Hi Louise - I had the same fears as you when taking my son of the medication (hydrocortisone and fludrocortisone) for 24 hours, but in the event he was fine.  The result showed he had zero cortisol and zero 17OHP levels though, indicating Hypoplasia (as I said before).  He was then given the long synacthen test, whereby he came off all his medication for 5 days and was given daily ACTH injections (this really worried me - I was sure he would collapse and possibly die!).  Surprisingly though, he was OK, although lacking in energy and very tired -he was well monitored in hospital with regular b/p checks and BM’s.   By the end of the fifth day, we had our first reading of 17OHP, which showed the adrenals were kicking back in, after being dormant.  His cortisol levels were zero for the whole 5 days though - hence the tiredness!  The DNA test confirmed the CAH diagnosis and since then we have had ’normal’ 17OHP profiles for a child with CAH.  He has the severe salt losing form, so produces no cortisol himself and is a heavy salt loser.

The DNA will confirm the diagnosis but if the adrenals have become dormant (as in my son’s case), will not kick-start them (for want of a better word!).  I therefore feel it would be worthwhile going ahead with the test - I am sure your son will be fine.  Good luck!