There are some who suffer from low cortisol but not low enough to be diagnosed with Addison's. People are calling it "Adrenal Fatigue", but I don't think it's yet recognized as a medical condition.
To answer you, OP---I am diagnosed AS and in the recent past I was being treated for Addison's Disease (first Adrenal Fatigue). My doctors have since decided that I DON'T have Addison's, but I still get my cortisol levels checked. They have just hit the number to be considered normal, but they are on the low, LOW end of normal.
_________________
After a time, you may find that having is not so pleasing a thing, after all, as wanting. It is not logical, but it is often true.
--Spock