What Does High Glucose Mean? Causes and Signs

High glucose, or hyperglycaemia, means the concentration of sugar circulating in your bloodstream is above the normal range. This can occur in people with type 1 or type 2 diabetes, as well as in those with non-diabetic hyperglycaemia (prediabetes), where levels are elevated but not yet diagnostic of diabetes. Understanding what high glucose means, how it is measured, what symptoms it causes, and when to seek medical advice is essential for protecting long-term health. Persistent hyperglycaemia can damage blood vessels, nerves, kidneys and eyes, making early identification and appropriate management a clinical priority.

What does high glucose mean and why does it happen?

High glucose means blood sugar is above the normal range due to insufficient or ineffective insulin, occurring in type 1 diabetes, type 2 diabetes, or non-diabetic hyperglycaemia, and can damage blood vessels and nerves if left unmanaged.

High glucose means that the concentration of glucose (sugar) circulating in the bloodstream is above the normal range, a state clinically referred to as hyperglycaemia. Glucose is the body's primary fuel source, derived from the carbohydrates we eat and from the liver's own glucose production. Under normal circumstances, the hormone insulin, produced by the pancreas, moves glucose from the blood into cells where it can be used for energy. When this process is disrupted, glucose accumulates in the blood rather than entering the cells.

The disruption can occur for several reasons. In type 1 diabetes, the immune system destroys the insulin-producing beta cells of the pancreas, leaving the body with little or no insulin. In type 2 diabetes, the body either does not produce enough insulin or the cells become resistant to its effects, meaning glucose cannot be absorbed efficiently. In both cases, blood glucose rises above healthy levels.

High glucose is not always linked to a formal diabetes diagnosis. NHS and NICE guidance use the term non-diabetic hyperglycaemia (NDH), sometimes also called prediabetes, to describe blood glucose levels that are elevated but not yet in the diabetic range. Left unaddressed, NDH significantly increases the risk of progressing to type 2 diabetes and is associated with cardiovascular risk even before that transition occurs, as set out in NICE guideline PH38 (Type 2 diabetes: prevention in people at high risk).^[1][2]

Understanding what high glucose means is important because persistent hyperglycaemia, over months and years, can damage blood vessels and nerves throughout the body, affecting the eyes, kidneys, heart and feet. Early identification and management are therefore central to preventing long-term complications.

What blood glucose levels are considered too high in the UK?

In the UK, a fasting plasma glucose of 7.0 mmol/L or above, or an HbA1c of 48 mmol/mol or above, is diagnostic of diabetes; levels between these and normal thresholds indicate non-diabetic hyperglycaemia.

In the UK, blood glucose is measured in millimoles per litre (mmol/L), and NHS and NICE guidance define specific thresholds that indicate when levels are too high. Formal diagnosis is based on venous plasma samples analysed in an accredited laboratory; home capillary blood glucose meters are useful for monitoring but are not used to make a diagnosis.

For fasting plasma glucose, a level of 5.5 mmol/L or below is generally considered normal. NICE (PH38) defines non-diabetic hyperglycaemia (NDH) as a fasting plasma glucose of 5.5 to 6.9 mmol/L or an HbA1c of 42 to 47 mmol/mol (6.0 to 6.4%). The WHO additionally defines impaired fasting glucose (IFG) as a fasting plasma glucose of 6.1 to 6.9 mmol/L. A fasting plasma glucose of 7.0 mmol/L or above on two separate occasions is consistent with a diagnosis of diabetes, unless symptoms are unequivocal, in which case a single result may suffice.

Impaired glucose tolerance (IGT) and the 2-hour diagnostic threshold for diabetes are determined by a formal 75 g oral glucose tolerance test (OGTT), carried out in a clinical setting. On an OGTT, a 2-hour plasma glucose of 7.8 to 11.0 mmol/L indicates IGT, and a 2-hour value of 11.1 mmol/L or above is diagnostic of diabetes regardless of symptoms. A random plasma glucose of 11.1 mmol/L or above is also diagnostic when accompanied by typical symptoms; without symptoms, a confirmatory test on a separate day is required. These OGTT thresholds are not the same as routine post-meal finger-prick readings, which are not used for diagnosis.

Another important measure is HbA1c, which reflects average blood glucose over the preceding two to three months. NICE guideline NG28 uses an HbA1c of 48 mmol/mol (6.5%) or above as the diagnostic threshold for type 2 diabetes. For people already living with diabetes, target ranges vary depending on individual circumstances and treatment. NICE recommends that most adults with type 2 diabetes aim for an HbA1c of 48 mmol/mol (6.5%) if managed by lifestyle or a single non-hypoglycaemia-inducing drug, and 53 mmol/mol (7.0%) for those on insulin or drugs associated with hypoglycaemia. Your GP or diabetes care team will advise on the most appropriate personal targets.

What symptoms should you look out for with high blood sugar?

High blood sugar typically causes increased thirst, frequent urination, fatigue and blurred vision; severe cases can lead to DKA or HHS, both medical emergencies requiring an immediate 999 call.

High blood sugar can produce a recognisable cluster of symptoms, though in the early stages of non-diabetic hyperglycaemia or mildly elevated glucose, many people experience no symptoms at all. The classic symptoms of hyperglycaemia include increased thirst (polydipsia), passing urine more frequently than usual (polyuria), fatigue, blurred vision, and headaches. These arise because the kidneys work harder to filter excess glucose from the blood, drawing water with it and causing dehydration.

Other symptoms that may accompany persistently high glucose include:

• Slow healing of cuts or wounds

• Recurrent infections, particularly thrush or urinary tract infections

• Unexplained weight loss (more common in type 1 diabetes)

• Tingling or numbness in the hands or feet

• Dry mouth and skin

It is worth noting that these symptoms can develop gradually and may be attributed to other causes, which is one reason type 2 diabetes is often diagnosed incidentally during routine blood tests. Diabetes UK estimates that a significant number of people in the UK are living with undiagnosed type 2 diabetes; current figures are published on the Diabetes UK website.

In more severe cases, particularly in type 1 diabetes or when glucose rises very rapidly, a dangerous condition called diabetic ketoacidosis (DKA) can develop. Symptoms of DKA include nausea, vomiting, abdominal pain, deep or rapid breathing, and a fruity smell on the breath.^[6] DKA is a medical emergency: if you or someone else develops these symptoms, call 999 immediately.

Older adults with type 2 diabetes are also at risk of a separate emergency called hyperosmolar hyperglycaemic state (HHS). Warning signs include very high blood glucose, severe thirst, confusion or drowsiness, and marked dehydration without the rapid breathing or fruity breath typical of DKA. HHS is also a medical emergency: call 999 if these features are present.

For less urgent concerns about blood sugar symptoms, contact your GP or call NHS 111 for guidance.

What causes blood glucose to rise unexpectedly?

Illness, stress, corticosteroids, reduced physical activity, missed diabetes medication and hormonal changes are common causes of unexpected blood glucose rises, even in people managing their condition carefully.

Blood glucose can rise unexpectedly for a range of reasons, even in people who are managing their diabetes or blood sugar carefully. One of the most common triggers is illness or infection. When the body is fighting an infection, stress hormones such as cortisol and adrenaline are released, which signal the liver to release more glucose and reduce the effectiveness of insulin.^[5][13] This is sometimes called sick-day hyperglycaemia and is a well-recognised phenomenon in diabetes management, with NHS and Diabetes UK sick-day guidance available for people with diabetes.

Other common causes of unexpected glucose rises include:

• Eating more carbohydrates than usual, or consuming foods with a high glycaemic index

• Reduced physical activity, since exercise helps muscles absorb glucose independently of insulin

• Psychological stress, which triggers the same hormonal response as physical illness

• Certain medicines, including corticosteroids (such as prednisolone), some antipsychotics, and thiazide diuretics, which can impair insulin action or increase glucose production^[5][13]

• Missed or reduced doses of diabetes medication or insulin

• Hormonal changes, including those associated with the menstrual cycle or menopause

For people without a diabetes diagnosis, a single elevated reading after a large meal is not necessarily cause for alarm, as blood glucose naturally rises after eating. However, consistently elevated readings, or readings that remain high several hours after eating, warrant further investigation.

Some people find that glucose rises overnight or in the early morning, a phenomenon known as the dawn phenomenon, caused by natural hormonal surges that occur before waking. Keeping a record of readings alongside food, activity and medication can help identify patterns and inform discussions with a healthcare professional.

When should you see a GP about your blood sugar levels?

See a GP if you have symptoms of hyperglycaemia, fasting readings consistently above 7.0 mmol/L, known risk factors for type 2 diabetes, or readings outside your agreed target range despite following your management plan.

You should see a GP about your blood sugar levels if you are experiencing symptoms of hyperglycaemia, such as persistent thirst, frequent urination, unexplained fatigue or blurred vision, particularly if these symptoms are new or worsening. A GP can arrange a fasting blood glucose test or HbA1c measurement to assess whether your levels fall within the normal, non-diabetic hyperglycaemia or diabetic range, and will guide next steps based on the results.

You should also speak to your GP if:

• A home blood glucose monitor consistently shows readings above 7.0 mmol/L when fasting, or above 11.1 mmol/L at any time

• You have risk factors for type 2 diabetes, including a BMI above 25 kg/m2 (or above 23 kg/m2 in people of South Asian, Chinese or other Asian heritage, in line with NICE PH46 and NHS guidance; thresholds for other ethnic groups should be discussed with your GP), a family history of diabetes, a history of gestational diabetes, or high blood pressure^[10]

• You have been told you have non-diabetic hyperglycaemia (prediabetes) and have not had a review in the past year

• You are already diagnosed with diabetes and your readings are consistently outside your agreed target range despite following your management plan

If a child or young person develops symptoms such as increased thirst, frequent urination, unexplained weight loss or excessive tiredness, seek same-day urgent medical advice, as these may indicate type 1 diabetes, which requires prompt assessment and treatment.

For people managing diabetes with medication, unexplained high readings that do not respond to usual adjustments should prompt a GP or diabetes nurse review rather than self-adjustment of doses without guidance.

If you are exploring whether lifestyle changes or medical treatment might help manage elevated blood glucose or weight, speaking to a healthcare professional is the right starting point. Bolt Pharmacy is a GPhC-registered online pharmacy in Great Britain offering online consultations with prescriber review and pharmacist support, where clinically appropriate options may be discussed. In an emergency, such as suspected DKA or HHS, call 999 immediately rather than waiting for a GP appointment.

Scientific References

1. Type 2 diabetes: prevention in people at high risk (PH38).
2. Type 2 diabetes: prevention in people at high risk – recommendations (PH38).
3. Type 2 diabetes in adults: management – blood glucose management (NG28).
4. Type 2 diabetes in adults: management – recommendations (NG28).
5. High blood sugar (hyperglycaemia).
6. Diabetic ketoacidosis.
7. Impaired fasting glucose: how low should it go?.
8. The threshold for diagnosing impaired fasting glucose.
9. Management of Hyperosmolar Hyperglycaemic State (HHS).
10. Appendix A1: Summary of evidence from surveillance (PH46).
11. Pathogenesis of the dawn phenomenon in patients with IDDM.
12. Pathogenesis and prevention of the dawn phenomenon in diabetic patients.
13. High blood glucose (hyperglycaemia) in type 2 diabetes – patient information (NG18).

Frequently Asked Questions