Hypoglycemia is abnormally low levels of sugar (glucose) in the blood.
Normally, the body maintains the level of glucose in the blood within a range of about 70 to 110 milligrams per deciliter (mg/dL) of blood. In hypoglycemia, the glucose level becomes too low. In diabetes mellitus, the glucose level in the blood becomes too high, a condition called hyperglycemia. Although diabetes is characterized by high levels of glucose in the blood, many people with diabetes periodically experience hypoglycemia due to side effects of diabetes treatment. Hypoglycemia is uncommon among people without diabetes.
Low levels of glucose in the blood interfere with the function of many organ systems. The brain is particularly sensitive to low glucose levels, because sugar is the brain's major energy source. If the glucose levels in the blood fall far below their usual range, the brain responds by stimulating the adrenal glands to release epinephrine (adrenaline) and cortisol, the pancreas to release glucagon, and the pituitary gland to release growth hormone, all of which cause the liver to release glucose into the blood.
Most cases of hypoglycemia occur in people with diabetes and are caused by the insulin or other drugs (for example, sulfonylureas) they take to lower the levels of glucose in their blood. People with diabetes sometimes call the hypoglycemia that can occur after taking insulin an "insulin reaction" or "being shaky." Insulin reactions are more common when intense efforts are made to keep the glucose levels in the blood as close to normal as possible. People with diabetes who reduce food intake to lose weight or who develop kidney failure are more likely to have hypoglycemia. Older people are more susceptible than younger people to hypoglycemia resulting from sulfonylurea drugs.
If, after taking a dose of a drug for diabetes, a person eats less than usual or is more physically active than normal, the drug may lower the level of glucose in the blood too much. People who have had severe diabetes for a long time are particularly prone to hypoglycemia in these situations because they may not produce enough glucagon or epinephrine. The amounts of glucagon and epinephrine that are released are often too low to counteract a low level of glucose in the blood.
Many drugs other than those for diabetes, most notably pentamidine, used to treat a form of pneumonia that occurs most often as part of AIDS, and quinine, used to treat muscle cramps, occasionally cause hypoglycemia.
An uncommon type of drug-related hypoglycemia sometimes occurs in people who secretly take insulin or other drugs that treat diabetes as part of a psychologic disorder such as Munchausen syndrome (see see Sidebar 2: Munchausen Syndrome: Faking Illness for Attention).
In otherwise healthy people, prolonged fasting (even up to several days) and prolonged strenuous exercise (even after a period of fasting) are unlikely to cause hypoglycemia.
However, there are several diseases or conditions in which the body fails to maintain adequate levels of glucose in the blood after a period without food (fasting hypoglycemia). In people who drink heavily without eating, alcohol can block the release of stored glucose from the liver. In people with liver disease, such as viral hepatitis, cirrhosis, or cancer, the liver may not store sufficient glucose. Infants and children who have an abnormality of the enzyme systems that control glucose use also may have fasting hypoglycemia.
A rare cause of fasting hypoglycemia is a tumor in the pancreas that produces large amounts of insulin. In some people, an autoimmune disorder lowers glucose levels in the blood by changing insulin secretion or by some other means. Disorders that lower hormone production by the pituitary and adrenal glands (most notably Addison disease) can cause hypoglycemia. Other severe diseases, such as kidney or heart failure, cancer, and shock, may also cause hypoglycemia, particularly in a person who is also being treated for diabetes.
Reaction to eating:
Hypoglycemia can occur after a person eats a meal containing a large amount of carbohydrates if, for some reason, the body produces more insulin than is needed. However, this type of reaction is probably rare. In some cases, people with normal blood glucose levels experience symptoms that can be confused with hypoglycemia.
After certain types of stomach surgery, such as removal of part of the stomach, sugars are absorbed very quickly, stimulating excess insulin production, which then may cause hypoglycemia. Problems with digestion of some sugars (fructose and galactose) and amino acids (leucine) may also cause hypoglycemia if an affected person eats foods containing those substances.
The symptoms of hypoglycemia rarely develop until the level of glucose in the blood falls below 60 milligrams per deciliter of blood. Some people develop symptoms at slightly higher levels, especially when blood glucose levels fall quickly, and some do not develop symptoms until the glucose levels in their blood are much lower.
The body first responds to a fall in the level of glucose in the blood by releasing epinephrine from the adrenal glands. Epinephrine stimulates the release of glucose from body stores but also causes symptoms similar to those of an anxiety attack: sweating, nervousness, shaking, faintness, palpitations, and hunger. More severe hypoglycemia reduces the glucose supply to the brain, causing dizziness, fatigue, weakness, headaches, inability to concentrate, confusion, inappropriate behavior that can be mistaken for drunkenness, slurred speech, blurred vision, seizures, and coma. Severe and prolonged hypoglycemia may permanently damage the brain. Symptoms can begin slowly or suddenly, progressing from mild discomfort to severe confusion or panic within minutes. Sometimes, people who have had diabetes for many years (especially if tightly controlled) are no longer able to sense the early symptoms of hypoglycemia, and faintness or even coma may develop without any other warning.
In a person with an insulin-producing pancreatic tumor, symptoms are likely to occur early in the morning after an overnight fast, especially if the glucose stores in the blood are further depleted by exercise before breakfast. At first, people with a tumor usually have only occasional episodes of hypoglycemia, but over months or years, episodes may become more frequent and severe.
In someone who is known to have diabetes, a doctor may suspect hypoglycemia when symptoms are described. The diagnosis may be confirmed when low glucose levels in the blood are measured while the person is experiencing symptoms.
In an otherwise healthy person who does not have diabetes, a doctor is usually able to recognize hypoglycemia based on the symptoms, medical history, a physical examination, and simple tests.
Doctors first measure the level of glucose in the blood. A low glucose level in the blood found at the time a person is experiencing typical symptoms of hypoglycemia confirms the diagnosis in a person without diabetes, especially if the relationship between a low glucose level in the blood and symptoms is demonstrated more than once. If symptoms are relieved as the glucose levels in the blood rise within a few minutes of ingesting sugar, the diagnosis is supported.
When the relationship between a person's symptoms and the level of glucose in the blood remains unclear in a person who does not have diabetes, additional tests may be needed. Often, the next step is measurement of the glucose level in the blood after a night of fasting in a hospital or other closely supervised setting. More extensive tests may also be needed.
If use of a drug such as pentamidine or quinine is thought to be the cause of hypoglycemia, the drug is stopped and blood glucose levels are measured to determine if they increase. If the cause remains unclear, other laboratory tests may be needed.
If an insulin-producing tumor is suspected, measurements of insulin levels in the blood during fasting (sometimes up to 72 hours) may be needed. If the insulin measurements suggest a tumor, the doctor will try to locate it before treatment.
People prone to hypoglycemia should carry or wear medical identification to inform health care practitioners of their condition.
The symptoms of hypoglycemia are relieved within minutes of consuming sugar in any form, such as candy, glucose tablets, or a sweet drink, such as a glass of fruit juice. People with recurring episodes of hypoglycemia, especially those with diabetes, often prefer to carry glucose tablets because the tablets take effect quickly and provide a consistent amount of sugar. People with hypoglycemia, whether or not they have diabetes, may benefit from consuming sugar followed by a food that provides longer-lasting carbohydrates (such as bread or crackers). When hypoglycemia is severe or prolonged and taking sugar by mouth is not possible, doctors quickly give glucose intravenously to prevent brain damage.
People who are known to be at risk of episodes of severe hypoglycemia may keep glucagon on hand for emergencies. Glucagon administration stimulates the liver to release large amounts of glucose. It is given by injection and generally restores blood glucose to an adequate level within 5 to 15 minutes. Glucagon kits are easy to use, and family members can be trained to administer the glucagon.
If a drug is causing hypoglycemia, the dose is adjusted or the drug is changed. Insulin-producing tumors should be removed surgically. However, because these tumors are small and difficult to locate, a specialist should do the surgery. Before surgery, the person may be given a drug such as octreotide or diazoxide to control symptoms. Sometimes more than one tumor is present, and if the surgeon does not find them all, a second operation may be necessary.
People who do not have diabetes but are prone to hypoglycemia often can avoid episodes by eating frequent small meals rather than the usual three meals a day.
Limiting intake of carbohydrates, especially simple sugars, is sometimes advocated to prevent "reactive hypoglycemia" (after a meal) but is of unclear benefit.
Last full review/revision July 2013 by Preeti Kishore, MD