Hyperglycemia doesn't cause symptoms until glucose values are significantly elevated — usually above 180 to 200 milligrams per deciliter (mg/dL), or 10 to 11 millimoles per liter (mmol/L). Symptoms of hyperglycemia develop slowly over several days or weeks. The longer blood sugar levels stay high, the more serious the symptoms become. However, some people who've had type 2 diabetes for a long time may not show any symptoms despite elevated blood sugar levels.

Early signs and symptoms

Recognizing early signs and symptoms of hyperglycemia can help you treat the condition promptly. Watch for:

• Frequent urination
• Increased thirst
• Blurred vision
• Fatigue
• Headache

Later signs and symptoms

If hyperglycemia goes untreated, it can cause toxic acids (ketones) to build up in your blood and urine (ketoacidosis). Signs and symptoms include:

• Fruity-smelling breath
• Nausea and vomiting
• Shortness of breath
• Dry mouth
• Weakness
• Confusion
• Coma
• Abdominal pain

During digestion, your body breaks down carbohydrates from foods — such as bread, rice and pasta — into various sugar molecules. One of these sugar molecules is glucose, a main energy source for your body. Glucose is absorbed directly into your bloodstream after you eat, but it can't enter the cells of most of your tissues without the help of insulin — a hormone secreted by your pancreas.

When the level of glucose in your blood rises, it signals your pancreas to release insulin. The insulin, in turn, unlocks your cells so that glucose can enter and provide the fuel your cells need to function properly. Any extra glucose is stored in your liver and muscles in the form of glycogen.

This process lowers the amount of glucose in your bloodstream and prevents it from reaching dangerously high levels. As your blood sugar level returns to normal, so does the secretion of insulin from your pancreas.

Diabetes drastically diminishes the effects of insulin on your body, either because your pancreas is unable to produce enough insulin (type 1 diabetes) or because your body is resistant to the effects of insulin or doesn't produce enough insulin to maintain a normal glucose level (type 2 diabetes). As a result, glucose tends to build up in your bloodstream (hyperglycemia) and may reach dangerously high levels if not treated properly. Insulin or other drugs are used to lower blood sugar levels.

Many factors can contribute to hyperglycemia, including:

• Not using enough insulin or oral diabetes medication
• Not injecting insulin properly or using expired insulin
• Not following your diabetes eating plan
• Being inactive
• Having an illness or infection
• Using certain medications, such as steroids
• Being injured or having surgery
• Experiencing emotional stress, such as family conflict or workplace challenges

Illness or stress can trigger hyperglycemia because hormones produced to combat illness or stress can also cause your blood sugar to rise. Even people who don't have diabetes may develop hyperglycemia during severe illness. But people with diabetes may need to take extra diabetes medication to keep blood glucose near normal during illness or stress.

Long-term complications

Keeping tight control of your blood sugar can help prevent many diabetes-related complications. Long-term complications of untreated hyperglycemia can include:

• Cardiovascular disease
• Nerve damage (neuropathy)
• Kidney damage (diabetic nephropathy) or kidney failure
• Damage to the blood vessels of the retina (diabetic retinopathy), potentially leading to blindness
• Clouding of the normally clear lens of your eye (cataract)
• Feet problems caused by damaged nerves or poor blood flow that can lead to serious skin infections, ulcerations, and in some severe cases, amputation
• Bone and joint problems
• Teeth and gum infections