High-dose dexamethasone suppression test in ACTH differentiation: which statement is true?

High-dose dexamethasone suppression test in ACTH differentiation: which statement is true?

• A. Suppresses ACTH from pituitary source; does not suppress ectopic
• B. Suppresses ACTH from ectopic sources only
• C. Suppresses both pituitary and ectopic sources
• D. Does not affect ACTH levels

Answer: (C)

The test relies on glucocorticoid feedback to distinguish pituitary ACTH production from ectopic ACTH production. Pituitary corticotroph adenomas (Cushing disease) usually retain some sensitivity to high-dose dexamethasone, so ACTH secretion falls and cortisol levels drop with a high-dose dexamethasone dose. In contrast, ectopic ACTH–producing tumors are typically resistant to glucocorticoid feedback, so ACTH (and cortisol) remain high despite the high-dose dexamethasone. Therefore, the typical true statement is that high-dose dexamethasone suppresses ACTH from the pituitary source but not from ectopic sources (with rare exceptions where partial suppression may occur).

The test relies on glucocorticoid feedback to distinguish pituitary ACTH production from ectopic ACTH production. Pituitary corticotroph adenomas (Cushing disease) usually retain some sensitivity to high-dose dexamethasone, so ACTH secretion falls and cortisol levels drop with a high-dose dexamethasone dose. In contrast, ectopic ACTH–producing tumors are typically resistant to glucocorticoid feedback, so ACTH (and cortisol) remain high despite the high-dose dexamethasone. Therefore, the typical true statement is that high-dose dexamethasone suppresses ACTH from the pituitary source but not from ectopic sources (with rare exceptions where partial suppression may occur).