What is the purpose of the low-dose and high-dose dexamethasone suppression tests in the evaluation of suspected Cushing's syndrome?

• A. Low-dose screens for hypercortisolism; high-dose helps differentiate pituitary from ectopic sources
• B. Low-dose confirms diabetes
• C. High-dose replaces cortisol
• D. They test thyroid function

Answer: (C)

The main idea being tested is how dexamethasone suppression tests are used to evaluate suspected Cushing’s syndrome by probing the feedback control of the HPA axis. A low-dose dexamethasone test acts as a screening: in a healthy person, a small dose of dexamethasone suppresses ACTH release and lowers cortisol. If cortisol remains high, it suggests hypercortisolism. If hypercortisolism is found, the high-dose dexamethasone test helps distinguish pituitary sources of ACTH (cushing disease) from ectopic ACTH production. Pituitary sources are often suppressible with a high dose, whereas ectopic sources usually are not, so cortisol stays elevated.

The idea that high-dose dexamethasone replaces cortisol isn’t accurate—dexamethasone is not a cortisol replacement; it’s used to test feedback control of the axis. The other options refer to diabetes or thyroid testing, which are unrelated to this diagnostic approach.