hypothyroidism and post-pregnancy

Hopefully someone can help me with this.

I am 35 and decided to have a complete physical a few weeks ago. Last week, I had my bloodwork done and this past Friday, got the results. I was very surprised to learn that I have hypothyroidism. I do have all the symptoms- mainly fatigue, but I attributed that to having a 7 month old infant and a 4 year old. After reading about how pregnancy can affect the thyroid, I asked the nurse if it pertained to me. She never really said much about it, just that I would have to be on medication for the rest of my life. I want to know if my pregnancy caused it (I'm otherwise healthy)- will it eventually go away? Or has it been too long (7 months)? Also, how would you know if it is caused by the autoimmune disorder?