With CAH - the 17-OHP is a very elevated and that is why they need cortef. The plasma renin blood test is also important - salt retention and salt/sugar balance. If not controlled - the CAH person is dizzy, light headed, loses weight, and can die from an adrenal crisis if not long term - in not under control and especially if there is an illness and enough cortef given immediately and repeated over 48 hours of the illness every 6 hours. Cortef is short lasting in the body. It is hard to mimic in the body. And the levels change during the day.  In Germany - the doctor looks at the 17-0HP - with three samples - 8 am; 3pm and 11pm (salvia tests, urine and blood).