Not all of the above manifestations occur in every case of hypoglycemia. There is no consistent order to the appearance of the symptoms. Specific manifestations vary by age and by the severity of the hypoglycemia. In older children and adults, moderately severe hypoglycemia can resemble mania, mental illness, drug intoxication, or drunkenness. In the elderly, hypoglycemia can produce focal stroke-like effects or a hard-to-define malaise. The symptoms of a single person do tend to be similar from episode to episode.

In the large majority of cases, hypoglycemia severe enough to cause seizures or unconsciousness can be reversed without obvious harm to the brain. Cases of death or permanent neurological damage occurring with a single episode have usually involved prolonged, untreated unconsciousness, interference with breathing, severe concurrent disease, or some other type of vulnerability. Nevertheless, brain damage or death has occasionally resulted from severe hypoglycemia.

Neuroglycopenia is a medical term that refers to a shortage of glucose (glycopenia) in the brain, usually due to hypoglycemia. Glycopenia affects the function of neurons, and alters brain function and behavior. Prolonged or recurrent neuroglycopenia can result in loss of consciousness, damage to the brain, and eventual death.

Hypoglycemic symptoms and manifestations can be divided into those produced by the counterregulatory hormones (epinephrine/adrenaline and glucagon) triggered by the falling glucose, and the neuroglycopenic effects produced by the reduced brain sugar.

- Shakiness, anxiety, nervousness

- Palpitations, tachycardia

- Sweating, feeling of warmth (sympathetic muscarinic rather than adrenergic)

- Pallor, coldness, clamminess

- Dilated pupils (mydriasis)

- Hunger, borborygmus

- Nausea, vomiting, abdominal discomfort

- Headache

Not all of the above manifestations occur in every case of hypoglycemia. There is no consistent order to the appearance of the symptoms, if symptoms even occur. Specific manifestations may also vary by age, by severity of the hypoglycemia and the speed of the decline. In young children, vomiting can sometimes accompany morning hypoglycemia with ketosis. In older children and adults, moderately severe hypoglycemia can resemble mania, mental illness, drug intoxication, or drunkenness. In the elderly, hypoglycemia can produce focal stroke-like effects or a hard-to-define malaise. The symptoms of a single person may be similar from episode to episode, but are not necessarily so and may be influenced by the speed at which glucose levels are dropping, as well as previous incidents.

In newborns, hypoglycemia can produce irritability, jitters, myoclonic jerks, cyanosis, respiratory distress, apneic episodes, sweating, hypothermia, somnolence, hypotonia, refusal to feed, and seizures or "spells." Hypoglycemia can resemble asphyxia, hypocalcemia, sepsis, or heart failure.

In both young and old patients, the brain may habituate to low glucose levels, with a reduction of noticeable symptoms despite neuroglycopenic impairment. In insulin-dependent diabetic patients this phenomenon is termed "hypoglycemia unawareness" and is a significant clinical problem when improved glycemic control is attempted. Another aspect of this phenomenon occurs in type I glycogenosis, when chronic hypoglycemia before diagnosis may be better tolerated than acute hypoglycemia after treatment is underway.

Hypoglycemic symptoms can also occur when one is sleeping. Examples of symptoms during sleep can include damp bed sheets or clothes from perspiration. Having nightmares or the act of crying out can be a sign of hypoglycemia. Once the individual is awake they may feel tired, irritable, or confused and these may be signs of hypoglycemia as well.

In nearly all cases, hypoglycemia that is severe enough to cause seizures or unconsciousness can be reversed without obvious harm to the brain. Cases of death or permanent neurological damage occurring with a single episode have usually involved prolonged, untreated unconsciousness, interference with breathing, severe concurrent disease, or some other type of vulnerability. Nevertheless, brain damage or death has occasionally resulted from severe hypoglycemia.

Research in healthy adults shows that mental efficiency declines slightly but measurably as blood glucose falls below 3.6 mM (65 mg/dL). Hormonal defense mechanisms (adrenaline and glucagon) are normally activated as it drops below a threshold level (about 55 mg/dL (3.0 mM) for most people), producing the typical hypoglycemic symptoms of shakiness and dysphoria. Obvious impairment may not occur until the glucose falls below 40 mg/dL (2.2 mM), and many healthy people may occasionally have glucose levels below 65 in the morning without apparent effects. Since the brain effects of hypoglycemia, termed neuroglycopenia, determine whether a given low glucose is a "problem" for that person, most doctors use the term "hypoglycemia" only when a moderately low glucose level is accompanied by symptoms or brain effects.

Determining the presence of both parts of this definition is not always straightforward, as hypoglycemic symptoms and effects are vague and can be produced by other conditions; people with recurrently low glucose levels can lose their threshold symptoms so that severe neuroglycopenic impairment can occur without much warning, and many measurement methods (especially glucose meters) are imprecise at low levels.

It may take longer to recover from severe hypoglycemia with unconsciousness or seizure even after restoration of normal blood glucose. When a person has not been unconscious, failure of carbohydrate to reverse the symptoms in 10–15 minutes increases the likelihood that hypoglycemia was not the cause of the symptoms. When severe hypoglycemia has persisted in a hospitalized person, the amount of glucose required to maintain satisfactory blood glucose levels becomes an important clue to the underlying etiology. Glucose requirements above 10 mg/kg/minute in infants, or 6 mg/kg/minute in children and adults are strong evidence for hyperinsulinism. In this context this is referred to as the "glucose infusion rate" (GIR). Finally, the blood glucose response to glucagon given when the glucose is low can also help distinguish among various types of hypoglycemia. A rise of blood glucose by more than 30 mg/dl (1.70 mmol/l) suggests insulin excess as the probable cause of the hypoglycemia.

Ketotic hypoglycemia more commonly refers to a common but mysterious "disease" of recurrent hypoglycemic symptoms with ketosis in young children. The cause and the homogeneity of the condition remain uncertain, but a characteristic presentation, precipitating factors, diagnostic test results, treatment, and natural history can be described. It remains one of the more common causes of hypoglycemia in the age range.

The typical patient with ketotic hypoglycemia is a young child between the ages of 10 months and 4 years. Episodes nearly always occur in the morning after an overnight fast, often one that is longer than usual. Symptoms include those of neuroglycopenia, ketosis, or both. The neuroglycopenic symptoms usually include lethargy and malaise, but may include unresponsiveness or seizures. The principal symptoms of ketosis are anorexia, abdominal discomfort, and nausea, sometimes progressing to vomiting.

If severe, parents usually take the child to a local emergency department, where blood is drawn. The glucose is usually found to be between 35 and 60 mg/dl (1.8-3.1 mMol/L). The total CO is usually somewhat low as well, (14-19 mMol/L is typical), and if urine is obtained, high levels of ketones are discovered. Ketones can also be measured in the blood at the bedside (Medisense glucometer). Other routine tests are normal. If given intravenous fluids with saline and dextrose, the child improves dramatically and is usually restored to normal health within a few hours. These symptoms are normally seen because of the child being unadapted to using fat as energy, typically when the child's daily glucose intake might be too high (more than 50g/day for a child). This is also associated with fluctuant glycemia throughout the day.

A first episode is usually attributed to a viral infection or acute gastroenteritis. However, in most of these children one or more additional episodes recur over next few years and become immediately recognizable to the parents. In mild cases, carbohydrates and a few hours of sleep will be enough to end the symptoms. Thus said, the required amount of carbohydrate intake of a child, as well as for an adult is close to 0, because the liver can supply the required glucose quantity needed for the body through gluconeogenesis.

Precipitating factors, conditions that trigger an episode, may include extended fasting (e.g., missing supper the night before), a low carbohydrate intake the previous day (e.g., a hot dog without a bun), or stress such as a viral infection. Most children affected by ketotic hypoglycemia have a slender build, many with a weight percentile below height percentile, though without other evidence of malnutrition. Overweight children are rarely affected.

Manifestations of hyperinsulinemic hypoglycemia vary by age and severity of the hypoglycemia. In general, most signs and symptoms can be attributed to (1) the effects on the brain of insufficient glucose (neuroglycopenia) or (2) to the adrenergic response of the autonomic nervous system to hypoglycemia. A few miscellaneous symptoms are harder to attribute to either of these causes. In most cases, all effects are reversed when normal glucose levels are restored.

There are uncommon cases of more persistent harm, and rarely even death due to severe hypoglycemia of this type. One reason hypoglycemia due to excessive insulin can be more dangerous is that insulin lowers the available amounts of most alternate brain fuels, such as ketones. Brain damage of various types ranging from stroke-like focal effects to impaired memory and thinking can occur. Children who have prolonged or recurrent hyperinsulinemic hypoglycemia in infancy can suffer harm to their brains and may be developmentally delayed.

There are several genetic forms of hyperinsulinemic hypoglycemia: