High blood sugar is called hyperglycemia. The term comes from Greek roots meaning “excessive sugar in the blood,” and it describes any situation where glucose levels rise above the normal range. A fasting blood sugar below 100 mg/dL is considered normal, so anything consistently above that threshold falls into hyperglycemia territory.
What the Numbers Mean
Blood sugar levels fall into distinct categories that determine whether you’re dealing with normal glucose, a warning zone, or diabetes. These ranges are based on a fasting blood test, meaning you haven’t eaten for at least eight hours:
- Normal: below 100 mg/dL
- Prediabetes: 100 to 125 mg/dL
- Diabetes: 126 mg/dL or higher on two separate tests
A random blood sugar reading of 200 mg/dL or higher, regardless of when you last ate, also suggests diabetes. After meals, blood sugar naturally rises, but in people without diabetes it rarely goes above 140 mg/dL and returns to baseline within two to three hours. For people already managing diabetes, the American Diabetes Association recommends keeping blood sugar between 80 and 130 mg/dL before meals and below 180 mg/dL one to two hours after eating.
There’s also a test called A1C that reflects your average blood sugar over roughly three months. An A1C below 5.7% is healthy. Between 5.7% and 6.4% indicates prediabetes, and 6.5% or higher on two separate tests means diabetes.
Why Blood Sugar Gets Too High
Your body runs on glucose. When you eat, food breaks down into sugars that enter your bloodstream, and your pancreas releases insulin in response. Insulin works like a key, unlocking your cells so they can absorb that sugar and use it for energy.
Hyperglycemia happens when this system breaks down. In type 1 diabetes, the pancreas produces little to no insulin, so glucose has no way into cells and accumulates in the blood. In type 2 diabetes, the process is more gradual. Cells get exposed to high insulin levels over a long period and eventually stop responding well to it. This is called insulin resistance. The pancreas compensates by producing even more insulin, but eventually it can’t keep up. Blood sugar climbs and stays elevated.
When blood sugar is persistently high, your body tries to stash the excess. It stores sugar in the liver and muscles first. Once those are full, the liver converts remaining sugar into body fat, which is one reason insulin resistance often goes hand in hand with weight gain.
Hyperglycemia Without Diabetes
You don’t need to have diabetes for your blood sugar to spike. This is called nondiabetic hyperglycemia, and it can appear suddenly during a serious illness, injury, or surgery. Burns, infections like pneumonia, and physical trauma can all trigger temporary glucose surges. Certain medications, particularly steroids and diuretics, can raise blood sugar as a side effect. Conditions like Cushing syndrome and polycystic ovarian syndrome also increase the risk.
Pregnancy brings its own form of high blood sugar called gestational diabetes, which develops when the body can’t produce enough insulin to meet the increased demands of pregnancy. Screening typically happens around 24 weeks, though it may be done earlier if risk factors are present. Gestational diabetes usually resolves after delivery but raises the long-term risk of developing type 2 diabetes later.
How High Blood Sugar Feels
Mild hyperglycemia often produces no symptoms at all, which is part of what makes it dangerous. Many people with type 2 diabetes walk around with elevated glucose for months or years before a blood test catches it. When symptoms do appear, the classic trio is frequent urination, increased thirst, and increased hunger. Your kidneys work harder to filter excess sugar, pulling more water from your body in the process, which makes you both dehydrated and thirsty.
Other common signs include unexplained weight loss, fatigue, blurry vision, irritability, and frequent urinary tract or yeast infections. Type 2 diabetes can also cause slow-healing wounds, numbness or tingling in the hands and feet, and dark patches of skin around the neck, armpits, or groin.
When Hyperglycemia Becomes an Emergency
At very high levels, hyperglycemia can trigger two life-threatening conditions. Diabetic ketoacidosis, or DKA, occurs most often in type 1 diabetes when the body has so little insulin that cells start burning fat for fuel instead. This produces acids called ketones that build up in the blood. Blood sugar during DKA typically exceeds 250 mg/dL and frequently reaches 350 to 500 mg/dL. Symptoms include nausea, vomiting, stomach pain, and a fruity smell on the breath.
The second emergency, called hyperglycemic hyperosmolar state, is more common in type 2 diabetes and involves extreme dehydration with blood sugar levels often exceeding 800 mg/dL. Both conditions require immediate medical attention, and both develop over hours to days, giving warning signs like extreme thirst, confusion, and very frequent urination before they become critical.
Related Terms You Might See
Hyperglycemia is the umbrella term, but you’ll encounter more specific language depending on when the high reading occurs. Fasting hyperglycemia refers to elevated blood sugar after not eating for at least eight hours. Postprandial hyperglycemia describes a spike after meals. Prediabetes is the label for blood sugar that’s above normal but not yet in the diabetic range, a zone where lifestyle changes can often reverse the trend.
The opposite of hyperglycemia is hypoglycemia, which means blood sugar has dropped too low, generally below 70 mg/dL. Despite sounding similar, the two conditions feel quite different and require opposite responses. Hypoglycemia causes shakiness, sweating, and confusion and needs fast-acting sugar to correct. Hyperglycemia builds more slowly and is managed through insulin, physical activity, or adjustments to what and how much you eat.