It can be weird like that. My girlfriend was diagnosed ~2.5 years ago (just had her dosage upped, actually), and she didn't really gain any weight as her symptoms got worse leading up to the diagnosis. She did have the crippling fatigue, though, as well as some hair loss and a goitre. I know it can affect appetite as well, which can result in weight being maintained despite the lower BMR.
Now, the cat we adopted that turned out to have pretty severe untreated hyperthyroidism? As soon as we treated her she like doubled in weight. When we first had her examined, the vet remarked that she had a lot of loose skin like she'd recently lost a lot of weight, but seeing her inflate to fill that extra skin was kind of hilarious. She still has some loose skin despite being quite chubby now (not catastrophically, plus she's 17, so we aren't exactly desperate to optimize her lifespan by making sure her weight is perfect), which really makes me wonder just how fat she was before her thyroid went crazy.