The opposite of hypothyroidism is hyperthyroidism, which is common in cats but rare in dogs. With hyperthyroidism, too much of the hormones are released. Hyperthyroidism is commonly linked to many types of cancer, hypothyroidism is generally associated only with thyroid cancer. The breeds that are most likely to get this include Golden Retrievers, Doberman Pinschers, and Irish Setters. The main symptoms of hypothyroidism include thinning of the fur, excess shedding, a dull coat, reduced activity, unexplained weight gain, and an inability to tolerate cold temperatures. In some cases, dogs may also experience thinning of the skin.