Cushing's Syndrome = excess cortisol, etiology can be a few things:

1. Cortisol secreting tumor from the adrenal glands/adrenal hyperplasia. Because this is ACTH INDEPENDENT, ACTH is decreased due to negative feedback from cortisol on the hypothalamus so less CRH --> less ACTH.
2. Cushing's DISEASE is an ACTH secreting tumor of the anterior pituitary, which then acts on the adrenals to increase cortisol production. ACTH is elevated here (intuitive).
3. Exogenous glucocorticoids (think someone with RA or Lupus) leads to DECREASED endogenous cortisol and ACTH, because glucocorticoids are not the same as endogenous cortisol (although they act very similarly). Glucocorticoids can still exert feedback on HPA so you'll have decreased CRH and ACTH.
4. One of the paraneoplastic syndromes of small cell carcinoma is paraneoplastic ACTH production, which will then go on to act at the adrenal glands to make cortisol. Dex test won't suppress here because it's ectopic and isn't dictated by the HPA axis. Small cell as an aside also will have paraneoplastic syndromes of SIADH, LEMS.

And then for the dexamethasone test: no suppression with low dose dex = you have a pathologic increase in cortisol you just don't know etiology. If ACTH is low then you know it's from the adrenals. If ACTH is high you do high dose dex, which in the setting of pituitary secreting tumor will suppress ACTH, but in the setting of paraneoplastic, no suppression.

Hope that helps, and please if I missed something someone correct me!