I’ll give it a try. As you probably know, the problem in CAH is that the adrenal gland can not product cortisol. The gland than get ’overstimulated’, the effect of that is high levels of 17-OH  and androgens. By supplying the body with enough cortisol the adrenal gland is not supposed to be ’overstimulated’ anymore and so the 17OH and androgen levels will drop, (which is good). This way the docter can tell wheter or not the patient get’s enough cortisol. (When he reaches puberty this testosteron will rise, since at that time it’s produced somewhere else (testis)).

I hope this helps you, (and that you can understand my writing since English is not my native langauge)