High Blood Sugar (Hyperglycemia): Causes, Symptoms, Levels and What to Do

What counts as high

Blood glucose moves through the day, rising after meals and settling between them. "High" therefore depends on when it is measured.

The threshold of about 180 mg/dL has a physiological meaning: above it, the kidneys can no longer reabsorb all the glucose passing through them, and glucose spills into the urine, dragging water with it. That is the origin of the classic symptoms.

For people with diabetes, target ranges are set individually. A common set is 80–130 mg/dL before meals and below 180 mg/dL after, but older adults or those prone to hypoglycemia may have looser targets.

What causes it

In someone without a diabetes diagnosis

• Undiagnosed prediabetes or type 2 diabetes — the most common explanation for repeated high readings.
• Acute illness, infection or surgery. Stress hormones raise glucose and reduce insulin sensitivity; this "stress hyperglycemia" usually resolves with recovery but can unmask underlying diabetes.
• Medications, especially corticosteroids (prednisone, dexamethasone), some diuretics, antipsychotics, beta-blockers and certain HIV and transplant drugs.
• Pregnancy (gestational diabetes).
• Pancreatic disease — pancreatitis, cystic fibrosis, or pancreatic surgery.
• Hormonal disorders such as Cushing's syndrome or an overactive thyroid.

In someone with diabetes

• Too much carbohydrate for the insulin or medication available
• A missed or reduced dose of medication or insulin
• Illness, infection or injury
• Physical inactivity
• Emotional stress or poor sleep
• Steroid medication
• The dawn phenomenon — early-morning hormone release raising fasting glucose
• Expired or improperly stored insulin, or a failed pump or pen

What happens inside the body

When glucose is high, cells that depend on insulin — muscle and fat — are short of fuel despite the abundance around them. The body responds as if it were starving: it releases more glucose from the liver, breaks down fat for fuel, and drives hunger. Meanwhile, tissues that take up glucose without insulin — the lens of the eye, nerves, kidneys, the lining of blood vessels — are flooded with it.

In the short term, this produces the symptoms below. Over months and years, excess glucose attaches to proteins throughout the body, thickens and weakens the walls of small blood vessels, and generates oxidative stress that damages nerves. The long-term effects on the body are covered separately.

Symptoms

Early hyperglycemia is often silent. As glucose rises above the kidney threshold, symptoms appear in a predictable sequence: frequent urination, then thirst, then fatigue and blurred vision, then — if glucose keeps rising — weight loss, nausea and drowsiness. The symptom guide covers each in detail.

When high blood sugar is an emergency

Two conditions can develop when glucose runs very high.

Diabetic ketoacidosis (DKA) occurs when there is not enough insulin for the body to use glucose at all, so it burns fat rapidly and produces acids called ketones. It is most common in type 1 diabetes but can occur in type 2, particularly during illness or with SGLT2 inhibitor medication (where it can occur at only moderately raised glucose). Signs: nausea and vomiting, abdominal pain, deep rapid breathing, fruity-smelling breath, drowsiness or confusion.

Hyperosmolar hyperglycemic state (HHS) develops more slowly, usually in older people with type 2 diabetes, when glucose climbs very high — often above 600 mg/dL (33 mmol/L) — and severe dehydration follows. Signs: extreme thirst, dry mouth, weakness, confusion, and eventually seizures or loss of consciousness.

How it is brought down

A moderate spike

For a reading above target but below the emergency range, in someone who feels well:

• Drink water to counter the fluid loss.
• Move. A 15–30 minute walk lowers glucose by increasing muscle uptake. Avoid intense exercise if glucose is above 250 mg/dL and ketones are present, as it can raise glucose further.
• Skip further carbohydrate until the level falls.
• Take medication as prescribed; people on insulin follow their correction plan.
• Re-check in one to two hours.

Persistent high readings

Readings that stay above target for more than a day or two despite the above mean the treatment plan needs adjusting. Contact your care team rather than repeatedly correcting. Common fixes are a medication dose change, adding a medication, reviewing carbohydrate intake, or treating an infection that is driving the rise.

Severe hyperglycemia

DKA and HHS are treated in hospital with intravenous fluids, insulin and electrolyte replacement, and by treating whatever triggered them.

Preventing high blood sugar

For people with diabetes, the fundamentals are consistent carbohydrate intake, taking medication reliably, regular activity, sleep, and a sick-day plan agreed in advance with their care team (which usually involves checking glucose more often, continuing insulin, and knowing when to seek help). For people without a diagnosis, repeated high readings are a reason to be tested, not managed at home.

Can supplements play a supportive role?

Some people explore supplements marketed for blood sugar. None is appropriate for bringing down a high reading, and none replaces medication. The ingredient evidence, and the interactions with diabetes drugs, are reviewed in Can supplements lower blood sugar?.

Frequently asked questions

References