The classic symptoms of untreated diabetes are weight loss, polyuria (increased urination), polydipsia (increased thirst), and polyphagia (increased hunger). Symptoms may develop rapidly (weeks or months) in type 1 DM, while they usually develop much more slowly and may be subtle or absent in type 2 DM.

Several other signs and symptoms can mark the onset of diabetes although they are not specific to the disease. In addition to the known ones above, they include blurry vision, headache, fatigue, slow healing of cuts, and itchy skin. Prolonged high blood glucose can cause glucose absorption in the lens of the eye, which leads to changes in its shape, resulting in vision changes. A number of skin rashes that can occur in diabetes are collectively known as diabetic dermadromes.

Low blood sugar is common in persons with type 1 and type 2 DM. Most cases are mild and are not considered medical emergencies. Effects can range from feelings of unease, sweating, trembling, and increased appetite in mild cases to more serious issues such as confusion, changes in behavior such as aggressiveness, seizures, unconsciousness, and (rarely) permanent brain damage or death in severe cases. Moderate hypoglycemia may easily be mistaken for drunkenness; rapid breathing and sweating, cold, pale skin are characteristic of hypoglycemia but not definitive. Mild to moderate cases are self-treated by eating or drinking something high in sugar. Severe cases can lead to unconsciousness and must be treated with intravenous glucose or injections with glucagon.

People (usually with type 1 DM) may also experience episodes of diabetic ketoacidosis, a metabolic disturbance characterized by nausea, vomiting and abdominal pain, the smell of acetone on the breath, deep breathing known as Kussmaul breathing, and in severe cases a decreased level of consciousness.

A rare but equally severe possibility is hyperosmolar hyperglycemic state, which is more common in type 2 DM and is mainly the result of dehydration.

The classic symptoms of diabetes are polyuria (frequent urination), polydipsia (increased thirst), polyphagia (increased hunger), and weight loss. Other symptoms that are commonly present at diagnosis include a history of blurred vision, itchiness, peripheral neuropathy, recurrent vaginal infections, and fatigue. Many people, however, have no symptoms during the first few years and are diagnosed on routine testing. A small number of people with type 2 diabetes mellitus can develop a hyperosmolar hyperglycemic state (a condition of very high blood sugar associated with a decreased level of consciousness and low blood pressure).

The classical symptoms of type 1 diabetes include: polyuria (excessive urination), polydipsia (increased thirst), dry mouth, polyphagia (increased hunger), fatigue, and weight loss.

Many type 1 diabetics are diagnosed when they present with diabetic ketoacidosis. The signs and symptoms of diabetic ketoacidosis include dry skin, rapid deep breathing, drowsiness, increased thirst, frequent urination, abdominal pain, and vomiting.

About 12 percent of people with type 1 diabetes have clinical depression.

About 6 percent of people with type 1 diabetes have celiac disease, but in most cases there are no digestive symptoms or are mistakenly attributed to poor control of diabetes, gastroparesis or diabetic neuropathy. In most cases, celiac disease is diagnosed after onset of type 1 diabetes. The association of celiac disease with type 1 diabetes increases the risk of complications, such as retinopathy and mortality. This association can be explained by shared genetic factors, and inflammation or nutritional deficiencies caused by untreated celiac disease, even if type 1 diabetes is diagnosed first.

Some people with type 1 diabetes experience dramatic and recurrent swings in glucose levels, often occurring for no apparent reason; this is called "unstable diabetes" or "labile diabetes", and sometimes "brittle diabetes", although this term is no longer used. The results of such swings can be irregular and unpredictable hyperglycemias, sometimes involving ketoacidosis, and sometimes serious hypoglycemias. Brittle diabetes occurs no more frequently than in 1% to 2% of diabetics.

Type 2 diabetes is typically a chronic disease associated with a ten-year-shorter life expectancy. This is partly due to a number of complications with which it is associated, including: two to four times the risk of cardiovascular disease, including ischemic heart disease and stroke; a 20-fold increase in lower limb amputations, and increased rates of hospitalizations. In the developed world, and increasingly elsewhere, type 2 diabetes is the largest cause of nontraumatic blindness and kidney failure. It has also been associated with an increased risk of cognitive dysfunction and dementia through disease processes such as Alzheimer's disease and vascular dementia. Other complications include acanthosis nigricans, sexual dysfunction, and frequent infections.

Prediabetes typically has no distinct signs or symptoms except the sole sign of high blood sugar. Patients should monitor for signs and symptoms of type 2 diabetes mellitus. These include the following:

- Constant hunger

- Unexplained weight loss

- Weight gain

- Flu-like symptoms, including weakness and fatigue

- Blurred vision

- Slow healing of cuts or bruises

- Tingling or loss of feeling in hands or feet

- Recurring gum or skin infections

- Recurring vaginal or bladder infections

- A high BMI (Body Mass Index) result

Diabetes mellitus type 1 (also known as type 1 diabetes) is a form of diabetes mellitus in which not enough insulin is produced. This results in high blood sugar levels in the body. The classical symptoms are frequent urination, increased thirst, increased hunger, and weight loss. Additional symptoms may include blurry vision, feeling tired, and poor healing. Symptoms typically develop over a short period of time.

The cause of type 1 diabetes is unknown. However, it is believed to involve a combination of genetic and environmental factors. Risk factors include having a family member with the condition. The underlying mechanism involves an autoimmune destruction of the insulin-producing beta cells in the pancreas. Diabetes is diagnosed by testing the level of sugar or A1C in the blood. Type 1 diabetes can be distinguished from type 2 by testing for the presence of autoantibodies.

There is no known way to prevent type 1 diabetes. Treatment with insulin is required for survival. Insulin therapy is usually given by injection just under the skin but can also be delivered by an insulin pump. A diabetic diet and exercise are an important part of management. Untreated, diabetes can cause many complications. Complications of relatively rapid onset include diabetic ketoacidosis and nonketotic hyperosmolar coma. Long-term complications include heart disease, stroke, kidney failure, foot ulcers and damage to the eyes. Furthermore, complications may arise from low blood sugar caused by excessive dosing of insulin.

Type 1 diabetes makes up an estimated 5–10% of all diabetes cases. The number of people affected globally is unknown, although it is estimated that about 80,000 children develop the disease each year. Within the United States the number of people affected is estimated at one to three million. Rates of disease vary widely with approximately 1 new case per 100,000 per year in East Asia and Latin America and around 30 new cases per 100,000 per year in Scandinavia and Kuwait. It typically begins in children and young adults.

Impaired glucose tolerance (IGT) is a pre-diabetic state of dysglycemia, that is associated with insulin resistance and increased risk of cardiovascular pathology. IGT may precede type 2 diabetes mellitus by many years. IGT is also a risk factor for mortality.

The symptoms of latent autoimmune diabetes of adults are similar to those of other forms of diabetes: polydipsia (excessive thirst and drinking), polyuria (excessive urination), and often blurred vision. Compared to juvenile type 1 diabetes, the symptoms develop comparatively slowly, over a period of at least six months.

It is estimated that more than 50% of persons diagnosed as having non-obesity-related type 2 diabetes may actually have LADA. Glutamic acid decarboxylase autoantibody (GADA), islet cell autoantibody (ICA), insulinoma-associated (IA-2) autoantibody, and zinc transporter autoantibody (ZnT8) testing should be performed on all adults who are not obese who are diagnosed with diabetes. However, some overweight patients are misdiagnosed with type 2 due to their weight. Moreover, it is now becoming evident that autoimmune diabetes may be highly underdiagnosed in many individuals who have diabetes, and that the body mass index levels may have rather limited use in connections with latent autoimmune diabetes.

Persons with LADA typically have low, although sometimes moderate, levels of C-peptide as the disease progresses. Those with insulin resistance or type 2 diabetes are more likely to have high levels of C-peptide due to an over production of insulin.

The following characteristics suggest the possibility of a diagnosis of MODY in hyperglycemic and diabetic patients:

- Mild to moderate hyperglycemia (typically 130–250 mg/dl, or 7–14 mmol/l) discovered before 30 years of age. However, anyone under 50 can develop MODY.

- A first-degree relative with a similar degree of diabetes.

- Absence of positive antibodies or other autoimmunity (e.g., thyroiditis) in patient and family. However, Urbanova et al. found that about one quarter of Central European MODY patients are positive for islet cell autoantibodies (GADA and IA2A). Their expression is transient but highly prevalent. The autoantibodies were found in patients with delayed diabetes onset, and in times of insufficient diabetes control. The islet cell autoantibodies are absent in MODY in at least some populations (Japanese, Britons).

- Persistence of a low insulin requirement (e.g., less than 0.5 u/kg/day) past the usual "honeymoon" period.

- Absence of obesity (although overweight or obese people can get MODY) or other problems associated with type 2 diabetes or metabolic syndrome (e.g., hypertension, hyperlipidemia, polycystic ovary syndrome).

- Insulin resistance very rarely happens.

- Cystic kidney disease in patient or close relatives.

- Non-transient neonatal diabetes, or apparent type 1 diabetes with onset before six months of age.

- Liver adenoma or hepatocellular carcinoma in MODY type 3

- Renal cysts, rudimentary or bicornuate uterus, vaginal aplasia, absence of the vas deferens, epidymal cysts in MODY type 5

The diagnosis of MODY is confirmed by specific gene testing available through commercial laboratories.

Diabetic coma is a medical emergency in which a person with diabetes mellitus is comatose (unconscious) because of one of the acute complications of diabetes:

1. Severe diabetic hypoglycemia

2. Diabetic ketoacidosis advanced enough to result in unconsciousness from a combination of severe hyperglycemia, dehydration and shock, and exhaustion

3. Hyperosmolar nonketotic coma in which extreme hyperglycemia and dehydration alone are sufficient to cause unconsciousness.

Gestational diabetes is formally defined as "any degree of glucose intolerance with onset or first recognition during pregnancy". This definition acknowledges the possibility that a woman may have previously undiagnosed diabetes mellitus, or may have developed diabetes coincidentally with pregnancy. Whether symptoms subside after pregnancy is also irrelevant to the diagnosis.

A woman is diagnosed with gestational diabetes when glucose intolerance continues beyond 24–28 weeks of gestation.

The White classification, named after Priscilla White, who pioneered research on the effect of diabetes types on perinatal outcome, is widely used to assess maternal and fetal risk. It distinguishes between gestational diabetes (type A) and pregestational diabetes (diabetes that existed prior to pregnancy). These two groups are further subdivided according to their associated risks and management.

The two subtypes of gestational diabetes under this classification system are:

- Type A1: abnormal oral glucose tolerance test (OGTT), but normal blood glucose levels during fasting and two hours after meals; diet modification is sufficient to control glucose levels

- Type A2: abnormal OGTT compounded by abnormal glucose levels during fasting and/or after meals; additional therapy with insulin or other medications is required

Diabetes which existed prior to pregnancy is also split up into several subtypes under this system:

- Type B: onset at age 20 or older and duration of less than 10 years.

- Type C: onset at age 10–19 or duration of 10–19 years.

- Type D: onset before age 10 or duration greater than 20 years.

- Type E: overt diabetes mellitus with calcified pelvic vessels.

- Type F: diabetic nephropathy.

- Type R: proliferative retinopathy.

- Type RF: retinopathy and nephropathy.

- Type H: ischemic heart disease.

- Type T: prior kidney transplant.

An early age of onset or long-standing disease comes with greater risks, hence the first three subtypes.

Two other sets of criteria are available for diagnosis of gestational diabetes, both based on blood-sugar levels.

Criteria for diagnosis of gestational diabetes, using the 100 gram Glucose Tolerance Test, according to Carpenter and Coustan:

- Fasting 95 mg/dl

- 1 hour 180 mg/dl

- 2 hours 155 mg/dl

- 3 hours 140 mg/dl

Criteria for diagnosis of gestational diabetes according to National Diabetes Data Group:

- Fasting 105 mg/dl

- 1 hour 190 mg/dl

- 2 hours 165 mg/dl

- 3 hours 145 mg/dl

Diabetic ketoacidosis (DKA) is an acute and dangerous complication that is always a medical emergency and requires prompt medical attention. Low insulin levels cause the liver to turn fatty acid to ketone for fuel (i.e., ketosis); ketone bodies are intermediate substrates in that metabolic sequence. This is normal when periodic, but can become a serious problem if sustained. Elevated levels of ketone bodies in the blood decrease the blood's pH, leading to DKA. On presentation at hospital, the patient in DKA is typically dehydrated, and breathing rapidly and deeply. Abdominal pain is common and may be severe. The level of consciousness is typically normal until late in the process, when lethargy may progress to coma. Ketoacidosis can easily become severe enough to cause hypotension, shock, and death. Urine analysis will reveal significant levels of ketone bodies (which have exceeded their renal threshold blood levels to appear in the urine, often before other overt symptoms). Prompt, proper treatment usually results in full recovery, though death can result from inadequate or delayed treatment, or from complications (e.g., brain edema). Ketoacidosis is much more common in type 1 diabetes than type 2.

Currently, MODY is the final diagnosis in 1%–2% of people initially diagnosed with diabetes. The prevalence is 70–110 per million population. 50% of first-degree relatives will inherit the same mutation, giving them a greater than 95% lifetime risk of developing MODY themselves. For this reason, correct diagnosis of this condition is important. Typically patients present with a strong family history of diabetes (any type) and the onset of symptoms is in the second to fifth decade.

There are two general types of clinical presentation.

- Some forms of MODY produce significant hyperglycemia and the typical signs and symptoms of diabetes: increased thirst and urination (polydipsia and polyuria).

- In contrast, many people with MODY have no signs or symptoms and are diagnosed either by accident, when a high glucose is discovered during testing for other reasons, or screening of relatives of a person discovered to have diabetes. Discovery of mild hyperglycemia during a routine glucose tolerance test for pregnancy is particularly characteristic.

MODY cases may make up as many as 5% of presumed type 1 and type 2 diabetes cases in a large clinic population. While the goals of diabetes management are the same no matter what type, there are two primary advantages of confirming a diagnosis of MODY.

- Insulin may not be necessary and it may be possible to switch a person from insulin injections to oral agents without loss of glycemic control.

- It may prompt screening of relatives and so help identify other cases in family members.

As it occurs infrequently, many cases of MODY are initially assumed to be more common forms of diabetes: type 1 if the patient is young and not overweight, type 2 if the patient is overweight, or gestational diabetes if the patient is pregnant. Standard diabetes treatments (insulin for type 1 and gestational diabetes, and oral hypoglycemic agents for type 2) are often initiated before the doctor suspects a more unusual form of diabetes.

According to the criteria of the World Health Organization and the American Diabetes Association, impaired glucose tolerance is defined as:

- two-hour glucose levels of 140 to 199 mg per dL (7.8 to 11.0 mmol/l) on the 75-g oral glucose tolerance test. A patient is said to be under the condition of IGT when he/she has an intermediately raised glucose level after 2 hours, but less than the level that would qualify for type 2 diabetes mellitus. The fasting glucose may be either normal or mildly elevated.

From 10 to 15 percent of adults in the United States have impaired glucose tolerance or impaired fasting glucose.

Impaired fasting glucose is often without any signs or symptoms, other than higher than normal glucose levels being detected in an individual's fasting blood sample. There may be signs and symptoms associated with elevated blood glucose, though these are likely to be minor, with significant symptoms suggestive of complete progression to type 2 diabetes. Such symptoms include:

- Increased thirst

- Increased urination, especially waking up in the night to urinate

- Tiredness and fatigue

- Blurred vision

- Slow healing of wounds

- Altered sensation, such as numbness or tingling, particularly of the hands and feet

- Recurrent, and difficult to clear infections, particularly of the urinary tract

Gestational diabetes is a condition in which a woman without diabetes develops high blood sugar levels during pregnancy. Gestational diabetes generally results in few symptoms; however, it does increase the risk of pre-eclampsia, depression, and requiring a Caesarean section. Babies born to mothers with poorly treated gestational diabetes are at increased risk of being too large, having low blood sugar after birth, and jaundice. If untreated, it can also result in a stillbirth. Long term, children are at higher risk of being overweight and developing type 2 diabetes.

Gestational diabetes is caused by not enough insulin in the setting of insulin resistance. Risk factors include being overweight, previously having gestational diabetes, a family history of type 2 diabetes, and having polycystic ovarian syndrome. Diagnosis is by blood tests. For those at normal risk screening is recommended between 24 and 28 weeks gestation. For those at high risk testing may occur at the first prenatal visit.

Prevention is by maintaining a healthy weight and exercising before pregnancy. Gestational diabetes is a treated with a diabetic diet, exercise, and possibly insulin injections. Most women are able to manage their blood sugar with a diet and exercise. Blood sugar testing among those who are affected is often recommended four times a day. Breastfeeding is recommended as soon as possible after birth.

Gestational diabetes affects 3–9% of pregnancies, depending on the population studied. It is especially common during the last three months of pregnancy. It affects 1% of those under the age of 20 and 13% of those over the age of 44. A number of ethnic groups including Asians, American Indians, Indigenous Australians, and Pacific Islanders are at higher risk. In 90% of people gestational diabetes will resolve after the baby is born. Women, however, are at an increased risk of developing type 2 diabetes.

Impaired glucose tolerance (IGT) is a pre-diabetic state of hyperglycemia that is associated with insulin resistance and increased risk of cardiovascular pathology. IGT may precede type 2 diabetes mellitus by many years. IGT is also a risk factor for mortality.

The main symptoms which occur in nearly all dogs with diabetes mellitus are:

- excessive water consumption, known as polydipsia,

- frequent and/or excessive urination, known as polyuria, often requiring the dog to be let outside to urinate during the night,

- greater than average appetite, known as polyphagia,

- weight loss.

Sometimes, the first sign of diabetes noticed by the owner may be that their dog either has become blind (due to the formation of cataracts in the eyes), or has vomiting, anorexia, lethargy and weakness (due to ketoacidosis).

Diabetes mellitus is a disease in which the beta cells of the endocrine pancreas either stop producing insulin or can no longer produce it in enough quantity for the body's needs. The condition is commonly divided into two types, depending on the origin of the condition: Type 1 diabetes, sometimes called "juvenile diabetes", is caused by destruction of the beta cells of the pancreas. The condition is also referred to as insulin-dependent diabetes, meaning exogenous insulin injections must replace the insulin the pancreas is no longer capable of producing for the body's needs. Dogs can have insulin-dependent, or Type 1, diabetes; research finds no Type 2 diabetes in dogs. Because of this, there is no possibility the permanently damaged pancreatic beta cells could re-activate to engender a remission as may be possible with some feline diabetes cases, where the primary type of diabetes is Type 2. There is another less common form of diabetes, diabetes insipidus, which is a condition of insufficient antidiuretic hormone or resistance to it.

This most common form of diabetes affects approximately 0.34% of dogs. The condition is treatable and need not shorten the animal's life span or interfere with quality of life. If left untreated, the condition can lead to cataracts, increasing weakness in the legs (neuropathy), malnutrition, ketoacidosis, dehydration, and death. Diabetes mainly affects middle-age and older dogs, but there are juvenile cases. The typical canine diabetes patient is middle-age, female, and overweight at diagnosis.

The number of dogs diagnosed with diabetes mellitus has increased three-fold in thirty years. In survival rates from almost the same time, only 50% survived the first 60 days after diagnosis and went on to be successfully treated at home. Currently, diabetic dogs receiving treatment have the same expected lifespan as non-diabetic dogs of the same age and gender.

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.

The White classification, named after Priscilla White who pioneered research on the effect of diabetes types on perinatal outcome, is widely used to assess maternal and fetal risk. It distinguishes between gestational diabetes (type A) and diabetes that existed before pregnancy (pregestational diabetes). These two groups are further subdivided according to their associated risks and management.

There are 2 classes of gestational diabetes (diabetes which began during pregnancy):

- Class A: gestational diabetes; diet controlled

- Class A: gestational diabetes; medication controlled

The second group of diabetes which existed before pregnancy can be split up into these classes:

- Class B: onset at age 20 or older or with duration of less than 10 years

- Class C: onset at age 10-19 or duration of 10–19 years

- Class D: onset before age 10 or duration greater than 20 years

- Class E: overt diabetes mellitus with calcified pelvic vessels

- Class F: diabetic nephropathy

- Class R: proliferative retinopathy

- Class RF: retinopathy and nephropathy

- Class H: ischemic heart disease

- Class T: prior kidney transplant

An early age of onset or long-standing disease comes with greater risks, hence the first three subtypes.

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.

Steroid diabetes (also "steroid-induced diabetes") is a medical term referring to prolonged hyperglycemia due to glucocorticoid therapy for another medical condition. It is usually, but not always, a transient condition.