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High blood sugar (hyperglycemia): symptoms, levels and what to do
Key takeaways
- For most adults with diabetes, targets are 80–130 mg/dL before meals and below 180 mg/dL 1–2 hours after (ADA).
- Typical symptoms: thirst, frequent urination, tiredness, blurry vision and headache.
- Above 240 mg/dL, people who use insulin should check ketones.
- Vomiting, belly pain, fruity breath or fast breathing with high glucose can mean DKA — an emergency.
What is hyperglycemia?
Hyperglycemia means there is too much glucose in the blood because the body has too little insulin or cannot use it well. Occasional highs are common in diabetes; staying high for months raises the risk of diabetes complications. Persistent high readings in someone without a diagnosis can be a first sign of diabetes — see diabetes symptoms.
High blood sugar levels
| Reading | What it suggests |
|---|---|
| Fasting 100–125 mg/dL | Prediabetes range (if you don't have diabetes) |
| Fasting 126 mg/dL or higher | Diabetes range (needs confirmation) |
| Above 180 mg/dL after meals | Above the usual target for adults with diabetes |
| Above 240 mg/dL | Check ketones if you use insulin; call your care team if it persists |
| Above 600 mg/dL | Risk of hyperosmolar hyperglycemic state (HHS) — seek care urgently |
Symptoms of high blood sugar
- Increased thirst and dry mouth
- Peeing more often, including at night
- Fatigue and weakness
- Blurred vision
- Headache, trouble concentrating
- Over time: weight loss, slow-healing cuts, frequent infections
Common causes
- Eating more carbohydrate than usual, or sugary drinks
- Missed or too little diabetes medicine or insulin; an insulin pump problem
- Illness or infection — the flu can push glucose up for days
- Stress, poor sleep, menstrual cycle changes
- Medicines such as steroids
- The dawn phenomenon: early-morning hormones raise fasting glucose
When high blood sugar is an emergency
Diabetic ketoacidosis (DKA) happens when the body burns fat for fuel and ketones build up. It is most common in type 1 diabetes but can occur in type 2. Warning signs:
- Nausea, vomiting or belly pain
- Fruity-smelling breath
- Fast, deep breathing
- Very dry mouth, extreme tiredness, confusion
If you have high glucose with moderate or large ketones, or any of these signs, contact your care team right away or go to the emergency room. HHS, seen mostly in type 2 diabetes, brings very high glucose with severe dehydration and confusion and also needs emergency care.
What to do about a high reading
- Drink water and recheck.
- Take medicine or correction insulin as your care plan says.
- If you feel well and have no ketones, light activity such as a walk can help — but don't exercise with ketones present.
- Note what may have caused it and look for patterns.
Spot your patterns
Cardivo keeps glucose readings with meal context, CGM data from Apple Health, heart rate and blood pressure in one journal — so you can see how meals, sleep, stress and illness move your numbers and share a PDF with your doctor. See also low blood sugar and the A1C chart.
This guide is general wellness education based on the public sources below. It is not medical advice, diagnosis, or treatment. Cardivo is not a medical device. If you think you are having a medical emergency, call 911.
Frequently asked questions
What blood sugar level is too high?
For adults with diabetes, the usual target is below 180 mg/dL one to two hours after a meal and 80–130 mg/dL before meals. Readings above 240 mg/dL call for a ketone check if you use insulin.
What are the first signs of high blood sugar?
Increased thirst, frequent urination, tiredness, blurred vision and headache.
When is high blood sugar an emergency?
High glucose with nausea, vomiting, belly pain, fruity breath, fast breathing or confusion can mean DKA or HHS and needs emergency care.
Sources
- American Diabetes Association — Hyperglycemia (high blood glucose)
- CDC — Diabetic ketoacidosis
- NIDDK — Know your blood sugar numbers