Increased Blood Sugar (Hyperglycemia)

Blood sugar ranges explained

For most people with diabetes, the American Diabetes Association suggests targets of about 80 to 130 mg/dL (4.4 to 7.2 mmol/L) before meals and under 180 mg/dL (10.0 mmol/L) 1 to 2 hours after starting a meal. Your own targets may differ.

Why blood sugar goes up

Insulin, made by the pancreas, moves glucose from the blood into cells. Blood sugar rises when there is too little insulin (type 1 diabetes, pancreatic damage) or the body does not respond to it well (insulin resistance in type 2 diabetes and prediabetes), or when hormones that oppose insulin are high.

Causes in people without known diabetes

• Undiagnosed type 2 diabetes or prediabetes: the most common cause in adults. Risk rises with excess weight, inactivity, family history, age over 35 and a history of gestational diabetes.
• Type 1 diabetes: often develops over weeks, especially in children and young adults, with thirst, weight loss and tiredness.
• Pregnancy: gestational diabetes, usually found on a test at 24 to 28 weeks.
• Stress hyperglycemia: severe illness, infection, surgery, a heart attack or stroke release stress hormones that raise sugar temporarily.
• Medicines: corticosteroids (prednisone, dexamethasone), some antipsychotics, thiazide diuretics, statins (small effect), some HIV medicines and tacrolimus.
• Hormonal conditions: Cushing syndrome, acromegaly and overactive thyroid.
• Pancreas disease: pancreatitis, pancreatic cancer, cystic fibrosis and hemochromatosis.

Causes in people with diabetes

• eating more carbohydrate than usual
• missed or too-low doses of insulin or tablets, or insulin damaged by heat
• less physical activity
• infection or illness, even a cold
• stress, poor sleep and menstrual cycle changes
• the "dawn phenomenon": early-morning hormone surges raising fasting sugar

Symptoms of high blood sugar

Many people have no symptoms until levels are well above 180 to 200 mg/dL (10 to 11 mmol/L). Then:

• excessive thirst and dry mouth
• frequent urination, including at night
• tiredness and weakness
• blurred vision
• headaches and poor concentration
• unintended weight loss
• recurrent infections: thrush, urinary infections, skin infections
• slow-healing cuts and sores

Dangerous complications

Diabetic ketoacidosis (DKA)

DKA happens mainly in type 1 diabetes when there is too little insulin. The body burns fat for fuel, producing acidic ketones. Signs develop over hours to a day or two: nausea, vomiting, abdominal pain, fruity breath, deep rapid breathing, drowsiness and confusion. It can occur with only moderately high sugar, especially with SGLT2 inhibitor medicines.

Hyperosmolar hyperglycemic state (HHS)

HHS occurs mostly in older people with type 2 diabetes, often triggered by infection. Sugar climbs very high, often over 600 mg/dL (33.3 mmol/L), over days, causing severe dehydration, confusion and drowsiness.

Long-term effects

Years of raised blood sugar damage blood vessels and nerves, increasing the risk of heart attack, stroke, kidney disease, eye damage (retinopathy), nerve damage, foot ulcers and erectile dysfunction. Keeping levels in range greatly reduces these risks.

How high blood sugar is diagnosed

A doctor confirms raised blood sugar with a lab test: fasting glucose, HbA1c or an oral glucose tolerance test, usually repeated on a second day unless symptoms are clear. They may also check urine for ketones, kidney function, cholesterol, and antibodies if type 1 diabetes is suspected.

Home glucose meters and continuous glucose monitors are used to track levels day to day in people already diagnosed.

How to lower high blood sugar

If you have diabetes and a high reading

• Follow your plan: take a correction dose of insulin if prescribed.
• Drink water to replace fluid lost in urine. Avoid sugary drinks.
• Check ketones if sugar is over 240 to 250 mg/dL (13.3 to 13.9 mmol/L), or as your team advises, especially with type 1 diabetes.
• Gentle activity such as a walk can lower sugar, but do not exercise if ketones are present.
• Sick-day rules: keep taking insulin when ill even if not eating, check sugar every 2 to 4 hours, and ask your team which tablets to pause during vomiting or dehydration.

Long-term control

• regular physical activity, about 150 minutes a week
• a diet lower in refined carbohydrates and sugary drinks, with more fiber, vegetables and whole grains
• losing 5% to 10% of body weight if overweight, which can return prediabetes to normal in many people
• medicines such as metformin, GLP-1 receptor agonists, SGLT2 inhibitors and insulin, chosen with your doctor
• regular HbA1c checks, usually every 3 to 6 months

Frequently asked questions

What are the symptoms of high blood sugar?

Common symptoms are increased thirst, frequent urination, tiredness, blurred vision, headaches and slow-healing cuts. Many people with mildly raised sugar have no symptoms, which is why screening blood tests are recommended for adults at risk.

What is a dangerous blood sugar level?

Levels over 250 to 300 mg/dL (13.9 to 16.7 mmol/L), especially with ketones, vomiting or drowsiness, need urgent attention. Levels over 600 mg/dL (33.3 mmol/L) can cause hyperosmolar hyperglycemic state. Symptoms matter as much as the number.

Can you have high blood sugar without diabetes?

Yes. Severe illness, infection, surgery, steroid medicines, pregnancy and hormonal conditions can raise blood sugar temporarily. A test that is high once should be repeated to see whether prediabetes or diabetes is present.

How can I lower my blood sugar quickly?

If you have diabetes, take correction insulin as prescribed, drink water and go for a gentle walk if there are no ketones. Do not try to lower sugar sharply without a plan from your care team. Seek urgent care for vomiting, ketones or confusion.

Does stress raise blood sugar?

Yes. Stress hormones such as cortisol and adrenaline release stored glucose and make the body less sensitive to insulin. People with diabetes often see higher readings during illness, emotional stress or poor sleep.