ChristinaDe
Laurie ~ I wish you tons of luck!
For me, choosing an option & sticking w/ it was the hardest part of all this.
And the weight gain stuff? I didn’t have RAI, but had a TT (I don’t think it’s the actual treatment option that causes the problem, I think it’s the going hypo afterward that does – in most cases anyway). I gained a little weight after my TT during the time that I was hypo. I didn’t gain a ton (about 5 lbs, maybe less)…but I was nervous about it like you are so watched how I ate, forced myself to exercise a couple of times a week, and quickly reported hypo symptoms to my dr so she could increase my Synthroid & not leave me hanging out in hypo land too long. Looking back, it’s that last thing I did that probably had the most impact. I spent very little time being super hypo, and the time that I spent mildly hypo, while longer, wasn’t that long either. Since my levels have normalized I haven’t had any issues w/ weight at all. But you know how weight is. It’s dependent on so many things – from genes, to age, to diet, to hormones – on and on.
Just be sure to listen to your body!! 
Christina