High levels of glucose in the blood is called high blood sugar (hyperglycemia). This occurs when the body makes too little insulin or the body cannot use insulin properly. Insulin is the hormone that helps the body move glucose from the blood and into muscles or fat, where it is used for energy. High blood sugar can occur in people with diabetes or pre-diabetes. It may be caused by not following your diabetes meal plan, inactivity, stress or trauma, or not taking diabetes medicines correctly. When glucose builds up in the blood it can cause symptoms such as being very thirsty, blurry vision, dry skin, feeling weak or tired, and having to urinate a lot. It is important to recognize early symptoms of hyperglycemia and treat it right away. Untreated hyperglycemia may affect the eyes, kidneys, nerves, and heart.