In patients with confirmed hypercortisolism with a high/normal unsuppressed ACTH who have both a pituitary adenoma and adrenal adenoma identified on imaging, can you reliably use DHEA-S to determine the source of cortisol production?

There is a difference between high-normal and unsuppressed ACTH. If ACTH is high-normal, this is ACTH-dependent Cushing. However, you can have mild adrenal Cushing without fully suppressed ACTH (e.g., ACTH in the 10-20 pg/ml range and sometimes even above 20). I don't think you can rely on DHEAS in ...

DHEAS alone may not be a reliable indicator of the source of hypercortisolism. If ACTH is not suppressed, it is likely that DHEAS may not be low. AVS and/or IPSS may be needed to confirm the source.