My mother has Hashimoto, and needs to take medication for life to balance her hypothyroidism, but I know that her doctor told her the weight stabilizes after you've started the treatment. I suppose you need to take some meds as well, so it's probably a matter of a few weeks before it isn't a problem for you any more (not that I know *exactly* how long it takes - for what I know, it may start to stabilize for you next week!).
In any case, hello and welcome here.