High Blood Sugar (Hyperglycemia): Levels & Signs

High blood sugar — hyperglycemia — means there is more glucose in the blood than the body can use, usually because there is too little insulin or the body does not respond to it well. Short spikes are common; levels that stay high damage blood vessels and nerves over time, and very high levels can become an emergency.

What counts as high?

Symptoms

Many people feel nothing until glucose is well above 200 mg/dL. Typical signs then are:

• Thirst, dry mouth and frequent urination
• Tiredness, difficulty concentrating
• Blurred vision
• Headache
• Unintended weight loss (with long-standing high levels)
• Frequent infections, slow-healing wounds

Causes

• Undiagnosed diabetes or prediabetes
• A carbohydrate-heavy meal, or more food than the insulin dose covers
• Missed or too-low doses of insulin or tablets; insulin that has gone bad through heat
• Illness, infection, surgery or injury (stress hormones raise glucose)
• Medications such as corticosteroids, some antipsychotics and diuretics
• Less activity than usual; emotional stress
• The dawn phenomenon — early-morning hormone release

When high blood sugar is an emergency

Diabetic ketoacidosis (DKA) develops mainly in type 1 diabetes and LADA when insulin is lacking. The body burns fat and produces acidic ketones. Warning signs: nausea, vomiting, abdominal pain, deep rapid breathing, fruity-smelling breath, confusion — usually with glucose above 250 mg/dL, though DKA can occur at lower levels with SGLT2 inhibitors.

Hyperosmolar hyperglycemic state (HHS) occurs mainly in type 2 diabetes: extremely high glucose (often above 600 mg/dL), severe dehydration, drowsiness and confusion.

What to do

1. Re-check after washing your hands — sugar on the fingers can cause falsely high readings.
2. Drink water or other sugar-free fluids.
3. If you use insulin, take a correction dose as prescribed, and test ketones above 240 mg/dL.
4. A walk can help if you feel well and ketones are negative — don't exercise with ketones.
5. With diabetes: contact your care team today if you are above 300 mg/dL or levels keep rising. Without known diabetes: see a doctor today from 250 mg/dL.

More: How to Lower High Blood Glucose Quickly. Without diabetes, repeated high readings are a reason to have your fasting glucose or HbA1c checked.

Why long-term high blood sugar matters

Years of high glucose damage small and large blood vessels: eyes (retinopathy), kidneys (nephropathy), nerves (neuropathy, especially in the feet), heart and brain. Keeping HbA1c near target — along with blood pressure and cholesterol — sharply reduces these risks.

Check a specific number: 180, 250 or 300 mg/dL.