Could be male pattern baldness. Hard to say. It is pretty unusual too be thinning so much on the sides. I would go to a different dermatologist and get a second opinion. Is your beard thinning as well? Also if you just went to a general practitioner, make sure to go to an actual dermatologist. I swear some general practitioners know less than yahoo answers. Dermatologists deal with this stuff for a living so have seen almost everything.