Some tablets can increase appetite which can lead to weight gain if the patient over eats? So you should be fine if this is the case, give your g.p a ring and ask if you are worried.
Like Daisy said, you're not going to have weight gain if you're not taking in extra calories, that's just impossible, right? As far as I'm aware, at least Maybe you might have water retention or something though? I started on tablets for my underactive thyroid 7 years ago and theyre supposed to make you lose your appetite/lose weight but I'm still waiting for that to happen!