The thyroid gland, located in your dog's neck, produces the hormone thyroxine, along with several other important hormones. When the gland is underproducing hormones, your dog suffers from hyperthyroidism, causing symptoms like lethargy, weight gain, changes in coat and skin, exercise intolerance, mental dullness, and more, according to the American Kennel Club. If diagnosed early, the impacts can be less severe, however, the only true way to avoid the disease is to look at issues within breeding. "Hypothyroidism is usually treated with an oral replacement hormone," Dr. Sarno explained. "Unfortunately, your pet will probably have to take daily medication for the rest of his/her life."