Hello,
Certainly low thyroid levels can lead to seizures and passing out or low energy. Delta was put on thyroid meds at about 10 yrs old. Are the seizures full, eyes rolled back in head, shaking, loosing control of bladder etc. or are they milder? If full, your vet may want to consider low dose phenobarb or some such. The more ominous possibility is some sort of brain tumor leading to the seizures. You may want to think about having that checked out.
Tom