What can cause extreme fatigue with no other symptoms & normal test results?

[pokiea122]

I have been struggling with fatigue for about a year.  I have seen my GP repeatedly along with an endocrinologist.  All of my tests are perfectly normal.  I'm tired enough that it is affecting my job and everyday responsibilities.

I'm a healthy 31 y/o woman other than this fatigue.  I have zero other symptoms that I can think of.  I eat well, I'm not stressed out or depressed, and I get 8 hours of sleep a night.  I do take Vitamin D, but that has been for a number of years and my current numbers are excellent.  I wake up semi-refreshed and then just get more and more tired as the day goes on.  Exercise doesn't make me more tired.  It might even help a little, but not enough.  I have had a ton of blood work done and everything was normal.  It included glucose & A1C, CBC, metabolic panel, various hormone tests, and thyroid (TSH and T4 Free).  I also had a negative test for sleep apnea.

My doctor prescribed me an ADHD medication when my fatigue got so bad I couldn't even think straight, and it helped immensely for the first couple weeks.  After that, it didn't help anymore than a cup (maybe two) of coffee.  My GP had me increase the meds and after trying it for a couple weeks, all that did was make me jittery and tired at the same time.  

My next step is to follow an elimination diet for food allergies.  I have a couple allergies and am aware that they can present in odd ways, so I figure it's harmless to take a look from that angle.

Can anyone here give me other ideas or questions I can ask my doctor?  My GP isn't bad, but I'm pretty sure he has run out of ideas and I'm going to start getting brushed off.