What Causes Thinning Of Hair?

I am 33 years old and my hair is A LOT thinner than it use to be. I have always had extrememly thick hair now it seems to be extrememly thin. I take oxy pro elite occasionally now, and i am wondering if this could cause it. ??
2 years ago I went through a very rough time in my life and during that time i was very stressed out and very addicted to adderall. I no longer take adderall but i am thinking maybe that is what caused my thin hair?? I hope it is not the oxy pro elite but if it is i should stop taking it as well.