A patient on long-term prednisolone for rheumatoid arthritis develops Cushing's syndrome. Which morning cortisol test result would confirm HPA axis suppression in this patient?

• A Elevated 24-hour urinary free cortisol above normal range
• B Low or absent morning plasma cortisol after overnight dexamethasone suppression test
• C Elevated ACTH level from loss of negative feedback
• D Low morning plasma cortisol (< 3 mcg/dL) indicating suppressed endogenous cortisol production despite exogenous steroids ✓

Explanation

Exogenous glucocorticoids suppress the HPA axis via negative feedback at the hypothalamus (reduced CRH) and pituitary (reduced ACTH). A morning plasma cortisol below 3 mcg/dL is diagnostic of significant HPA suppression, indicating the adrenal glands can no longer mount an adequate cortisol response to stress. This is clinically important during drug tapering or surgical stress. The dexamethasone suppression test is used to diagnose endogenous Cushing's, not to assess HPA suppression from therapeutic steroids. In exogenous Cushing's, endogenous cortisol is LOW (suppressed), while ACTH is also low (not elevated), and urinary free cortisol may reflect the synthetic steroid contribution variably depending on the assay.

Reference: KD Tripathi, Essentials of Medical Pharmacology, 8th ed.

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