She was always trim until about 4 months ago. It was at that time that she developed siezures ( small ones .... lips tremble ).... The vet put her on Potassium Bromide. In those for months she has gained 20 pounds even though I have reduced her food amount by a third. Side effects for this drug include a voracious appetite, but it doesn't list weight gain.... and, like I said, we cut her food back. She has become somewhat lackadaisical. She is 6 years old and used to have an absolute abundance of energy. It was only this week that the thought that it "might" be the PB occurred to me.
I agree. It might even be that the seizures are a symptom of the same thing that's causing the weight gain - hypothyroidism absolutely lowers the seizure threshold in humans, and I wouldn't be surprised if it's the same with dogs. Either way, weight gain and behavior changes without changes in diet or environment scream something medical underlying, and I agree that thyroid should be one of the first things checked.