The reason your dog is losing weight will depend upon its cause. For example:
Change of Diet
A change of diet may make a dog turn his nose and rebel against his new food. A new diet may have fewer calories, which will cause weight loss. A few pieces of boiled chicken or broth on top of the new diet may entice him to eat his new diet.
Anxiety
Anxiety can cause a dog to lose his appetite. A recent move, the death of a loved one, a new pet, or construction work in the neighborhood may cause a dog to be to be stressed and not want to eat. He will need lots of love and reassurance. Hand feeding him in a quiet room may get him to start eating.
Periodontal Disease
Periodontal disease can include gingivitis, and loss of bone and soft tissue. Periodontal disease can lead to loose painful teeth, which can deter your dog from eating.
Parasites
Dogs with intestinal parasites can experience weight loss. Parasites such as tapeworms and hookworms rob your dog of nutrients. Even though your dog is eating, he is rapidly losing weight.
Malabsorption Disorder
A malabsorption disorder causes the deficiency of nutrients in the GI tract. Conditions such as inflammatory bowel disease, intestinal cancer, intestinal bacterial overgrowth and villous atrophy can cause weight loss.
Pancreatitis
Pancreatitis is the inflammation of the pancreas. The pancreas produces enzymes, which aid in food digestion, and also makes hormones such as insulin. When the pancreas is inflamed it can overflow enzyme into the abdominal cavity. These enzymes can cause damage to the other organs such as the liver, stomach, gallbladder, bile duct and intestines.
Diabetes
Insulin-deficiency diabetes is the most common type of diabetes in dogs. This condition is caused when your dog’s pancreas is not producing enough insulin. The pancreas may be damaged and/or not functioning properly.