Addison's Disease, also known as adrenal insufficiency, is the point where you have lost most of the function of your adrenal glands. Though reaching this point is extremely rare, living at a high stress level is sadly taken as "normal" in our modern society which is why many of us suffer from adrenal exhaustion.
There are several physical conditions which can severely damage the adrenal glands. Few people will ever reach the point where their adrenal glands no longer function, but these possibilities should be discussed with your doctor and the required tests given, so that they can be eliminated as causes for your adrenal problems. About 70% of the time when adrenal glands are destroyed, it is because the immune system has attacked and destroyed at least 90% of the adrenal glands. Also, infections such as tuberculosis, HIV, or fungal infections can damage the adrenal glands, so that there is little function left. Direct blows to the adrenal glands (lower back) which cause hemorrhage can cause the adrenal glands to be come so damaged that they lose the ability to produce adrenal hormones. Adrenal tumors also can cause severe adrenal gland damage. If you have problems with other glands in your endocrine system it will affect the adrenal glands. If the pituitary gland is not sending out it's hormonal messages to the adrenal glands (Secondary Adrenal Insufficiency), the adrenal glands will not function well. Some of these conditions can be treated and some may not. Your doctor can help to determine what can be done, or if your adrenal glands have been too severely damaged.
As I've said, such physical damage of our adrenal glands is rare. For most of us who suffer low adrenal symptoms, we are simply experiencing the effects of over-stress.