Awhile before I was diagnosed, I started noticing the top of my head was thinning...I assumed it was stress or hormones. I've since read that insulin resistance can contribute to this (as well as a host of other lovely problems/symptoms). After being on metformin for awhile and getting my blood sugar under control, my hair density has finally returned to normal.
Incidentally, I do transcription for a dermatologist who sees lots of women for hair loss. Usually they will do a thyroid panel, ferritin/iron level, hormone panel to r/o underlying causes. Sometimes stress, crash dieting and/or heredity can also be a culprit.
If however, it is determined that it is the metformin, stopping it should take care of the problem.