How long do dogs live is a common question. The answer requires you to consider many factors, which include breed mix and size, and environmental influences. In general, smaller dogs live longer, from 14-22 years, with medium sized dogs living 11-14 years and large dogs living 8-12 years. Mixed breed dogs tend to live longer as many purebreds suffer from breed specific genetic problems that may shorten lifespan.
Other times, lethargy can be caused by lack of exercise, alcohol use, or eating disorders. In these cases, the less you do, the more lethargic you can feel, and this can become a vicious cycle that is difficult to overcome. This could lead to disorders, such as depression or anxiety, which also have related lethargy.
Lethargy is one of the more common symptoms of certain thyroid problems, especially hypothyroidism. A dog with this condition will no longer have the energy to get up and play, go for a walk or even seek out a favorite toy.