Dehydration can be a serious, life-threatening illness for dogs. To test for dehydration, pick up a fold of skin on his middle back area, then release it. Normally, the skin will snap back. If it falls back slowly or remains up, this means your dog is dehydrated and you should call your veterinarian immediately. Among the causes of extreme water loss are diarrhea, vomiting, and excessive urination associated with diabetes, heart, and renal disease.