I've always been really thin even when I was hypo and unmedicated. However, I did gain some weight when I was hypo. It was a small amount to most people, but since I'm on the low end of weight it was a lot for me. I gained about 5-7% of my body weight. And before I started having hypo symptoms I had been at the same weight no matter what. So it was a little odd to gain anything for me.
After I got on medication and got at a right dose, I did eventually drop the weight. I really don't know how long though. But I pretty much always maintain that same weight, except while pregnant of course. I was a few pounds heavier for a while post partum, but around 5 months I was back down to that same weight.