High blood sugar (hyperglycemia) is most often seen in people who have diabetes that isn’t well controlled. The symptoms of high blood sugar can be mild, moderate, or severe.

Mild high blood sugar

If your blood sugar levels are consistently higher than your target range (usually 200 milligrams per deciliter (mg/dL) to 350 mg/dL in adults and 200 mg/dL to 240 mg/dL in children), you may have mild symptoms of high blood sugar. You may urinate more than usual if you are drinking plenty of liquids. Some people who have diabetes may not notice any symptoms when their blood sugar level is in this range. The main symptoms of high blood sugar are:

• Increased thirst.
• Increased urination.
• Weight loss.
• Fatigue.
• Increased appetite.

Hyperglycemia means high (hyper) glucose (gly) in the blood (emia). Your body needs glucose to properly function. Your cells rely on glucose for energy. Hyperglycemia is a defining characteristic of diabetes—when the blood glucose level is too high because the body isn’t properly using or doesn’t make the hormone insulin.

If you’ve had diabetes for any length of time at all, you’ve probably seen lists of the signs and symptoms of high blood glucose dozens of times. Doctors and diabetes educators hand them out. Hundreds of websites reprint them. Most diabetes books list them. You likely know some of the items on the list by heart: thirst, frequent urination, blurry vision, slow healing of cuts, and more.

But have you ever stopped to wonder why these symptoms occur? How does high blood glucose cause frequent urination, make your vision go blurry, or cause all of those other things to happen? Here are some answers to explain what’s going on in your body when you have high blood glucose.