The main risk factor is a history of diabetes mellitus type 2. Occasionally it may occur in those without a prior history of diabetes or those with diabetes mellitus type 1. Triggers include infections, stroke, trauma, certain medications, and heart attacks.

Other risk factors:

- Lack of sufficient insulin (but enough to prevent ketosis)

- Poor kidney function

- Poor fluid intake (dehydration)

- Older age (50–70 years)

- Certain medical conditions (cerebral vascular injury, myocardial infarction, sepsis)

- Some medications (glucocorticoids, beta-blockers, thiazide diuretics, calcium channel blockers, phenytoin)

Chronic hyperglycemia that persists even in fasting states is most commonly caused by diabetes mellitus. In fact, chronic hyperglycemia is the defining characteristic of the disease. Intermittent hyperglycemia may be present in prediabetic states. Acute episodes of hyperglycemia without an obvious cause may indicate developing diabetes or a predisposition to the disorder.

In diabetes mellitus, hyperglycemia is usually caused by low insulin levels (Diabetes mellitus type 1) and/or by resistance to insulin at the cellular level (Diabetes mellitus type 2), depending on the type and state of the disease. Low insulin levels and/or insulin resistance prevent the body from converting glucose into glycogen (a starch-like source of energy stored mostly in the liver), which in turn makes it difficult or impossible to remove excess glucose from the blood. With normal glucose levels, the total amount of glucose in the blood at any given moment is only enough to provide energy to the body for 20–30 minutes, and so glucose levels must be precisely maintained by the body's internal control mechanisms. When the mechanisms fail in a way that allows glucose to rise to abnormal levels, hyperglycemia is the result.

Ketoacidosis may be the first symptom of immune-mediated diabetes, particularly in children and adolescents. Also, patients with immune-mediated diabetes, can change from modest fasting hyperglycemia to severe hyperglycemia and even ketoacidosis as a result of stress or an infection.

Certain medications increase the risk of hyperglycemia, including corticosteroids, octreotide, beta blockers, epinephrine, thiazide diuretics, niacin, pentamidine, protease inhibitors, L-asparaginase, and some antipsychotic agents. The acute administration of stimulants such as amphetamine typically produces hyperglycemia; chronic use, however, produces hypoglycemia. Some of the newer psychotropic medications, such as Zyprexa (Olanzapine) and Cymbalta (Duloxetine), can also cause significant hyperglycemia.

Thiazides are used to treat type 2 diabetes but it also causes severe hyperglycemia.

Attacks of DKA can be prevented in those known to have diabetes to an extent by adherence to "sick day rules"; these are clear-cut instructions to person on how to treat themselves when unwell. Instructions include advice on how much extra insulin to take when sugar levels appear uncontrolled, an easily digestible diet rich in salt and carbohydrates, means to suppress fever and treat infection, and recommendations when to call for medical help.

People with diabetes can monitor their own ketone levels when unwell and seek help if they are elevated.

DKA most frequently occurs in those who already have diabetes, but it may also be the first presentation in someone who had not previously been known to be diabetic. There is often a particular underlying problem that has led to the DKA episode; this may be intercurrent illness (pneumonia, influenza, gastroenteritis, a urinary tract infection), pregnancy, inadequate insulin administration (e.g. defective insulin pen device), myocardial infarction (heart attack), stroke or the use of cocaine. Young people with recurrent episodes of DKA may have an underlying eating disorder, or may be using insufficient insulin for fear that it will cause weight gain.

Diabetic ketoacidosis may occur in those previously known to have diabetes mellitus type 2 or in those who on further investigations turn out to have features of type 2 diabetes (e.g. obesity, strong family history); this is more common in African, African-American and Hispanic people. Their condition is then labeled "ketosis-prone type 2 diabetes".

Drugs in the gliflozin class (SGLT2 inhibitors), which are generally used for type 2 diabetes, have been associated with cases of diabetic ketoacidosis where the blood sugars are not significantly elevated ("euglycemic DKA"). This may be because they were being used in people with type 1 diabetes, but in those with type 2 diabetes it may be as a result of an increase in glucagon levels.

HHS is usually precipitated by an infection, myocardial infarction, stroke or another acute illness. A relative insulin deficiency leads to a serum glucose that is usually higher than 33 mmol/L (600 mg/dL), and a resulting serum osmolarity that is greater than 320 mOsm. This leads to excessive urination (more specifically an osmotic diuresis), which, in turn, leads to volume depletion and hemoconcentration that causes a further increase in blood glucose level. Ketosis is absent because the presence of some insulin inhibits hormone-sensitive lipase mediated fat tissue breakdown.

In non-diabetic persons, ketonuria may occur during acute illness or severe stress. Approximately 15% of hospitalized patients may have ketonuria, even though they do not have diabetes. In a diabetic patient, ketone bodies in the urine suggest that the patient is not adequately controlled and that adjustments of medication, diet, or both should be made promptly. In the non diabetic patient, ketonuria reflects a reduced carbohydrate metabolism and an increased fat metabolism.

Diabetic coma is a reversible form of coma found in people with diabetes mellitus. It is a medical emergency.

Three different types of diabetic coma are identified:

1. Severe low blood sugar in a diabetic person

2. Diabetic ketoacidosis (usually type 1) advanced enough to result in unconsciousness from a combination of a severely increased blood sugar level, dehydration and shock, and exhaustion

3. Hyperosmolar nonketotic coma (usually type 2) in which an extremely high blood sugar level and dehydration alone are sufficient to cause unconsciousness.

In most medical contexts, the term diabetic coma refers to the diagnostical dilemma posed when a physician is confronted with an unconscious patient about whom nothing is known except that they have diabetes. An example might be a physician working in an emergency department who receives an unconscious patient wearing a medical identification tag saying DIABETIC. Paramedics may be called to rescue an unconscious person by friends who identify them as diabetic. Brief descriptions of the three major conditions are followed by a discussion of the diagnostic process used to distinguish among them, as well as a few other conditions which must be considered.

An estimated 2 to 15 percent of diabetics will suffer from at least one episode of diabetic coma in their lifetimes as a result of severe hypoglycemia.

Ketoacidosis is a metabolic state associated with high concentrations of ketone bodies, formed by the breakdown of fatty acids and the deamination of amino acids. The two common ketones produced in humans are acetoacetic acid and β-hydroxybutyrate.

Ketoacidosis is a pathological metabolic state marked by extreme and uncontrolled ketosis. In ketoacidosis, the body fails to adequately regulate ketone production causing such a severe accumulation of keto acids that the pH of the blood is substantially decreased. In extreme cases ketoacidosis can be fatal.

Ketoacidosis is most common in untreated type 1 diabetes mellitus, when the liver breaks down fat and proteins in response to a perceived need for respiratory substrate. Prolonged alcoholism may lead to alcoholic ketoacidosis.

Ketoacidosis can be smelled on a person's breath. This is due to acetone, a direct by-product of the spontaneous decomposition of acetoacetic acid. It is often described as smelling like fruit or nail polish remover. Ketosis may also give off an odor, but the odor is usually more subtle due to lower concentrations of acetone.

Treatment consists most simply of correcting blood sugar and insulin levels, which will halt ketone production. If the severity of the case warrants more aggressive measures, intravenous sodium bicarbonate infusion can be given to raise blood pH back to an acceptable range. However, serious caution must be exercised with IV sodium bicarbonate to avoid the risk of equally life-threatening hypernatremia.

Three common causes of ketoacidosis are alcohol, starvation, and diabetes, resulting in alcoholic ketoacidosis, starvation ketoacidosis, and diabetic ketoacidosis respectively.

In diabetic ketoacidosis, a high concentration of ketone bodies is usually accompanied by insulin deficiency, hyperglycemia, and dehydration. Particularly in type 1 diabetics the lack of insulin in the bloodstream prevents glucose absorption, thereby inhibiting the production of oxaloacetate through reduced levels of pyruvate, and can cause unchecked ketone body production (through fatty acid metabolism) potentially leading to dangerous glucose and ketone levels in the blood. Hyperglycemia results in glucose overloading the kidneys and spilling into the urine (transport maximum for glucose is exceeded). Dehydration results following the osmotic movement of water into urine (Osmotic diuresis), exacerbating the acidosis.

In alcoholic ketoacidosis, alcohol causes dehydration and blocks the first step of gluconeogenesis by depleting oxaloacetate. The body is unable to synthesize enough glucose to meet its needs, thus creating an energy crisis resulting in fatty acid metabolism, and ketone body formation.

A 1988 study over 41 months found that improved glucose control led to initial "worsening of complications" but was not followed by the expected improvement in complications. In 1993 it was discovered that the serum of diabetics with neuropathy is toxic to nerves, even if its blood sugar content is normal.

Research from 1995 also challenged the theory of hyperglycemia as the cause of diabetic complications. The fact that 40% of diabetics who carefully controlled their blood sugar nevertheless developed neuropathy made clear other factors were involved.

In a 2013 meta-analysis of 6 randomized controlled trials involving 27,654 patients, tight blood glucose control reduced the risk for some macrovascular and microvascular events but without effect on all-cause mortality and cardiovascular mortality.

People with type 1 diabetes mellitus who must take insulin in full replacement doses are most vulnerable to episodes of hypoglycemia. It is usually mild enough to reverse by eating or drinking carbohydrates, but blood glucose occasionally can fall fast enough and low enough to produce unconsciousness before hypoglycemia can be recognized and reversed. Hypoglycemia can be severe enough to cause unconsciousness during sleep. Predisposing factors can include eating less than usual or prolonged exercise earlier in the day. Some people with diabetes can lose their ability to recognize the symptoms of early hypoglycemia.

Unconsciousness due to hypoglycemia can occur within 20 minutes to an hour after early symptoms and is not usually preceded by other illness or symptoms. Twitching or convulsions may occur. A person unconscious from hypoglycemia is usually pale, has a rapid heart beat, and is soaked in sweat: all signs of the adrenaline response to hypoglycemia. The individual is not usually dehydrated and breathing is normal or shallow. Their blood sugar level, measured by a glucose meter or laboratory measurement at the time of discovery, is usually low but not always severely, and in some cases may have already risen from the nadir that triggered the unconsciousness.

Unconsciousness due to hypoglycemia is treated by raising the blood glucose with intravenous glucose or injected glucagon.

Research from 2007 suggested that in type 1 diabetics, the continuing autoimmune disease which initially destroyed the beta cells of the pancreas may also cause retinopathy, neuropathy, and nephropathy.

In 2008 it was even suggested to treat retinopathy with drugs to suppress the abnormal immune response rather than by blood sugar control.

Screening for ketonuria is done frequently for acutely ill patients, presurgical patients, and pregnant women. Any diabetic patient who has elevated levels of blood and urine glucose should be tested for urinary ketones. In addition, when diabetic treatment is being switched from insulin to oral hypoglycemic agents, the patient's urine should be monitored for ketonuria. The development of ketonuria within 24 hours after insulin withdrawal usually indicates a poor response to the oral hypoglycemic agents. Diabetic patients should have their urine tested regularly for glucose and ketones, particularly when acute infection or other illness develops.

In conditions associated with acidosis, urinary ketones are tested to assess the severity of acidosis and to monitor treatment response. Urine ketones appear before there is any significant increase in blood ketones; therefore, urine ketone measurement is especially helpful in emergency situations.

Ketosis is deliberately induced by use of a ketogenic diet as a medical intervention in cases of intractable epilepsy. Other uses of low-carbohydrate diets remain controversial. Carbohydrate deprivation to the point of ketosis has been argued to have both negative and positive effects on health.

Ketone bodies are acidic, but acid-base homeostasis in the blood is normally maintained through bicarbonate buffering, respiratory compensation to vary the amount of CO in the bloodstream, hydrogen ion absorption by tissue proteins and bone, and renal compensation through increased excretion of dihydrogen phosphate and ammonium ions. Prolonged excess of ketone bodies can overwhelm normal compensatory mechanisms, defined as acidosis if blood pH falls below 7.35.

There are two major causes of ketoacidosis:

- Most commonly, ketoacidosis is diabetic ketoacidosis (DKA), resulting from increased fat metabolism due to a shortage of insulin. It is associated primarily with type I diabetes, and may result in a diabetic coma if left untreated.

- Alcoholic ketoacidosis (AKA) presents infrequently, but can occur with acute alcohol intoxication, most often following a binge in alcoholics with acute or chronic liver or pancreatic disorders. Alcoholic ketoacidosis occurs more frequently following methanol or ethylene glycol intoxication than following intoxication with uncontaminated ethanol.

A mild acidosis may result from prolonged fasting or when following a ketogenic diet or a very low calorie diet.

Causes of increased anion gap include:

- Lactic acidosis

- Ketoacidosis

- Chronic renal failure (accumulation of sulfates, phosphates, urea)

- Intoxication:

- Organic acids, salicylates, ethanol, methanol, formaldehyde, ethylene glycol, paraldehyde, isoniazid

- Sulfates, metformin

- Massive rhabdomyolysis

A mnemonic can also be used - MUDPILES

- M-Methanol

- U-Uremia (chronic kidney failure)

- D-Diabetic ketoacidosis

- P-Paraldehyde

- I-Infection, Iron, Isoniazid, Inborn errors of metabolism

- L-Lactic acidosis

- E-Ethylene glycol (Note: Ethanol is sometimes included in this mnemonic, as well, although the acidosis caused by ethanol is actually primarily due to the increased production of lactic acid found in such intoxication.)

- S-Salicylates

Causes include:

The newest mnemonic was proposed in "The Lancet" reflecting current causes of anion gap metabolic acidosis:

- G — glycols (ethylene glycol & propylene glycol)

- O — oxoproline, a metabolite of paracetamol

- L — L-lactate, the chemical responsible for lactic acidosis

- D — D-lactate

- M — methanol

- A — aspirin

- R — renal failure

- K — ketoacidosis, ketones generated from starvation, alcohol, and diabetic ketoacidosis

The mnemonic MUDPILES is commonly used to remember the causes of increased anion gap metabolic acidosis.

- M — Methanol

- U — Uremia (chronic kidney failure)

- D — Diabetic ketoacidosis

- P — Paracetamol, Propylene glycol (used as an inactive stabilizer in many medications; historically, the "P" also stood for Paraldehyde, though this substance is not commonly used today)

- I — Infection, Iron, Isoniazid (which can cause lactic acidosis in overdose), Inborn errors of metabolism (an especially important consideration in pediatric patients)

- L — Lactic acidosis

- E — Ethylene glycol (Note: Ethanol is sometimes included in this mnemonic as well, although the acidosis caused by ethanol is actually primarily due to the increased production of lactic acid found in such intoxication.)

- S — Salicylates

Another frequently used mnemonic is KARMEL.

- K — Ketoacidosis

- A — aspirin

- R — Renal failure

- M — Methanol

- E — Ethylene glycol

- L — Lactic acidosis

Another frequently used mnemonic is KULT.

- K — Ketoacidosis (DKA, AKA)

- U — Uremia

- L — Lactic acidosis

- T — Toxins (Ethylene glycol, methanol, as well as drugs, such as aspirin, Metformin)

The preferred mnemonic of D. Robert Dufour, the chief of the Pathology and Laboratory Medicine Service, Veterans Affairs Medical Center, is DUMPSALE, which omits the I of MUDPILES as the proposed values of *I* are exceedingly rare in clinical practice.

- D — Diabetic ketoacidosis

- U — Uremia

- M — Methanol

- P — Paraldehyde

- S — Salicylates

- A — Alcoholic ketoacidosis

- L — Lactic acidosis

- E — Ethylene Glycol

The mnemonic for the [rare, in comparison] toxins is ACE GIFTs: Aspirin, Cyanide, Ethanolic ketosis, Glycols [ ethylene and propylene ], Isoniazid, Ferrous iron, Toluene. Most of these cause a lactic acidosis.

The several different causes of lactic acidosis include:

- Genetic conditions

- Biotinidase deficiency, multiple carboxylase deficiency, or nongenetic deficiencies of biotin

- Diabetes mellitus and deafness

- Fructose 1,6-bisphosphatase deficiency

- Glucose-6-phosphatase deficiency

- GRACILE syndrome

- Mitochondrial encephalomyopathy, lactic acidosis, and stroke-like episodes

- Pyruvate dehydrogenase deficiency

- Pyruvate carboxylase deficiency

- Drugs

- Linezolid

- Phenformin

- Metformin

- Isoniazid toxicity

- Propofol

- Propylene glycol (D-lactic acidosis)

- Nucleoside reverse transcriptase inhibitors

- Abacavir/dolutegravir/lamivudine

- Emtricitabine/tenofovir

- Potassium cyanide (cyanide poisoning)

- Fialuridine

- Other

- Impaired delivery of oxygen to cells in the tissues (e.g., from impaired blood flow (hypoperfusion))

- Bleeding

- Polymyositis

- Ethanol toxicity

- Sepsis

- Shock

- Advanced liver disease

- Diabetic ketoacidosis

- Excessive exercise (overtraining)

- Regional hypoperfusion (e.g., bowel ischemia or marked cellulitis)

- Cancers such as Non-Hodgkin's and Burkitt lymphomas

- Pheochromocytoma

Metabolic acidosis occurs when the body produces too much acid, or when the kidneys are not removing enough acid from the body. Several types of metabolic acidosis occur. The main causes are best grouped by their influence on the anion gap.

The anion gap can be spuriously normal in sampling errors of the sodium level, e.g. in extreme hypertriglyceridemia. The anion gap can be increased due to relatively low levels of cations other than sodium and potassium (e.g. calcium or magnesium).

The signs of diabetes mellitus are caused by a persistently high blood glucose concentration, which may be caused by either insufficient insulin, or by a lack of response to insulin. Most cats have a type of diabetes mellitus similar to human diabetes mellitus type 2, with β-cell dysfunction and insulin resistance. Factors which contribute to insulin resistance include obesity and endocrine diseases such as acromegaly. Acromegaly affects 20–30% of diabetic cats; it can be diagnosed by measuring the concentration of insulin-like growth factor-1 (IGF-1) in the blood.

The underlying cause determines the prognosis of lactic acidosis. In sepsis, elevated lactate levels correlate with mortality. The mortality of lactic acidosis in people taking metformin was previously reported to be 50%, but in more recent reports this was closer to 25%.

Nonketotic hyperosmolar syndrome (also known as hyperglycemic hyperosmolar syndrome) is a rare but extremely serious complication of untreated canine diabetes, which is a medical emergency. It shares the symptoms of extreme hyperglycemia, dehydration, and lethargy with ketoacidosis; because there is some insulin in the system, the body does not begin to turn to using fat as its energy source and there is no ketone production. There is not sufficient insulin available to the body for proper uptake of glucose, but there is enough to prevent ketone formation. The problem of dehydration in NHS is more profound than in diabetic ketoacidosis. Seizures and coma are possible. Treatment is similar to that of ketoacidosis, with the exceptions being that NHS requires that the blood glucose levels and rehydration be normalized at a slower rate than for DKA; cerebral edema is possible if the treatment progresses too rapidly.

The back legs may become weak and the gait may become stilted or wobbly, due to diabetic neuropathy, which is caused by damage to the myelin sheath of the peripheral nerves due to glucose toxicity and cell starvation, which are in turn caused by chronic hyperglycemia. Most common in cats, the back legs become weaker until the cat displays a plantigrade stance, standing on its hocks instead of on its toes as normal. The cat may also have trouble walking and jumping, and may need to sit down after a few steps. Some recommend a specific form of vitamin B called methylcobalamin to heal the nerve damage. Neuropathy sometimes heals on its own within 6–10 weeks once blood sugar is regulated, but anecdotal evidence points to a faster and more complete recovery with methylcobalamin supplements.

Hypoglycemia, or low blood glucose, can happen even with care, since insulin requirements can change without warning. Some common reasons for hypoglycemia include increased or unplanned exercise, illness, or medication interactions, where another medication the effects of the insulin. Vomiting and diarrhea episodes can bring on a hypoglycemia reaction, due to dehydration or simply a case of too much insulin and not enough properly digested food. Symptoms of hypoglycemia need to be taken seriously and addressed promptly. Since serious hypoglycemia can be fatal, it is better to treat a suspected incident than to fail to respond quickly to the signs of actual hypoglycemia. Dr. Audrey Cook addressed the issue in her 2007 article on diabetes mellitus: "Hypoglycemia is deadly; hyperglycemia is not. Owners must clearly understand that too much insulin can kill, and that they should call a veterinarian or halve the dose if they have any concerns about a pet's well-being or appetite. Tell owners to offer food immediately if the pet is weak or is behaving strangely."