomas will suppress their ACTH secretion (decreasing cortisol by >50%), whereas ectopic ACTH-producing tumors will not suppress.

Adım Adım Çözüm

1
Evaluate the plasma ACTH level in the context of cushingoid features.
A suppressed ACTH level indicates an ACTH-independent process (adrenal or exogenous). An elevated or inappropriately normal ACTH level indicates an ACTH-dependent process (pituitary or ectopic).
Cortisol normally exerts negative feedback on the pituitary. If cortisol is high and ACTH is suppressed, the source of cortisol is outside the pituitary axis.
2
Differentiate the ACTH-independent causes based on cortisol levels.
High endogenous cortisol with low ACTH points to an adrenal adenoma or carcinoma. Low endogenous cortisol with low ACTH in a patient with cushingoid signs strongly points to exogenous synthetic glucocorticoid use.
Synthetic glucocorticoids (like prednisone or dexamethasone) cause tissue manifestations of Cushing syndrome and suppress the HPA axis but are not cross-reactive on standard serum/urine cortisol assays, leading to low measured levels.
3
Differentiate the ACTH-dependent causes using the high-dose dexamethasone suppression test.
Suppression of serum cortisol indicates a pituitary adenoma (Cushing disease). Lack of suppression indicates an ectopic ACTH source (e.g., small cell lung cancer).
Pituitary corticotroph adenomas retain partial sensitivity to glucocorticoid negative feedback, whereas ectopic neuroendocrine tumors typically lack functional glucocorticoid receptors and secrete ACTH entirely autonomously.

Anahtar Kavram

The diagnostic algorithm for Cushing syndrome (hypercortisolism) utilizes ACTH levels to determine dependency, followed by high-dose dexamethasone suppression to localize the source of ACTH.
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Endocrinology and Metabolism

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