Yup, it could be. The same thing happened with my dog, Chum. His thyroid produces nothing on it's own. He would be running around and just fall over. His thyroid levels were so low it was affecting his heart. After the first month on thyroid supplements, he was hyperthyroid. It took a couple adjustments to get the dose right, but he's fine now. I'm sure your dog will be too once you find the right level of supplement.