How can you differentiate Cushing disease from other causes of Cushing syndrome?

• A. ACTH-dependent hypercortisolism with pituitary source; high-dose dexamethasone suppression or ACTH levels with MRI can help differentiate
• B. ACTH-independent adrenal tumor with cortisol suppression on high-dose dexamethasone
• C. Low ACTH with high cortisol
• D. MRI alone is sufficient

Answer: (A)

The key idea is that distinguishing Cushing disease from other causes hinges on whether the excess cortisol is ACTH-driven and how the axis responds to glucocorticoid feedback. In Cushing disease the pituitary overproduces ACTH, so cortisol excess is ACTH-dependent. To separate pituitary from other ACTH-dependent sources (like ectopic ACTH) or from ACTH-independent adrenal disease, we use the high-dose dexamethasone suppression test and ACTH measurements. If cortisol falls with a high-dose dexamethasone dose, this suggests a pituitary source because pituitary tumors often retain some feedback sensitivity, whereas ectopic ACTH-secreting tumors typically show little or no suppression. Measuring ACTH helps categorize the process: an inappropriately normal or elevated ACTH points to ACTH-dependent disease, while a suppressed or low ACTH points to an adrenal (ACTH-independent) cause. Imaging then aids localization: MRI of the pituitary can reveal a microadenoma consistent with Cushing disease, and if the MRI is inconclusive but the biochemical pattern supports a pituitary source, further testing like sampling from the inferior petrosal sinuses can confirm central ACTH production. In contrast, an ACTH-independent scenario with low ACTH directs you toward adrenal pathology rather than pituitary disease.