I've been through the same thing recently as a result of an ovarian cyst. I had the cyst removed laproscopicaly and my OB put me on low dose birth control and my weight shot up 15 pounds over 2 months. I've been on thyroid medication for some time now and it suddenly felt like all the low thyroid symptoms were coming back again. After being to 2 different docs and hearing that I must be eating too much (I'm not, nothing had changed in my eating habits) I finally stumbled onto information on estrogen dominance. Too much estrogen (mine was from the birth control pills but it can occur naturally also) can interfere with thyroid function and cause rapid weight gain. You might try Googling estrogen dominance and see if any of the info helps. There's a simple saliva test that you can get that will tell if it's a problem. Hope this helps.