Not sure

I have had symptoms of low thyroid for years.  It runs in my family-almost everyone has it.  I have been tested over and over again and the results always come back ok.  I am so tired of being tired.  I know there are more detailed tests for thyroid that detect issues that the standard tests don't find, but my doctor thinks the usual tests are fine.  Almost every day, (days that I'm not super busy running around), I completely crash by about 1:00 and I have been known to be in a coma like sleep for up to 6 hours.  It is really hard to wake up from that kind of sleep.  I always think, "Oh, I will just lie down and read or watch a tv show or rest my eyes..." and BAM!  I'm out and basically incapacitated for hours until I can shake off the exhaustion.  It happened today, worse than it has been in a while.  I am really frustrated.  Why is it that I have classic symptoms of things that can be helped with medication, but none of my tests come back positive?  ARGH!!