So you have a patient with Cushinoid features, striae, buffalo hump etc.. and you want to figure what is the source of the excess cortisol that is causing the symptoms. Is it a tumor of the pituitary, adrenal ademoma or even paraneoplastic syndrome (small cell CA of lung)?. In a normal person if you inject them with dexamethasone (cortisol like substance that the pituitary registers as cortisol) this will serve as negative feedback to the pituitary and less ACTH will be excreted and hence cortisol levels will go down. So in a patient with cushings features you don't know what's causing the excess cortisol. If you give them the "standard" dose and the the measured cortisol is lowered by the next morning they don't have a tumor. If the cortisol levels have not been lowered there is a tumor somewhere. Next you do a "high" dose dex admninistration. If the cortisol levels are lowered that means the problem lies in the pituitary. Pituitary adenomas secreting ACTH can be inhibited by "high dose" dex. Finally if the high dose test doesn't reduce their cortisol levels that means you have an adrenal cortical adeoma pumping out cortisol or paraneoplastic syndrome. I'm pretty sure this is all correct and I hope it helped.
