It was thought my son may have an pituitary tumour as he has high ACTH levels with marked ACTH pigmentation (as well as high 17 OHP).  He has CAH which has been difficult to control due to his body metabolising medication quickly.  To check whether this high ACTH was from a tumour or due to the CAH he had the formal 4 day dexamethasone suppression test (as well as an MRI of the pituitary and CT of the adrenals).  The specialist told me that he has never known the ACTH not to suppress with this test if the high ACTH  level is due to CAH. Are your 17 OHP levels suppressed?  I know your problem is a little different as a tumour has been found.  You may find something helpful if you post a message on the Addison’s Board http://www.hopeoasis.org/  Hope this helps, good luck!.