Sometimes a senior dog will sometimes seem to get old and sluggish.  Often the reason is not his advanced age but hypothyroidism. This is a condition where the thyroid gland stops producing enough hormones.  Hypothyroidism can cause hair loss and weight gain even in dogs with a diminished appetite.  Some researchers think that canine hypothyroidism has a genetic basis.  This makes it very different from feline thyroid problems.  Cats often develop hyperthyroidism. This is a condition where they produce too much thyroid hormone.  Veterinarians suspect that feline hyperthyroidism which causes weight loss not gain, is triggered by environmental factors.  There is pet medication than can help to control either hypothyroidism or hyperthyroidism. Of course all medications have to be prescribed by your veterinarian.