It is perfectly normal for a healthy dog to have an increased appetite during cold weather, increased activity, pregnancy and lactation. However, an increased appetite can also indicate the beginnings of a more serious condition.
Diabetes is one of the first diseases that come to mind with an increased appetite. Warning signs, besides increased hunger, are increased urination and water intake, weight loss, depression and vomiting.
Hyperthyroidism, while rare in dogs, has many of the same symptoms as diabetes.
Cushing's Syndrome is a possibility in older dogs. Besides increased hunger, dogs that may have Cushing's will exhibit increased thirst and urination and may have symmetrical hair loss.
Certain worms or other intestinal parasites can affect your dog's appetite because they rob the body of necessary nutrients. Periodic stool testing, as recommended by your vet, will determine if your dog has parasites.
Once at the vet, you can expect a complete exam that may include blood, urine and fecal analysis, endoscopies, X-rays, electrocardiograms or biopsies to determine the cause and proper treatment. If your dog is dehydrated, he may need to receive subcutaneous fluids, and, if necessary, he may be force fed. Your vet will send your dog home with a list of at-home care instructions appropriate for the condition he was diagnosed with.