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 often no visible symptoms of hyperinsulinemia unless hypoglycaemia (low blood sugar) is present.

Some patients may experience a variety of symptoms when hypoglycaemia is present, including:

- Temporary muscle weakness

- Brain fog

- Fatigue

- Temporary thought disorder, or inability to concentrate

- Visual problems such as blurred vision or double vision

- Headaches

- Shaking/Trembling

- Thirst

If a person experiences any of these symptoms, a visit to a qualified medical practitioner is advised, and diagnostic blood testing may be required.

There are several genetic forms of hyperinsulinemic hypoglycemia:

Hyperinsulinism due to reduced insulin sensitivity is usually asymptomatic. In contrast, hyperinsulinemic hypoglycemia can produce any of the entire range of hypoglycemic symptoms, from shakiness and weakness, to seizures or coma.

Although many factors influence insulin secretion, the most important control is the amount of glucose moving from the blood into the beta cells of the pancreas. In healthy people, even small rises in blood glucose result in increased insulin secretion. As long as the pancreatic beta cells are able to sense the glucose level and produce insulin, the amount of insulin secreted is usually the amount required to maintain a fasting blood glucose between 70 and 100 mg/dL (3.9-5.6 mmol/L) and a non-fasting glucose level below 140 mg/dL (<7.8 mmol/L).

When liver cells and other cells that remove glucose from the blood become less sensitive (more resistant) to the insulin, the pancreas increases secretion and the level of insulin in the blood rises. This increased secretion can compensate for reduced sensitivity for many years, with maintenance of normal glucose levels. However, if insulin resistance worsens or insulin secretion ability declines, the glucose levels will begin to rise. Persistent elevation of glucose levels is termed diabetes mellitus.

Typical fasting insulin levels found in this type of hyperinsulinism are above 20 μU/mL. When resistance is severe, levels can exceed 100 μU/mL.

In addition to being a risk factor for type 2 diabetes, hyperinsulinism due to insulin resistance may increase blood pressure and contribute to hypertension by direct action on vascular endothelial cells (the cells lining blood vessels). Hyperinsulinism has also been implicated as a contributing factor in the excessive production of androgens in polycystic ovary syndrome.

The principal treatments of hyperinsulinism due to insulin resistance are measures that improve insulin sensitivity, such as weight loss, physical exercise, and drugs such as thiazolidinediones or metformin.

Hypoglycemia in early infancy can cause jitteriness, lethargy, unresponsiveness, or seizures. The most severe forms may cause macrosomia in utero, producing a large birth weight, often accompanied by abnormality of the pancreas. Milder hypoglycemia in infancy causes hunger every few hours, with increasing jitteriness or lethargy. Milder forms have occasionally been detected by investigation of family members of infants with severe forms, adults with the mildest degrees of congenital hyperinsulinism have a decreased tolerance for prolonged fasting. Other presentations are:

The variable ages of presentations and courses suggest that some forms of congenital hyperinsulinism, especially those involving abnormalities of K channel function, can worsen or improve with time the potential harm from hyperinsulinemic hypoglycemia depends on the severity, and duration. Children who have recurrent hyperinsulinemic hypoglycemia in infancy can suffer harm to the brain

Possible causes include:

- Neoplasm

- Pancreatic cancer

- Polycystic ovary syndrome (PCOS)

- Trans fats

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

These depend on poorly understood variations in individual biology and consequently may not be found with all people diagnosed with insulin resistance.

- Increased hunger

- Lethargy (tiredness)

- Brain fogginess and inability to focus

- High blood sugar

- Weight gain, fat storage, difficulty losing weight – for most people, excess weight is from high subcutaneous fat storage; the fat in IR is generally stored in and around abdominal organs in both males and females; it is currently suspected that hormones produced in that fat are a precipitating cause of insulin resistance

- Increased blood cholesterol levels

- Increased blood pressure; many people with hypertension are either diabetic or pre-diabetic and have elevated insulin levels due to insulin resistance; one of insulin's effects is to control arterial wall tension throughout the body

Congenital hyperinsulinism is a medical term referring to a variety of congenital disorders in which hypoglycemia is caused by excessive insulin secretion. Congenital forms of hyperinsulinemic hypoglycemia can be transient or persistent, mild or severe. These conditions are present at birth and most become apparent in early infancy. Mild cases can be treated by frequent feedings, more severe cases can be controlled by medications that reduce insulin secretion or effects

Oxyhyperglycemia is a special type of impaired glucose tolerance characterized by a rapid and transient hyperglycemia (i.e. rise in blood glucose) spike after an oral intake of glucose, the peak of this spike being high enough to cause transient, symptom free glycosuria (i.e. detectable glucose in urine), but this hyperglycemia reverses rapidly and may even go to hypoglycemia in the later phase. This sharp downstroke overshooting towards hypoglycemia distinguishes this pathologic phenomenon from the artificial hyperglycemia inducible by an intravenous bolus dose of a large amount of glucose solution. Early dumping syndrome patients usually have oxyhyperglycemia associated with any meal or OGTT.

The Greek root "oxy" means "sharp" or "pointy". The OGTT curve in this condition appears sharp and somewhat pointy (at least relative to the other forms of hyperglycemia)- hence this name.

Dorlands dictionary defines oxyhyperglycemia as:

A blood level of approximately 180 mg/dL is the renal glucose threshold below which all glucose is reabsorbed from glomerular filtrate. But at blood concentrations above the renal threshold sugar starts appearing in the urine.

Oxyhyperglycemia, like other forms of Impaired glucose tolerance has also been suggested to be a prediabetic condition

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.

Symptoms vary according to individuals' hydration level and sensitivity to the rate and/or magnitude of decline of their blood glucose concentration.

A crash is usually felt within four hours or less of heavy carbohydrate consumption. Symptoms of reactive hypoglycemia include:

- double vision or blurry vision

- unclear thinking

- insomnia

- heart palpitation or fibrillation

- fatigue

- dizziness

- light-headedness

- sweating

- headaches

- depression

- nervousness

- muscle twitches

- irritability

- tremors

- flushing

- craving sweets

- increased appetite

- rhinitis

- nausea, vomiting

- panic attack

- numbness/coldness in the extremities

- confusion

- irrationality

- bad temper

- paleness

- cold hands

- disorientation

- the need to sleep or 'crash'

- coma can be a result in severe untreated episodes

The majority of these symptoms, often correlated with feelings of hunger, mimic the effect of inadequate sugar intake as the biology of a crash is similar in itself to the body’s response to low blood sugar levels following periods of glucose deficiency.

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.

Insulin resistance (IR) is a pathological condition in which cells fail to respond normally to the hormone insulin. The body produces insulin when glucose starts to be released into the bloodstream from the digestion of carbohydrates in the diet. Normally this insulin response triggers glucose being taken into body cells, to be used for energy, and inhibits the body from using fat for energy. The concentration of glucose in the blood decreases as a result, staying within the normal range even when a large amount of carbohydrates is consumed. When the body produces insulin under conditions of insulin resistance, the cells are resistant to the insulin and are unable to use it as effectively, leading to high blood sugar. Beta cells in the pancreas subsequently increase their production of insulin, further contributing to a high blood insulin level. This often remains undetected and can contribute to the development of type 2 diabetes or latent autoimmune diabetes of adults. Although this type of chronic insulin resistance is harmful, during acute illness it is actually a well-evolved protective mechanism. Recent investigations have revealed that insulin resistance helps to conserve the brain's glucose supply by preventing muscles from taking up excessive glucose. In theory, insulin resistance should even be strengthened under harsh metabolic conditions such as pregnancy, during which the expanding fetal brain demands more glucose.

People who develop type 2 diabetes usually pass through earlier stages of insulin resistance and prediabetes, although those often go undiagnosed. Insulin resistance is a syndrome (a set of signs and symptoms) resulting from reduced insulin activity; it is also part of a larger constellation of symptoms called the metabolic syndrome.

Insulin resistance may also develop in patients who have recently experienced abdominal or bariatric procedures. This acute form of insulin resistance that may result post-operatively tends to increase over the short term, with sensitivity to insulin typically returning to patients after about five days.

Chronic Somogyi rebound is a contested explanation of phenomena of elevated blood sugars in the morning. Also called the Somogyi effect and posthypoglycemic hyperglycemia, it is a rebounding high blood sugar that is a response to low blood sugar. When managing the blood glucose level with insulin injections, this effect is counter-intuitive to insulin users who experience high blood sugar in the morning as a result of an overabundance of insulin at night.

This theoretical phenomenon was named after Michael Somogyi, a Hungarian-born professor of biochemistry at the Washington University and Jewish Hospital of St. Louis, who prepared the first insulin treatment given to a child with diabetes in the USA in October 1922. Somogyi showed that excessive insulin makes diabetes unstable and first published his findings in 1938.

Compare with the dawn phenomenon, which is a morning rise in blood sugar in response to waning insulin and a growth hormone surge (that further antagonizes insulin).

Some of the subtypes of Glycogen storage disease show ketotic hypoglycemia after fasting periods. Especially Glycogen storage disease type IX can be a common cause for ketotic hypoglycemia, with the most common sub-type IXa mainly affecting boys.

A person with type 1 diabetes should balance insulin delivery to manage their blood glucose level. Occasionally, insufficient insulin can result in hyperglycemia. The appropriate response is to take a correction dose of insulin to reduce the blood sugar level and to consider adjusting the insulin regimen to deliver additional insulin in the future to prevent hyperglycemia. Conversely, excessive insulin delivery may result in hypoglycemia. The appropriate response is to treat the hypoglycemia and to consider adjusting the regimen to reduce insulin in the future.

Somogyi and others have claimed that if prolonged hypoglycemia is untreated, then stress due to low blood sugar can result in a high blood glucose rebound. The physiological mechanisms driving the rebound are defensive. When the blood glucose level falls below normal, the body responds by releasing the endocrine hormone glucagon as well as the stress hormones epinephrine, cortisol and growth hormone. Glucagon facilitates release of glucose from the liver that raises the blood glucose immediately, and the stress hormones cause insulin resistance for several hours, sustaining the elevated blood sugar.

Diabetic hypoglycemia can be mild, recognized easily by the patient, and reversed with a small amount of carbohydrates eaten or drunk, or it may be severe enough to cause unconsciousness requiring intravenous dextrose or an injection of glucagon. Severe hypoglycemic unconsciousness is one form of diabetic coma. A common medical definition of severe hypoglycemia is "hypoglycemia severe enough that the person needs assistance in dealing with it". A co-morbidity is the issue of hypoglycemia unawareness. Recent research using machine learning methods have proved to be successful in predicting such severe hypoglycemia episodes.

Symptoms of diabetic hypoglycemia, when they occur, are those of hypoglycemia: neuroglycopenic, adrenergic, and abdominal. Symptoms and effects can be mild, moderate or severe, depending on how low the glucose falls and a variety of other factors. It is rare but possible for diabetic hypoglycemia to result in brain damage or death. Indeed, an estimated 2-4% of deaths of people with type 1 diabetes mellitus have been attributed to hypoglycemia.

In North America a mild episode of diabetic hypoglycemia is sometimes termed a "low" or an "insulin reaction," and in Europe a "hypo", although all of these terms are occasionally used interchangeably in North America, Europe, Australia and New Zealand. A severe episode is sometimes also referred to as "insulin shock".

In a counter-intuitive manifestation, hypoglycemia can trigger a Somogyi effect, resulting in a rebounding high blood sugar or hyperglycemia.

A commonly used "number" to define the lower limit of normal glucose is 70 mg/dl (3.9 mmol/l), though in someone with diabetes, hypoglycemic symptoms can sometimes occur at higher glucose levels, or may fail to occur at lower. Some textbooks for nursing and pre-hospital care use the range 80 mg/dl to 120 mg/dl (4.4 mmol/l to 6.7 mmol/l). This variability is further compounded by the imprecision of glucose meter measurements at low levels, or the ability of glucose levels to change rapidly.

Oxyhyperglycemia is most commonly caused by early dumping syndrome, but it can rarely caused by other conditions like Graves' disease. It was first described by Lawrence et al. in 1936 as often happening after gastroenterostomy. It is seen in most forms of gastrectomy, gastric bypass and gastrostomy procedures, all of which are surgical causes of dumping syndrome.

Cats will generally show a gradual onset of the disease over a few weeks or months, and it may escape notice for even longer.

The first outward symptoms are a sudden weight loss (or occasionally gain), accompanied by excessive drinking and urination; for example, cats can appear to develop an obsession with water and lurk around faucets or water bowls. Appetite is suddenly either ravenous (up to three-times normal) or absent. These symptoms arise from the body being unable to use glucose as an energy source.

A fasting glucose blood test will normally be suggestive of diabetes at this point. The same home blood test monitors used in humans are used on cats, usually by obtaining blood from the ear edges or paw pads. As the disease progresses, ketone bodies will be present in the urine, which can be detected with the same urine strips as in humans.

In the final stages, the cat starts wasting and the body will breaking down its own fat and muscle to survive. Lethargy or limpness, and acetone-smelling breath are acute symptoms of ketoacidosis and/or dehydration and is a medical emergency.

Untreated, diabetes leads to coma and then death.

Too little insulin over time can cause tissue starvation (as glucose can't reach the brain or body). In combination with dehydration, fasting, infection, or other body stresses, this can turn over a few hours into diabetic ketoacidosis, a medical emergency with a high fatality rate, that cannot be treated at home. Many undiagnosed diabetic cats first come to the vet in this state, since they haven't been receiving insulin. Symptoms include lethargy, acetone or fruity smell on breath, shortness of breath, high blood sugar, huge thirst drive. Emergency care includes fluid therapy, insulin, management of presenting symptoms and 24-hour hospitalization.

Patients with insulinomas usually develop neuroglycopenic symptoms. These include recurrent headache, lethargy, diplopia, and blurred vision, particularly with exercise or fasting. Severe hypoglycemia may result in seizures, coma, and permanent neurological damage. Symptoms resulting from the catecholaminergic response to hypoglycemia (i.e. tremulousness, palpitations, tachycardia, sweating, hunger, anxiety, nausea) are not as common. Sudden weight gain is sometimes seen.

Reactive hypoglycemia, postprandial hypoglycemia, or sugar crash is a term describing recurrent episodes of symptomatic hypoglycemia occurring within 4 hours after a high carbohydrate meal in people who do not have diabetes.

The condition is related to homeostatic systems utilised by the body to control blood sugar levels. It is variously described as a sense of tiredness, lethargy, irritation, or hangover, although the effects can be less if one has undertaken a lot of physical activity within the next few hours after consumption.

The alleged mechanism for the feeling of a crash is correlated with an abnormally rapid rise in blood glucose after eating. This normally leads to insulin secretion (known as an "insulin spike"), which in turn initiates rapid glucose uptake by tissues either accumulating it as glycogen or utilizing it for energy production. The consequent fall in blood glucose is indicated as the reason for the "sugar crash".. A deeper cause might be hysteresis effect of insulin action, i.e., the effect of insulin is still prominent even if both plasma glucose and insulin levels were already low, causing a plasma glucose level eventually much lower than the baseline level.

Sugar crashes are not to be confused with the after-effects of consuming large amounts of "protein", which produces fatigue akin to a sugar crash, but are instead the result of the body prioritising the digestion of ingested food.

The prevalence of this condition is difficult to ascertain because a number of stricter or looser definitions have been used. It is recommended that the term reactive hypoglycemia be reserved for the pattern of postprandial hypoglycemia which meets the Whipple criteria (symptoms correspond to measurably low glucose and are relieved by raising the glucose), and that the term idiopathic postprandial syndrome be used for similar patterns of symptoms where abnormally low glucose levels at the time of symptoms cannot be documented.

To assist diagnosis, a doctor can order an HbA1c test, which measures the blood sugar average over the two or three months before the test. The more specific 6-hour glucose tolerance test can be used to chart changes in the patient's blood sugar levels before ingestion of a special glucose drink and at regular intervals during the six hours following to see if an unusual rise or drop in blood glucose levels occurs.

According to the U.S. National Institute of Health (NIH), a blood glucose level below 70 mg/dL (3.9 mmol/L) at the time of symptoms followed by relief after eating confirms a diagnosis for reactive hypoglycemia.