Well, yeah. the thyroid regulates metabolism. If she is on thryroid hormone replacement, then her metabolism will have kicked up from its lethargic past. She's burning more calories faster etc. I'd expect her energy level to also be up. Now, you do want to make sure that she's not swung too far the other way into hyperthyroidism, which can happen if the dose is too high.
Since she's losing weight, I wouldn't worry about modifying her diet yet.
When does she go back in for a check up? I'd certainly give the vet a call, if for no other reason than to calm your worries.
I'm by NO means implying that your dog IS on too high a doseage. In humans, it can take a while to find the right dosage, I'm not sure how that is for dogs.