Hypoglycemia is abnormally low glucose (blood sugar). It can occur when a person’s levels of insulin and glucose are out of balance.

As with hyperglycemia (high blood glucose), low blood glucose can happen to anyone with diabetes. It occurs most often in people who use insulin. All patients with type 1 diabetes and some patients with type 2 diabetes use insulin. Those who take certain other diabetes medications (antidiabetic agents) are also at an increased risk.

When a person eats food, much of it is broken down into glucose, the form of sugar that is used to fuel the body. The glucose is passed into the bloodstream where it is used by cells for energy. For the glucose to reach the cells, the hormone insulin must be present.

Insulin is produced by beta cells in the pancreas, a glandular organ located behind the stomach. As the insulin enters the blood, it stimulates the receptors in muscle, fat and other cells to remove glucose from the bloodstream. Hypoglycemia occurs when the body’s glucose is used up too quickly, when glucose is released into the bloodstream too slowly or when too much insulin is released into the bloodstream.

The American Diabetes Association (ADA) has established guidelines for glucose goals for people with diabetes. The guidelines list normal blood glucose levels for whole blood and plasma. Different tests measure glucose in different ways. Most glucose meters measure the glucose level in a blood sample as whole blood (with all components intact). Most laboratory tests measure the glucose in plasma (the fluid portion of the blood that contains water, minerals and proteins). In addition, some glucose meters translate whole blood readings into plasma readings. Because plasma readings are higher than whole blood glucose measurements, it is important for patients to know whether their test results are presented as whole blood or plasma measurements.

The ADA’s guidelines for glucose goals for people with diabetes are:  

However, these ranges are not appropriate for everyone. Target blood glucose range is a personalized number given by a physician. It is usually based on factors such as age and presence of diabetic complications or other medical conditions. Hypoglycemia unawareness (a condition in which the normal symptoms associated with hypoglycemia are not felt or noticed) is also taken into consideration when determining the patient’s range.  People with diabetes should aim to keep their glucose levels within their personalized range by following their diabetes management plan.

The normal body can detect when it has enough insulin and will automatically stop releasing it. People with diabetes who take insulin or some other diabetes medications (antidiabetic agents) must calculate how much medicine they will need to counteract the effects of exercise and diet on their glucose levels. This is an important factor because the body's mechanisms to regulate insulin release in individuals with diabetes are defective. Low glucose usually occurs just before meals, when insulin is peaking, or during and after exercise. Too little food or too much exercise in comparison to medication dosage may result in low glucose.

Low glucose is also common in the middle of the night. Nighttime hypoglycemia is dangerous because the patient may not be awake enough to feel the symptoms and could slip into a coma. Nighttime (nocturnal) hypoglycemia can be confirmed by testing blood glucose levels at 2 or 3 a.m. Patients may be encouraged to wake for such testing if they have any signs or symptoms of nighttime hypoglycemia. If there is evidence of nighttime hypoglycemia, the patient should immediately discuss it with the physician because changes in the diabetes management plan may be necessary.

Nocturnal hypoglycemia can cause rebound hyperglycemia the next morning. This is known as the Somogyi effect.

It is essential for people with diabetes to treat mild or moderate hypoglycemia quickly because it can rapidly develop into severe hypoglycemia, sometimes known as insulin shock. Severe hypoglycemia is the point at which people with diabetes are unable to treat their low blood glucose independently.

Low blood glucose first affects the autonomic nervous system. As glucose levels continue to drop, the brain begins to suffer from not receiving enough glucose, its major source of energy. Severe hypoglycemia can lead to seizures, unconsciousness, permanent brain damage and a potentially deadly diabetic coma.

The effects of low blood glucose on the autonomic nervous system and brain can increase a person’s risk of injuries from falls and motor vehicle accidents. It is advised that people with diabetes check their glucose level before driving, or engaging in other activities that require a high level of concentration, even if they do not suspect that it is low.

Patients should try to determine the cause of low glucose readings and report their findings to their physician. If low readings continue, a physician may make adjustments to the patient’s diabetes management plan, including changes in insulin dose, diet, or eating or exercise schedule. If poor control of glucose continues, other options may be available for such cases of unstable diabetes. 

Ketotic hypoglycemia is a rare childhood disorder combining ketosis (higher-than-normal levels of a waste product called ketones) and hypoglycemia. The condition is unrelated to diabetes.