It was on this forum that I first learned about the signs of thyroid disease. In our case, the one that's hypothyroid is our Shiba, not our B. I documented his symptoms on a blog post here. There are lots of useful threads on this board relating to hypothyroidism and thyroid testing as well.
In our Shiba's case, I'd say the most obvious marker was his REALLY sharp temperament (short fuse), awful skin issues, and frequent infections. Yes, there was weight gain as well, and it took him a full year to lose 10% – now that he's on Soloxine, and back at an ideal weight.
How old is Becca? Could the weight gain be a gradual result of her spay, as well as the approaching winter weather? How much more overweight than "usual" does she appear to you?
Anyway, good luck, and please let us know what the vet says! Ordering the full thyroid panel certainly couldn't hurt.