I lost gobs of hair for about a year pp.... pre pregnancy I lost a fair amount of hair with every shower (I have very thick hair) and it stopped almost completely while I was pregnant. I read that during PP all that hair that you didn't loose during the pregnancy comes out anyway, so it is really a lot for quite a while, but goes back to normal (eventually). And my hair is still thick.
My hair dresser (an extended breastfeeder herself!!) says that it is caused by pregnancy induced alopecia, which is just a fancy way of saying it is from hormones. I lost so much hair with each of my 4 babies that I had visible bald spots. Of course I have very fine hair to begin with. Good luck to you!
Hypothyroidism can hit during pregnancy or in the postpartum period--all the extra nutritional demands. I had this with my 2nd pregnancy, the hair loss didn't stop like it did with the first. If you suspect it, there are lots of different ways to treat it besides the prescription drug route, but you may want a good alternative healthcare provider to help.