What Causes High Blood Sugar? Signs and Treatment

High blood sugar, or hyperglycaemia, is a common clinical concern that affects millions of people across the UK. Understanding what causes high blood sugar is essential for early detection, effective management, and preventing serious complications. Blood glucose rises when the body cannot produce enough insulin, cannot use it properly, or both. While diabetes is the most well-known cause, illness, certain medicines, hormonal conditions, and lifestyle factors can all contribute. This article explains the key causes, warning signs, diagnostic tests used on the NHS, and when to seek urgent medical help.

What causes high blood sugar and why does it happen?

High blood sugar occurs when the body cannot produce enough insulin or cannot use it effectively, causing glucose to accumulate in the bloodstream rather than enter cells for energy.

High blood sugar, known medically as hyperglycaemia, occurs when the body cannot produce enough insulin, cannot use insulin effectively, or both. Insulin is a hormone produced by the pancreas that acts like a key, allowing glucose from food to enter cells and be used for energy. When this process breaks down, glucose accumulates in the bloodstream instead of being absorbed by tissues.

The underlying mechanism varies depending on the cause. 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, cells become resistant to insulin's effects over time, and the pancreas gradually loses its ability to compensate by producing more. In both cases, blood glucose rises above the normal fasting range of approximately 4.0 to 5.9 mmol/L, as referenced by NHS guidance on blood sugar levels.

High blood sugar is not always linked to diabetes. Temporary rises can occur in anyone during illness, physical stress, or after eating large amounts of carbohydrate-rich food. However, persistently elevated levels are a clinical concern that warrants investigation. Understanding the root cause is essential because the appropriate management differs considerably depending on whether hyperglycaemia is transient or part of an underlying condition such as diabetes or prediabetes.

Which medical conditions and lifestyle factors raise blood sugar?

Type 2 diabetes is the most common cause, but PCOS, Cushing's syndrome, pancreatitis, gestational diabetes, and medicines such as corticosteroids can also raise blood glucose.

Several medical conditions and everyday lifestyle factors can raise blood sugar, either temporarily or over the long term. The most common underlying condition is type 2 diabetes, which accounts for around 90 per cent of all diabetes diagnoses in the UK, according to Diabetes UK prevalence data.^[2] Other conditions that can cause or contribute to hyperglycaemia include:

• Type 1 diabetes, an autoimmune condition requiring lifelong insulin therapy

• Gestational diabetes, which develops during pregnancy and usually resolves after birth but increases future risk of type 2 diabetes

• Polycystic ovary syndrome (PCOS), which is associated with insulin resistance

• Cushing's syndrome, where excess cortisol impairs insulin action

• Pancreatitis or pancreatic cancer, which can damage insulin-producing cells

• Certain medicines, including corticosteroids, some antipsychotics, and thiazide diuretics, which can raise blood glucose as a recognised side effect; this list is illustrative rather than exhaustive, and the BNF or NICE Clinical Knowledge Summaries provide further detail on drug-induced hyperglycaemia^[3]

Lifestyle factors play a significant role, particularly in the development of type 2 diabetes and prediabetes. A diet high in refined carbohydrates and added sugars, physical inactivity, excess body weight (especially abdominal fat), chronic psychological stress, and poor sleep quality are all associated with impaired insulin sensitivity. Alcohol consumption can also cause unpredictable fluctuations in blood glucose, sometimes causing a rise followed by a delayed fall.

Genetics and ethnicity also influence risk. People of South Asian, Black African, or Black Caribbean heritage are at higher risk of developing type 2 diabetes at a lower body mass index than the general population. NICE guidance (PH46) sets lower BMI thresholds for these groups when assessing diabetes risk and determining eligibility for prevention programmes.

What are the warning signs that your blood sugar is too high?

The classic warning signs are increased thirst, frequent urination, fatigue, and blurred vision, though many people with early type 2 diabetes or prediabetes have no symptoms.

The classic warning signs of high blood sugar include increased thirst, frequent urination, fatigue, and blurred vision. These symptoms reflect the body's attempts to manage excess glucose in the bloodstream. When blood glucose rises above the kidney's reabsorption threshold (approximately 10 mmol/L), glucose spills into the urine, drawing water with it and causing the characteristic increased urination and thirst.^[1]

Other symptoms that may indicate persistently elevated blood sugar include:

• Unexplained weight loss, particularly in type 1 diabetes, where the body breaks down fat and muscle for energy

• Slow-healing cuts or wounds, because high glucose impairs immune function and circulation

• Recurrent infections, such as thrush or urinary tract infections

• Headaches and difficulty concentrating

• Tingling or numbness in the hands or feet, which may suggest early nerve involvement

It is important to note that many people with mildly elevated blood sugar, particularly in the early stages of type 2 diabetes or prediabetes, experience no symptoms at all. This is why routine screening is so important. The NHS Health Check programme, available to adults aged 40 to 74 in England, includes an assessment of diabetes risk and can identify raised blood sugar before symptoms develop.^[6][7]

In children, young people, and some adults, type 1 diabetes can develop rapidly. The Diabetes UK '4Ts' campaign highlights the key signs to look out for: Toilet (frequent urination), Thirsty (increased thirst), Tired (unusual fatigue), and Thinner (unexplained weight loss). If these symptoms appear together, particularly in a child or young person, seek same-day urgent medical assessment rather than waiting for a routine appointment.

If you notice several of these symptoms together, particularly increased thirst and urination, contact your GP for a blood test. Do not wait for symptoms to worsen before seeking advice, as prolonged hyperglycaemia can cause complications affecting the eyes, kidneys, nerves, and cardiovascular system.

How is high blood sugar diagnosed and monitored on the NHS?

HbA1c is the primary diagnostic test; a result of 48 mmol/mol or above on two occasions confirms type 2 diabetes, while 42 to 47 mmol/mol indicates prediabetes under NICE NG28.

High blood sugar is diagnosed on the NHS using standardised blood tests, with the HbA1c (glycated haemoglobin) test being the most commonly used for diagnosing type 2 diabetes and prediabetes. HbA1c reflects average blood glucose over the preceding two to three months. According to NICE guidance (NG28), an HbA1c of 48 mmol/mol or above on two separate occasions confirms a diagnosis of type 2 diabetes, while a result of 42 to 47 mmol/mol indicates prediabetes (non-diabetic hyperglycaemia).^[8][9]

HbA1c is not appropriate for diagnosis in all situations. It should not be used as the sole diagnostic test in children and young people, during pregnancy, where type 1 diabetes is suspected (because of the rapid onset of symptoms), or in people with conditions that affect red blood cell turnover, such as haemoglobinopathies or anaemia. In these circumstances, fasting plasma glucose or an oral glucose tolerance test (OGTT) is used instead, and clinical judgement guides the choice of test.

Other diagnostic tests include:

• Fasting plasma glucose, measured after at least eight hours without food; a result of 7.0 mmol/L or above indicates diabetes

• Random plasma glucose, where a result of 11.1 mmol/L or above alongside symptoms is diagnostic

• Oral glucose tolerance test (OGTT), in which blood glucose is measured two hours after a standard 75 g glucose drink; a two-hour result of 11.1 mmol/L or above indicates diabetes, and 7.8 to 11.0 mmol/L indicates impaired glucose tolerance. The OGTT is used routinely in pregnancy, where different thresholds apply under NICE guidance (NG3)

Once diagnosed, blood sugar is monitored regularly. People with type 2 diabetes managed by diet alone or with oral medication typically have their HbA1c checked every three to six months until stable, then annually. Those on insulin or at higher risk of hypoglycaemia may use self-monitoring of blood glucose (SMBG) with a finger-prick device, or in some cases a continuous glucose monitor (CGM).

NICE recommends that people with type 1 diabetes are offered CGM as a standard of care (NICE NG17).^[13][14] For type 2 diabetes, CGM availability on the NHS depends on individual clinical need and local ICB commissioning decisions. Your GP or diabetes team can advise on which monitoring approach is most appropriate for your situation.

What treatments and lifestyle changes can bring blood sugar down?

Lifestyle modification including dietary changes and physical activity is the foundation of treatment, with metformin as first-line medication and further options guided by NICE NG28 pathways.

Bringing high blood sugar down depends on the underlying cause, but lifestyle modification is the foundation of management for most people with type 2 diabetes or prediabetes. NICE guidance (NG28) recommends that all newly diagnosed individuals receive structured education, dietary advice, and support to increase physical activity before or alongside medication. Even modest weight loss of 5 to 10 per cent of body weight can significantly improve insulin sensitivity and reduce HbA1c, as supported by NICE NG28 and evidence from the DiRECT trial, which underpins the NHS Type 2 Diabetes Path to Remission programme.^[10][16]

Key lifestyle changes include:

• Dietary adjustment: reducing refined carbohydrates, sugary drinks, and ultra-processed foods; increasing fibre, vegetables, and lean protein

• Regular physical activity: at least 150 minutes of moderate-intensity exercise per week, in line with the UK Chief Medical Officers' physical activity guidelines^[18][19]

• Weight management: for people with obesity and type 2 diabetes, structured weight-loss programmes can lead to remission in some cases

• Reducing alcohol intake and stopping smoking, both of which worsen insulin resistance and cardiovascular risk

When lifestyle changes alone are insufficient, medication is introduced following NICE NG28 treatment pathways, with the choice of medicine guided by individual factors including cardiovascular and renal risk. Metformin remains the first-line oral medication for type 2 diabetes in most people. Other commonly used classes in UK practice include SGLT2 inhibitors (such as dapagliflozin or empagliflozin), DPP-4 inhibitors, sulfonylureas, pioglitazone, GLP-1 receptor agonists (such as semaglutide, available as Ozempic for type 2 diabetes or Wegovy for weight management), and dual GIP/GLP-1 receptor agonists such as tirzepatide (Mounjaro), which is MHRA-authorised both for type 2 diabetes and for chronic weight management.^[23][24] People with type 1 diabetes require insulin therapy. This list is illustrative; your prescriber will advise on the most appropriate option for your circumstances.

For those exploring weight management options as part of blood sugar control, Bolt Pharmacy is a GPhC-registered UK online pharmacy offering online weight-management consultations, with prescriber review and pharmacist support, where a range of clinically appropriate options may be considered.

When should you seek urgent medical help for high blood sugar?

Call 999 or go to A&E immediately if signs of DKA or HHS are present, including fruity breath, vomiting, confusion, or rapid breathing, as both conditions are life-threatening.

You should call 999 or go to your nearest accident and emergency department immediately if you or someone else with diabetes develops signs of diabetic ketoacidosis (DKA) or hyperosmolar hyperglycaemic state (HHS), both of which are life-threatening emergencies. DKA occurs most commonly in type 1 diabetes when severe insulin deficiency causes the body to break down fat rapidly, producing acidic ketones. HHS tends to occur in type 2 diabetes and involves extreme dehydration and very high blood glucose.

Emergency warning signs requiring 999 or A&E include:

• Rapid or laboured breathing, or breath that smells fruity or of pear drops (a sign of ketones)

• Vomiting that prevents you from keeping fluids down

• Severe abdominal pain

• Confusion, drowsiness, or loss of consciousness

If your blood glucose is persistently elevated and you are also detecting ketones on a urine or blood ketone test, seek urgent clinical advice from NHS 111 or your diabetes team rather than waiting for a routine appointment. They can assess whether you need emergency care or can be managed with prompt outpatient support.

People taking SGLT2 inhibitors (such as dapagliflozin or empagliflozin) should be aware that DKA can occasionally occur even when blood glucose is not very high (euglycaemic DKA).^[25][26] If you feel unwell and have symptoms that could suggest DKA, seek urgent clinical assessment even if your glucose reading appears near normal.

For less severe but still concerning symptoms, such as blood glucose that remains elevated despite your usual medication, or new symptoms you are unsure about, contact NHS 111 for urgent advice or speak to your GP. Do not attempt to manage a suspected DKA or HHS at home.

If you experience a side effect from a diabetes or weight-management medicine that concerns you, report it to your healthcare team. Suspected adverse drug reactions can also be reported to the MHRA via the Yellow Card scheme at yellowcard.mhra.gov.uk. Early reporting helps the MHRA monitor the safety of medicines used across the UK population and contributes to ongoing pharmacovigilance.

Scientific References

1. High blood sugar (hyperglycaemia).
2. Diabetes profile: statistical commentary, March 2025.
3. Diabetes | Treatment summaries — BNF.
4. Appendix A1: Summary of evidence from surveillance (PH46/NG246) — BMI thresholds for South Asian and other ethnic groups.
5. PH46 Expert report 2: Analyses from the SABRE cohort study — BMI equivalence in South Asians and African Caribbeans.
6. NHS Health Check.
7. First national evaluation of NHS Health Check programme published.
8. Type 2 diabetes in adults: management (NG28) — Blood glucose management chapter.
9. Type 2 diabetes: prevention in people at high risk (PH38) — Recommendations.
10. Type 2 diabetes in adults: management (NG28).
11. Diabetes in pregnancy: management from preconception to the postnatal period (NG3).
12. Diabetes in pregnancy: management from preconception to the postnatal period (NG3) — Recommendations.
13. Type 1 diabetes in adults: diagnosis and management (NG17).
14. Type 1 diabetes in adults: diagnosis and management (NG17) — Recommendations.
15. Continuous glucose monitoring | Type 1 diabetes in adults (QS208).
16. Early findings from the NHS Type 2 Diabetes Path to Remission programme (Valabhji et al., The Lancet Diabetes & Endocrinology).
17. 5-year follow-up of the randomised Diabetes Remission Clinical Trial (DiRECT).
18. UK Chief Medical Officers' physical activity guidelines report.
19. UK Chief Medical Officers' physical activity guidelines (full text).
20. Physical activity guidelines for adults aged 19 to 64.
21. Type 2 diabetes in adults: management (NG28) — Initial medicines chapter.
22. Type 2 diabetes in adults: choosing, reviewing and stopping medicines — visual summary (NG28).
23. MHRA authorises diabetes drug Mounjaro (tirzepatide) for weight management and weight loss.
24. Tirzepatide | Drugs | BNF.
25. SGLT2 inhibitors: updated advice on the risk of diabetic ketoacidosis.
26. Empagliflozin 10 mg film-coated tablets — SGLT2 inhibitors: advice on the risk of diabetic ketoacidosis (safety alert).
27. EMA confirms recommendations to minimise ketoacidosis risk with SGLT2 inhibitors in diabetes.

Frequently Asked Questions