Dexamethasone suppression testing

At this point in the case, the diagnosis of Cushing's syndrome is well-established. It fits the clinical presentation and the laboratory workup is clear. However, it has not been established whether the problem is primary to the adrenal, or secondary (e.g., excess pituitary ACTH output or a paraneoplastic syndrome). The mildly elevated ACTH in the face of markedly elevated serum cortisol suggests a pituitary problem. To confirm that, we can carry out dexamethasone suppression tests at low and high dexamethasone doses.

Dexamethasone suppression testing

Dexamethasone was given every 6 hr for 48 hr, with a 24 hr urine collection for 17-OH-corticosteroids prior to the test (baseline) and on 2nd day of the test.

             Patient        Reference
Baseline    11.9  mg/d      3.0 - 8.0
Low dose      8.0 mg/d          <3
High dose     3.1 mg/d         >50% suppression w/ pit. dis.

Questions

How would you interpret these results?