Like Megan said, I was told that you could not get an accurate reading of cortisol whilst taking dex and pred.  Hydrocortisone is the synthetic replacement for cortisol and works differently to pred and dex. The androstendione shows your overall control for 24 hours whereas 17 OHP is a snapshot of the level at that moment. I have read that the  DHEA level often stays low once treatment has started for CAH.  Dex is known to be up to 80 times stronger than hydrocortisone, some docs still use the old conversion of 40.  Have you tried hydrocortisone?