Pets — especially dogs — get underactive thyroids too. Interestingly, the dose of thyroid hormone needed to treat dogs is much higher than a person’s dose. Therefore, if dogs accidentally get into thyroid hormones at home, it rarely results in problems. However, large acute overdoses in cats and dogs can cause muscle tremors, nervousness, panting, a rapid heart rate and aggression. When in doubt, contact Pet Poison Helpline to see if your dog or cat ingested a toxic, poisonous amount.