ANswer is given in first aid 2016 page-317
Cushing Ds refers to hypercortisolism because of increased ACTH secretion from pituitary.! Dexamethsone is potent synthetic glucocorticoid. High dose dexamethasone(8mg)will supress ACTH release from Pituitary by feedback inhibition and hence decreases cortisol level.
This feedback does not work when ACTH is produced outside pituitary e.g., ectopic source.
Hope this adresses your doubt.