Hypoglycemia or hypoglycaemia is the medical term for a pathologic state produced by a lower than normal level of glucose (sugar) in the blood. The term hypoglycemia literally means "under-sweet blood" (Gr. hypo-, glykys, haima). The term also refers to a putative condition that is scientifically disputed and which is perhaps more properly considered as a part of "alternative" medicine. This is covered at the end of this article.

Hypoglycemia can produce a variety of symptoms and effects but the principal problems arise from an inadequate supply of glucose as fuel to the brain, resulting in impairment of function (neuroglycopenia). Derangements of function can range from vaguely "feeling bad" to coma and (rarely) permanent brain damage or death. Hypoglycemia can arise from many causes and can occur at any age.

The most common forms of moderate and severe hypoglycemia occur as a complication of treatment of diabetes mellitus with insulin or certain oral medications. Hypoglycemia is usually treated by the ingestion or administration of dextrose, or foods digestible to glucose.

Endocrinologists (specialists in hormones, including those which regulate glucose metabolism) typically consider the following criteria (referred to as Whipple's triad) as proving that individual's symptoms can be attributed to hypoglycemia:

1. Symptoms known to be caused by hypoglycemia
2. Low glucose at the time the symptoms occur
3. Reversal or improvement of symptoms or problems when the glucose is restored to normal

However, not everyone has accepted these suggested diagnostic criteria, and even the level of glucose low enough to define hypoglycemia has been a source of controversy in several contexts. For many purposes, plasma glucose levels below 70 mg/dl or 3.9 mmol/L are considered hypoglycemic; these issues are detailed below.

Defining hypoglycemia

No single glucose value alone serves to define the medical condition termed hypoglycemia for all people and purposes. Throughout the 24 hour cycles of eating, digestion, and fasting, blood plasma glucose levels are generally maintained within a range of 70-150 mg/dL (3.9-7.8 mmol/L) for healthy humans. Although 60 or 70 mg/dL (3.3 or 3.9 mmol/L) is commonly cited as the lower limit of normal glucose, different values (typically below 40, 50, 60, or 70 mg/dL) have been defined as low for different populations, clinical purposes, or circumstances.

The precise level of glucose considered low enough to define hypoglycemia is dependent on (1) the measurement method, (2) the age of the person, (3) presence or absence of effects, and (4) the purpose of the definition. While there is no disagreement as to the normal range of blood sugar, debate continues as to what degree of hypoglycemia warrants medical evaluation or treatment, or can cause harm.