Think about a patient who has cushing. Their adrenals will kick out enormous amounts of cortisol almost non stop and not once do they "get tired". So let me the ask, if someones adrenals with cushings, which is reciving a constant stimulus, and their adrenals continue to function just fine, what makes you think it is possible for our adrenals to get fatigued?
Adrenal fatigue is more then just a mislabel because along with this term comes the idea that if you give them a "rest" somehow everything will get better. Now is it possible for someone to have a adrenal insufficiency? Absolutely and it is called addisons disease but more than likely when someone uses the horrible term "adrenal fatigue" they are usually describing HPA dysfunction. When this happens, yes the body can stop naturally producing enough hormones BUT it isnt because the adrenals are tired but usually a deeper underlying cause which is causing something to become out of balance and if I remember correctly the typical treatment given for this is glucocorticoids but even then it has had a fairly limited success rate. As coop said this is relatively new and there are still many unanswered questions.
Think of it this way (is steroid terms )
I am going to try and compare the HPA axis to the HPTA
Here we have you HPA axis
Now my understanding of it is the hypothalamus will sense serum cortisol and modulate CRH/ACTH output in so to keep glucocortcoid levels constant.
Now to compare this to the HPTA
the way this works is somewhat similar, the hypothalamus releases GnHR and GHRH to our pituitary which in turn releases LH and FSH to the testes which release the testosterone.
Now with HPA dysfunction, it is not that the adrenals become "fatigued" but it is that a mechanism exists by which adrenal response is chronically down-regulated (usually after a prolonged period of stress) and this mechanism as far as I know has yet to be identified (although there are some theories floating about).
Now back to the HPTA comparison, with HPA dysfunction basically something is out of wack and you end up with a low hypothalamic CRH production which leads to low cortisol production. Now lets compare to someone whose HPTA is out of wack shall we. We all know that when coming off a cycle of exogenous androgens our body down regulates testosterone production (via negative feedback of the androgens). What we end up with is too many androgen receptors of the hypothalamus have become activated so there is a decrease in GnRH release which ultimately equals a decrease in testosterone production.
Now that said, does anything in the HPTA become "fatigued"? Or is it there was a disruption of the balance (i.e the introduction of exogenous androgens)? Would the solution to low test be to give your testes or leydig cells a rest? No it wont be, the solution is to somehow stimulate GnHR release. So if the solution to low test output isnt rest then why is the solution to low adrenal output that it needs rest?
The main problem with HPA dysfunction is we dont know exactly what the mechanism for adrenal fatigue is. Again, as Coop said above , the developments in the understanding in all of this is fairly recent and there is much that is still unknown. Oh and Mr coop if I got anything wrong please feel free to correct me as I dont have a full grasp of it entirely yet.