Hyperglycaemia (a high blood glucose level) usually only happens in people with diabetes.

The food that you eat is digested and broken down into other substances, including a simple sugar called glucose that then enters your bloodstream.

The amount of glucose in your blood is controlled by a hormone called insulin, which is produced by your pancreas (an organ lying behind the stomach). Insulin takes glucose out of your blood and moves it into your cells, where it is broken down to produce energy.

Diabetes

In people with diabetes, the body is unable to break glucose down into energy. This is because there is either not enough insulin to move the glucose, or because the insulin that is there does not work properly. The glucose remains in the blood, causing a high blood glucose level.

The high blood glucose level means that glucose enters the urine. The glucose takes extra water and electrolytes (minerals and salts in the blood) with it into the urine.

This causes the symptoms of hyperglycaemia, such as increased thirst and the need to urinate frequently. Eventually, this can lead to dehydration.

Hyperglycaemia triggers

There are some events that can trigger an increase in blood glucose if you have diabetes, including:

• stress
• changing your diet, or eating too much
• a wrong (or missed dose) of insulin
• over-treating an episode of hypoglycaemia (low blood sugar)
• an illness, such as a cold
• a change of medication (see below)

If you have diabetes, your GP will give you advice about how to manage your blood glucose levels.

Medications and hyperglycaemia

Some medications can cause hyperglycaemia as a side effect in people with diabetes. Examples of these medications include olanzapine and risperidone (used to treat mental health conditions), and steroid medication (used to relieve inflammation)

If you are prescribed a medication that could cause hyperglycaemia, your GP may arrange for your weight and blood glucose levels to be monitored during treatment in case your blood glucose level starts to increase.

Pregnant women

Some pregnant women experience hyperglycaemia as a result of gestational diabetes. This is a type of diabetes that can develop if your body is unable to produce enough extra insulin to meet the demands of pregnancy, leading to an increased level of glucose in the blood.

Most cases of gestational diabetes disappear after the baby is born.

Read more about diabetes and pregnancy.

Children

Children with hyperglycaemia may have undiagnosed diabetes. This will usually be type 1 diabetes but it could be type 2 diabetes if the child is obese (although this is rare).

Newborn babies

Some babies develop hyperglycaemia soon after being born. This is particularly common in babies born prematurely (before 37 weeks of pregnancy) and with a very low birthweight.

Some cases occur as a result of an underlying problem, such as blood poisoning (sepsis) or a condition called neonatal respiratory distress syndrome (NRDS), but very few cases are caused by diabetes.

Babies with hyperglycaemia often get better without treatment and without any lasting problems, although a few cases are serious and can be life-threatening.

Heart conditions

Hyperglycaemia can also occur in people who have experienced a condition called acute coronary syndrome (ACS).

This is a group of heart problems that includes heart attacks and unstable angina.

If left untreated, hyperglycaemia can lead to serious complications that can be life-threatening.

Diabetic ketoacidosis

Diabetic ketoacidosis (DKA) is a complication that can affect people with type 1 diabetes. It occurs when a lack of insulin means that cells cannot take glucose from the blood to use as energy. Instead, the cells start to break down fats to use as energy.

This causes chemicals called ketones to build-up in the blood, which increases the blood acidity.

The signs and symptoms of ketoacidosis include:

• an elevated blood glucose level
• a high number of ketones in your urine (ketone testing strips are available on prescription)
• nausea or vomiting
• stomach pain
• a fruity smell on your breath, which may smell like pear drops or nail varnish
• rapid breathing (hyperventilation)
• dehydration (symptoms of which include a headache, dry skin, and a weak, rapid heartbeat)
• drowsiness or confusion
• loss of consciousness

If you experience these signs or symptoms, seek urgent medical attention. If left untreated, ketoacidosis can lead to coma and death.

Diabetic ketoacidosis needs to be treated urgently in hospital, where you will usually be given replacement fluids and insulin into a vein to treat dehydration and lower your blood glucose level.

Hyperosmolar hyperglycaemic state

A hyperosmolar hyperglycaemic state (HHS) is similar to DKA, but tends to affect people with type 2 diabetes.

HHS occurs when insulin is present, but it is ineffective at removing glucose from the blood. This causes your body to become dehydrated as it tries to get rid of the excess sugar.

As well as an elevated blood sugar level, warning signs of HHS can include:

• frequent urination
• dry mouth
• extreme thirst
• feeling sick
• warm, dry skin
• confusion
• weakness down one side of the body
• drowsiness and a gradual loss of consciousness

Unlike DKA, HHS does not usually lead to a build-up of ketones in your urine.

If you experience symptoms of HHS, seek urgent medical attention. Like DKA, HHS can lead to coma and even death if not treated.

HHS is usually treated in hospital, using replacement fluids and insulin given directly into a vein to treat dehydration and lower your blood glucose level.

Long-term complications

In the long-term, hyperglycaemia can increase the likelihood of complications developing as a result of diabetes.

This is because high levels of glucose in the blood can damage blood vessels, nerves and organs. If your blood glucose level is not well controlled, you may be more at risk of developing:

• damage to your kidneys
• damage to your eyes
• damage to the nerves of your feet
• heart disease (when the heart’s blood supply is blocked)
• erection problems (in men)

Hyperglycaemia (a high blood sugar level) occurs when the body can’t remove glucose from the blood and turn it into energy. It usually only happens in people with diabetes.

The symptoms of hyperglycaemia tend to develop over several days or weeks, and can include:

• increased thirst
• a dry mouth
• needing to urinate frequently, particularly at night
• tiredness
• recurrent infections, such as thrush

Over time there may be further symptoms including weight loss and blurred vision.

If you haven’t been diagnosed with diabetes, visit your GP if you or your child experience symptoms of hyperglycaemia. Your GP will usually be able to diagnose the condition based on your symptoms. They may confirm the diagnosis by testing the level of glucose (sugar) in your blood.

What causes hyperglycaemia?

Hyperglycaemia usually occurs in people with diabetes because people with the condition have problems with insulin – the hormone that helps remove glucose from the blood and converts it to energy.

There are two types of diabetes, described below.

• Type 1 diabetes (insulin-dependent) – where the body produces little or no insulin. People with type 1 diabetes require lifelong treatment to replace the insulin. They also need to check their blood glucose level regularly to prevent complications developing.
• Type 2 diabetes (non-insulin dependent) – where either the body does not make enough insulin, or it cannot use the insulin properly (insulin resistance). This type of diabetes is often linked to obesity or being overweight and mostly occurs in people aged over 40.

If you have diabetes, there are some situations that can trigger an increase in blood glucose, including stress, missing a dose of insulin, eating too much or being ill.

Read more about the causes of hyperglycaemia.

Preventing and treating hyperglycaemia

If you have diabetes, your diabetes care team will explain how to monitor and manage your blood glucose levels. If you have type 1 diabetes, it is important not to miss or alter your dose of insulin and to maintain your fluid and food intake.

If hyperglycaemia occurs, your blood glucose levels will need to be lowered again following the advice of your care team. Increasing your dose of insulin or making changes to your diet are two ways of doing this.

It is important that you test your blood glucose levels regularly. You will be advised about when and how often your blood should be tested.

Contact your diabetes care team for further advice if you regularly experience episodes of hyperglycaemia.

Read more about how hyperglycaemia is treated.

When to seek urgent medical attention

You should seek immediate medical attention if you, or a friend or family member with diabetes, experience the following symptoms:

• nausea or vomiting 
• stomach pain
• a fruity smell on your breath, which may smell like pear drops or nail varnish
• drowsiness or confusion
• rapid breathing (hyperventilation)
• dehydration (signs of which include a headache, dry skin, and a weak, rapid heartbeat)
• loss of consciousness

These symptoms may be a sign of diabetic ketoacidosis, a serious and potentially life-threatening complication of hyperglycaemia.

Read more about the complications of hyperglycaemia.

The Map of Medicine is used by doctors throughout the NHS to determine the best treatment options for their patients. NHS Choices offers everyone in England exclusive and free access to this cutting-edge internet resource, which lets you see exactly what your doctor sees.

The information in the Map has been approved by the UK’s leading clinical experts, is based on the best available clinical evidence, and is continually updated. To take advantage of this unique resource go to:

Map of Medicine: diabetes

How you treat hyperglycaemia will depend on what type of diabetes you have, and how you have been advised to manage your blood glucose levels.

If you have diabetes

If you have diabetes, you should have a diabetes care team (a team of specialists who help monitor and treat your condition). They should explain what to do if you develop hyperglycaemia.

You may be advised to:

• increase your dose of insulin (a hormone that removes glucose from your blood so that your cells can break it down into energy)
• change your diet – for example, you may be advised to avoid foods that cause your glucose levels to rise, such as cakes or sugary drinks
• get more exercise, as this can reduce your blood glucose level
• monitor your glucose level – you may be given a home testing kit to check the level of glucose in your blood

Read more about treating type 1 diabetes and treating type 2 diabetes.

If you do not have diabetes

You should visit your GP if you experience the symptoms of hyperglycaemia because you may have undiagnosed diabetes.

Your GP will test your blood glucose levels and discuss the results with you. If you have diabetes, you will be given advice about how to manage the condition.

If you have hyperglycaemia as a result of a heart condition called acute coronary syndrome (ACS), you may need to be treated with insulin. You will be tested for diabetes after treatment and at regular intervals (at least once a year) thereafter, as people with ACS have an increased risk of developing type 2 diabetes.

When to seek medical attention

If you have diabetes, contact your diabetes care team for advice if you regularly experience episodes of hyperglycaemia.

You should seek medical attention urgently if you, or someone you know who has diabetes, starts to experience any of the following symptoms:

• nausea or vomiting (feeling or being sick)
• stomach pain
• a fruity smell on your breath, which may smell like pear drops or nail varnish
• drowsiness or confusion
• rapid breathing (hyperventilation)
• dehydration (when the normal water content of your body is reduced, which can cause a headache, dry skin and a weak, rapid heartbeat)
• loss of consciousness

If you have these symptoms, you may have diabetic ketoacidosis, which will need hospital treatment. Read more about the complications of hyperglycaemia.