Causes of Hypoglycemia
Hypoglycemia can be caused by a number of things. The basic mechanisms of hypoglycemia are either: 1) an abnormally large rate of removal of glucose from the bloodstream, 2) an abnormally low level of glucose in the blood from dietary intake, or 3) decreased production of glucose from glycogen stores by the liver. Most cases of hypoglycemia are caused by excessive use of glucose by normal body cells or by rapidly-dividing cancer cells.
Hypoglycemia often involves abnormally low levels of the hormone, insulin. In healthy dogs, insulin is made by specialized cells of the pancreas called the beta cells of the pancreatic islets of Langerhans. Insulin has a number of important functions, one of the most critical of which is regulating the uptake and storage of glucose from the bloodstream. Blood glucose normally comes from the dog’s intake of food – especially of carbohydrates. There must be a sufficient amount of insulin in circulation for glucose to be able to be moved out of the blood and into cells and tissues, where it is needed as the primary source of a dog’s energy. If the pancreatic beta cells produce too much insulin, then excessive amounts of glucose will be removed from the blood, causing the dog to become hypoglycemic (i.e., to have low blood sugar).
Insulin-secreting tumors of the pancreas, called insulinomas or beta-cell tumors, can cause the islet cells to overproduce insulin. Insulinomas are probably the most common cause of hypoglycemia in older dogs. Extra-pancreatic cancer, such as liver carcinoma or sarcoma, leukemia, malignant melanoma or other forms of neoplasia, can be responsible for hypoglycemia. Accelerated utilization of glucose can be caused by over-administration of insulin. This is referred to as an “iatrogenic insulin overdose.” An iatrogenic insulin overdose almost always happens when a dog is being treated for diabetes mellitus, and its owner is overzealous in administering intramuscular insulin through a needle and syringe. Excessive metabolic use of glucose can also be caused by pregnancy, cancer or sepsis. Sepsis is a systemic infection of the blood and other tissues that usually is caused by bacteria. It also can be caused by infection of other microorganisms, such as viruses, fungi or protozoa.
Most glucose originally enters a dog’s system through its diet. Eating a meal stimulates the pancreas to secrete insulin, which in turn promotes the uptake of glucose from the blood into bodily cells and tissues. Severe malnutrition and starvation are both direct causes of hypoglycemia. When a dog has inadequate levels of carbohydrates to meet its body’s metabolic demands, it will have low circulating dietary blood sugar levels. When that happens, the liver normally starts producing glucose to provide the dog with a source of energy. Under these circumstances, if the liver is functioning properly, it will convert glycogen (the form of glucose that is stored in liver and muscle cells) into glucose, through metabolic processes known as gluconeogenesis and glycogenolysis. If the liver is damaged or diseased, it will not be able to make enough glucose to compensate for the dog’s inadequate dietary glucose supply. As a result, the dog will become hypoglycemic, meaning that it will have low levels of circulating blood sugar. This can be the result of liver fibrosis or cirrhosis, severe hepatitis, glycogen storage diseases, liver cancer and portosystemic shunts, among other things. Prolonged exercise can also deplete liver levels of glycogen and lead to hypoglycemia. This is fairly common in adult dogs that are used for the strenuous sport of field hunting and field trials.
The adrenal glands are responsible for secreting a number of hormones that are precursors of the materials that the liver uses to make glucose when there is not enough of it in a dog’s diet to sustain its energy demands. If these adrenal hormones – especially glucocorticoids – are abnormally low for whatever reason, the liver will not have enough glycogen stores to convert into glucose, and hypoglycemia will be the result. Addison’s disease (hypoadrenocorticism) is an important cause of low blood glucocorticoid levels and hypoglycemia.
Other reported causes of hypoglycemia in dogs are pancreatitis, renal (kidney) failure, prolonged anorexia or starvation and ingestion of ethylene glycol (antifreeze).
Prevention of Hypoglycemia
Puppies and toy breeds are especially prone to developing hypoglycemia, because they are less able to use energy sources other than glucose and therefore have a higher glucose intake requirement than do older or larger dogs. Puppies and toy breeds should be fed frequently to prevent this problem, and their diet should be high in complex carbohydrates, high-quality proteins and fat. Hunting dogs should be fed a high protein and high fat diet several hours before they are sent to work, so that their blood glucose levels are elevated enough to sustain them during rigorous exercise.
The livers of very young animals, especially newborns, do not have a fully-developed ability to perform gluconeogenesis when an insufficient amount of carbohydrates are part of the dogs’ diets. As a result, even fairly short periods of fasting can cause puppies to become hypoglycemic. Stress can also contribute to hypoglycemia, especially in puppies and toy breeds. Repeated hypoglycemic attacks in toy breeds and in very young puppies can lead to irreversible brain damage.