Several years ago, I was getting episodes of hair loss. They gradually became so severe that I could see my scalp on top of my head, and began researching wigs. Like you, doctors and tests yielded nothing.
I then did a lot of research on vitamins/supplements for hair loss, and started taking them. After about 3-4 months on the following regimen, my hair loss stopped, and has never returned.
This is what I take each day:
B-complex - 50mg
Biotin - 5,000 mcg
MSM - 4-5,000 mg
Activated Silica - Jarrow Forumula's "JarroSil" - 15 drops daily
Best of luck to you!