I had a full thyroidectomy 20 years ago. The first two years are the worst, but unless you aggressively pursue better treatment, it probably won't happen. Every few years, I gain weight because my levels drop. It's just what happens, and you have to stay on top of getting it checked regularly, as well as watching your weight on a scale. They told me every 2 years to have it checked, but every year is better, though sometimes the drop can happen right after an appointment. I learned to notice a few pounds creeping on (within a week or two), then immediately make an appointment to have the levels checked.
Being at the right level of Sythroid helped me maintain my weight, but I couldn't lose weight at all until I finally convinced my doctor to let me try a mix of Synthroid and Cytomel. Moreover, the mix makes me feel a whole lot better than just being on Synthroid. I take .175mg of Synthroid and 5mg of Cytomel. Too much Cytomel will make you sleepless and have hot flashes, but the right amount didn't make my hands and feet freeze all the time like they were just on Synthroid.