Unexplained Weight Loss
It's important for dogs to stay fit and trim, but if your dog starts to lose weight without a diet change or increase in exercise, something may be wrong. Weight loss occurs in one of two ways — either insufficient calories are being taken in, or extra calories are being burned. If your dog's appetite and eating habits are still normal, the extra energy must be being used somehow, if not with exercise then possibly with a great metabolic use of calories.
Diabetes mellitus is one example of a disease in which a dog may have a good (if not voracious) appetite but still lose weight. This disease affects the production of insulin so the dog can't use all the calories consumed.
Dogs with cancer may have a loss of appetite that results in weight loss, or they may be quite hungry but still lose weight as the cancer cells consume lots of energy.
Because the causes of weight loss can be so serious, be sure to get it checked out. Certainly an unexplained weight loss of 10 percent of your dog's body weight is cause for a veterinary visit. Your veterinarian will want to check a fecal sample for parasites that can drain nutrition, then possibly do some blood work or check for abnormalities with X rays or an ultrasound.