Why am I tired all the time?

I have a very healthy diet, including all food groups and portion sizes, I drink 8-10 cups of water a day, I exercise 5-6 times a week doing a mix of cardio and weight training, I get 7-8 hours of sleep a night, and yet I feel tired all the time.
I currently not taking any vitamins, but if I were to take a multivitamin, I'm not sure which "brands" are good to take. Furthermore, I've been tested for iron and other areas (such as thyroid) to see if that's the problem, but the tests showed nothing.
Any ideas/suggestions?Why am I tired all the time?
Take some prenatal vitamins they work great for me. I feel really good and plus they make your hair and nails grow .
it seems as if you're doing all of the things possible to stay energized, you might need to get a second opinion from another doctor.
Mom of B & D