I agree with ArmsOfLove, thyroid problems can cause significant symptoms even at subclinical levels. I've been tested for thyroid problems many times due to chronic fatigue, so tired you feel like you have no bones in your body, and the result always came back clear, however I recently got much worse with other problems and I tested positive for Hypothyroidism. I am now on Thyroxine and the difference in my energy levels is remarkable!!!
I think I have had this for YEARS but it is only recently it became significant enough to show up on those darned tests. I am certain I have Adrenal Fatigue also but I have a jerk of a doctor who won't do tests at my suggestion.