measure ACTH from the beginning, as well as CRH, DHEA, renin, aldosterone, and electrolytes. Then the ACTH stimulation test (cosyntropin test), etc. As you mentioned, there are more aspects to the system than ACTH and cortisol.
I mention using metyrapone for Cushing's Dx because I see that some of what people have written are directions for interpreting that test rather than for adrenal insufficiency.
As far as adrenal insufficiency:
- high ACTH and low 11DOC after metyrapone - indicates primary adrenal insufficiency.
- low ACTH and low 11DOC - indicates secondary or tertiary adrenal insufficiency
- 11DOC at baseline (normal) - control; you did not administer sufficient metyrapone to block 11β-hydroxylase activity
I'm pretty sure that's all we can know about adrenal insufficiency from this test.