High Blood Glucose: Causes, Symptoms and Treatment

High blood glucose concentration, known clinically as hyperglycaemia, occurs when glucose builds up in the bloodstream faster than the body can use or store it. This can happen for many reasons, from dietary habits and physical inactivity to underlying conditions such as type 1 or type 2 diabetes. Left unaddressed, persistently elevated levels can damage blood vessels, nerves and organs over time. This article explains what high blood glucose means, what causes it, which symptoms to watch for, how it is treated, and what long-term risks arise if it goes unmanaged.

What does a high blood glucose concentration actually mean?

High blood glucose concentration means glucose is accumulating in the bloodstream beyond what the body can use, diagnosed at a fasting plasma glucose of 7.0 mmol/L or above, or an HbA1c of 48 mmol/mol or above under NICE criteria.

A high blood glucose concentration means there is more glucose circulating in the bloodstream than the body can effectively use or store, a state clinically referred to as hyperglycaemia. Glucose is the body's primary fuel source, derived mainly from carbohydrates in food. After a meal, the pancreas releases insulin, a hormone that allows cells to absorb glucose and keeps blood levels within a healthy range. When this system is disrupted, glucose accumulates in the blood.

In clinical practice, blood glucose is measured in millimoles per litre (mmol/L). For most adults without diabetes, a fasting blood glucose level below 6.1 mmol/L is considered normal. According to NICE guidance (NG28) and WHO criteria, a fasting plasma glucose of 7.0 mmol/L or above on two separate occasions is used to diagnose type 2 diabetes in people without symptoms. Where symptoms of hyperglycaemia are present, a single raised result may be sufficient to confirm the diagnosis. A random plasma glucose of 11.1 mmol/L or above with symptoms, or a two-hour plasma glucose of 11.1 mmol/L or above on an oral glucose tolerance test (OGTT), are also accepted diagnostic thresholds.

Levels between 6.1 and 6.9 mmol/L fasting may indicate impaired fasting glucose. In UK clinical practice, this intermediate state is more commonly described as non-diabetic hyperglycaemia (NDH), which also encompasses an HbA1c of 42 to 47 mmol/mol. People with NDH are at increased risk of developing type 2 diabetes and are eligible for referral to the NHS Diabetes Prevention Programme.

HbA1c (glycated haemoglobin) reflects average blood glucose over the preceding two to three months. A HbA1c of 48 mmol/mol or above is the NICE threshold for diagnosing type 2 diabetes.^[1][2] However, HbA1c should not be used as the sole diagnostic test in certain situations: it is not appropriate during pregnancy, in suspected new-onset type 1 diabetes, in children, or where conditions affecting red blood cell turnover (such as haemoglobinopathies or anaemia) may give misleading results. In these circumstances, plasma glucose measurements are used instead.

Understanding what constitutes a high blood glucose concentration is the first step towards recognising when levels may be causing harm and when medical assessment is warranted.

What causes blood glucose levels to rise too high?

Blood glucose rises too high when the body produces insufficient insulin or cannot use it effectively, as in type 1 and type 2 diabetes, and is worsened by obesity, inactivity, corticosteroids, hormonal conditions and illness.

Blood glucose levels rise too high when the body either does not produce enough insulin, cannot use insulin effectively, or both. The most common underlying conditions are type 1 diabetes, in which the immune system destroys insulin-producing beta cells in the pancreas, and type 2 diabetes, in which cells become resistant to insulin's effects. Non-diabetic hyperglycaemia (NDH) represents an intermediate state where glucose regulation is impaired but has not yet reached diagnostic thresholds for diabetes.

Beyond these primary conditions, several other factors can drive blood glucose upwards:

• Diet: Consuming large quantities of refined carbohydrates and sugary foods and drinks causes rapid glucose spikes.

• Physical inactivity: Muscles use glucose during exercise; a sedentary lifestyle reduces this demand and can worsen insulin resistance.

• Obesity: Excess body fat, particularly around the abdomen, is strongly associated with insulin resistance.

• Illness and infection: Physical stress triggers the release of hormones such as cortisol and adrenaline, which raise blood glucose as part of the body's stress response.

• Certain medications: Corticosteroids (such as prednisolone), some antipsychotics and thiazide diuretics are among the medicines known to elevate blood glucose.^[4][5] The degree of effect varies by drug, dose and individual patient factors; anyone concerned about a medicine they are taking should discuss this with their GP or pharmacist.

• Hormonal conditions: Disorders such as Cushing's syndrome, acromegaly and polycystic ovary syndrome (PCOS) can impair glucose regulation. In PCOS, increased insulin resistance is a recognised feature, though not all people with PCOS will develop hyperglycaemia.^[6][16]

• Gestational diabetes: Pregnancy-related hormonal changes can cause temporary insulin resistance in some women.

Identifying the underlying cause is essential because it directly shapes the most appropriate management approach. A GP can arrange blood tests and a full clinical assessment to determine what is driving elevated levels in any individual.

What symptoms should I look out for and when should I get help?

Key symptoms include increased thirst, frequent urination, fatigue and blurred vision; DKA or HHS features such as vomiting, confusion or rapid breathing require immediate 999 or A&E attendance.

High blood glucose concentration can produce a recognisable cluster of symptoms, though in some people, particularly those with gradually rising levels, there may be few or no obvious signs in the early stages. The most commonly reported symptoms include:

• Increased thirst (polydipsia) and a dry mouth

• Frequent urination (polyuria), including waking at night to pass urine

• Fatigue and lack of energy

• Blurred vision

• Headaches

• Difficulty concentrating

• Slow healing of cuts or wounds

• Recurrent infections, such as thrush or urinary tract infections

These symptoms arise because the kidneys work to filter excess glucose from the blood, drawing water with it and causing increased urination and thirst. Cells deprived of glucose may signal hunger and fatigue even when calorie intake is adequate.

In people with type 1 diabetes or those with very high glucose levels, a serious complication called diabetic ketoacidosis (DKA) can develop. Warning signs include nausea, vomiting, abdominal pain, rapid breathing and a fruity smell on the breath. DKA is a medical emergency: call 999 or go to A&E immediately if these features are present. People with diabetes who are unwell are advised to check blood or urine ketones; NHS guidance on sick-day rules provides further detail.

A different but equally serious emergency, hyperosmolar hyperglycaemic state (HHS), can occur in people with type 2 diabetes. HHS typically develops more gradually than DKA and does not usually involve ketones, but it causes severe dehydration and can lead to confusion, drowsiness and, in severe cases, seizures or loss of consciousness. If these features are present, call 999 or go to A&E immediately.

If you are an adult with new symptoms of polyuria, polydipsia and unexplained weight loss, particularly if they have come on rapidly, seek same-day urgent medical assessment, as these features may indicate new-onset type 1 diabetes.

For symptoms that are new, persistent or worsening but not immediately life-threatening, contact your GP promptly. If you are unsure whether your symptoms require urgent attention, NHS 111 can provide guidance. People already diagnosed with diabetes who notice consistently elevated readings on their glucose monitor should also contact their GP or diabetes care team for a medication or management review.

What treatments and lifestyle changes can bring blood glucose down?

Treatment combines dietary changes, at least 150 minutes of weekly moderate exercise and weight management; metformin is the NICE first-line medication for type 2 diabetes when lifestyle changes are insufficient.

Bringing a high blood glucose concentration down depends on the underlying cause, the severity of elevation and whether a diagnosis of diabetes has been established. Management typically combines lifestyle modification with, where necessary, medication.

Lifestyle measures form the foundation of treatment for NDH and type 2 diabetes and can meaningfully reduce blood glucose:

• Dietary changes: Reducing refined carbohydrates and added sugars, increasing fibre intake and following a balanced diet can lower post-meal glucose spikes. The NHS Eatwell Guide provides a practical framework.

• Regular physical activity: The UK Chief Medical Officers' Physical Activity Guidelines recommend at least 150 minutes of moderate-intensity activity per week for adults.^[8][9] Exercise improves insulin sensitivity and helps muscles absorb glucose more efficiently.

• Weight management: Even modest weight loss of 5 to 10 per cent of body weight can significantly improve glucose control in people with type 2 diabetes or NDH.^[1][2]

• Reducing alcohol intake and stopping smoking also support better metabolic health.

People identified with NDH may be eligible for referral to the NHS Diabetes Prevention Programme (NDPP), which provides structured support for lifestyle change.

Medications are prescribed when lifestyle changes alone are insufficient. Metformin remains the first-line oral medication recommended by NICE (NG28) for type 2 diabetes.^[3][5] Other classes include SGLT-2 inhibitors (such as dapagliflozin) and GLP-1 receptor agonists. In the UK, semaglutide is available as two distinct licensed products with different indications: Ozempic is authorised for the management of type 2 diabetes, while Wegovy is authorised for chronic weight management.^[19][20] Tirzepatide (Mounjaro), a dual GIP/GLP-1 receptor agonist, is MHRA-authorised both for type 2 diabetes and for chronic weight management. People with type 1 diabetes require insulin therapy. Eligibility, contraindications and suitability for any of these medicines (including during pregnancy or breastfeeding) are assessed by a prescriber on an individual basis.

Bolt Pharmacy is a GPhC-registered UK online pharmacy offering online weight-management consultations, with prescriber review and pharmacist support. Medicines such as Mounjaro or Wegovy are one option a prescriber may consider following an online consultation with Bolt Pharmacy, and are only prescribed where clinically appropriate after individual assessment.

All medication decisions should be made in partnership with a GP or specialist, who can tailor treatment to individual circumstances, comorbidities and preferences.

What are the long-term health risks if high blood glucose is left untreated?

Untreated high blood glucose causes progressive damage to blood vessels and nerves, leading to cardiovascular disease, kidney disease, retinopathy, peripheral neuropathy and increased infection risk.

Persistently high blood glucose concentration, if left unmanaged, causes progressive damage to blood vessels and nerves throughout the body, leading to a range of serious complications. These risks apply both to people with undiagnosed or poorly controlled diabetes and to those with sustained non-diabetic hyperglycaemia (NDH).

Cardiovascular disease is the leading cause of death in people with type 2 diabetes.^[14][15] Chronic hyperglycaemia accelerates atherosclerosis, increasing the risk of heart attack, stroke and peripheral arterial disease. NICE guidance (NG28) emphasises cardiovascular risk reduction as a central goal of diabetes management.

Diabetic kidney disease (nephropathy) develops when high glucose damages the small blood vessels in the kidneys, impairing their ability to filter waste. Over time, this can progress to chronic kidney disease and, in severe cases, kidney failure requiring dialysis or transplantation.

Diabetic eye disease (retinopathy) results from damage to the retinal blood vessels. People with pre-existing diabetic eye disease, particularly those with pre-proliferative or proliferative retinopathy, face a risk of worsening if blood glucose falls rapidly after a period of poor control.^[17][18] Anyone with existing retinopathy should discuss glucose targets and monitoring frequency with their diabetes team before making significant changes to their management. Regular NHS diabetic eye screening is recommended for all people with diabetes to detect changes early.

Peripheral neuropathy causes numbness, tingling or pain, most commonly in the feet and legs, and increases the risk of foot ulcers and, in severe cases, amputation.

Increased infection risk and poor wound healing are also well-recognised consequences of chronic hyperglycaemia.

Routine annual care processes, including blood pressure measurement, lipid testing, kidney function and urine albumin checks, and foot examination, are recommended for people with diabetes in line with NICE quality standards and UK care processes.^[13] These checks help detect complications early and guide treatment adjustments.

Many of these complications are preventable or can be significantly delayed with timely diagnosis and effective management. Regular monitoring, structured diabetes reviews with a GP or diabetes team, and adherence to agreed treatment plans are the most important protective steps a person can take.

Scientific References

1. Type 2 diabetes in adults: management (NG28) – Recommendations.
2. Blood glucose management | Type 2 diabetes in adults (NG28).
3. Initial medicines | Type 2 diabetes in adults: management (NG28).
4. High blood sugar (hyperglycaemia).
5. Diabetes | Treatment summaries – BNF.
6. Polycystic ovary syndrome | Treatment summaries – BNF.
7. Tirzepatide | Drugs – BNF.
8. Physical activity guidelines: UK Chief Medical Officers' report.
9. UK Chief Medical Officers' physical activity guidelines.
10. Physical activity guidelines for adults aged 19 to 64.
11. MHRA authorises diabetes drug Mounjaro (tirzepatide) for weight management and weight loss.
12. Tirzepatide – NHS medicines information.
13. 9 key care processes | Type 2 diabetes in adults (QS209).
14. Cardiovascular disease and diabetes profiles: statistical commentary.
15. Trends in predominant causes of death in individuals with and without diabetes. The Lancet Diabetes & Endocrinology.
16. Metabolic inflexibility in women with polycystic ovary syndrome: a systematic review. Gynecological Endocrinology.
17. Beyond Glycemic Control: Ocular Effects of Glucagon-like Peptide-1 Receptor Agonists. Vision (Basel).
18. Retinal Outcomes in Diabetes: Antihyperglycemic Therapy, EWDR, and Perioperative Considerations. Biomedicines.
19. Semaglutide – NHS medicines information.
20. GLP-1 medicines for weight loss and diabetes: what you need to know.

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