J.S.
Normal, normal normal.
When you are pregnant your body releases different hormones at different amounts causing abnormal hair retention. When you are not pregnant you lose hair daily, due to the body's natural system.
The release of extra hormones during pregnancy makes you hold onto hair that you would normally 'shed'. After you give birth these hormones normalize and you see massive hair loss.
My son is almost two and my new hairdresser immediately asked how long it had been since I had a child because I had so much new growth. (those baby annoying short hairs all around your face that stick out in all different directions!)
It can be due to thyroid, but a good doctor would also tell you this is normal. A routine check of your thyroid after birth is pretty par for the course, have it checked, but even if it is normal you will still go through the hair loss cycle it's okay, other women go through it and it will eventually stop.
There is an article about this exact issue in either this months Parents magazine or Oprah, (I cannot recall which).
Again, it's normal, so don't fret. Enjoy that baby. Your hair will start to grow back soon.