High blood sugar is a finding in a number of conditions, most notably diabetes mellitus. Elevated blood glucose leads to spillage of glucose into the urine (glucosuria) so that the urine is sugary. (The term diabetes mellitus means "sweet urine.")
Aside from diabetes, the many other causes of high blood sugar include just eating more sugar (or food) than usual, the presence of an infection or another illness, an injury and the stress of surgery.
High blood sugar may produce few or no symptoms. When there are symptoms, they may be dry mouth, thirst, frequent urination, urination during the night, blurry vision, fatigue or drowsiness, weight loss, or increased appetite.
An elevated level of blood sugar may be a useful independent indicator of heart disease risk. The risk of dying from heart disease has been found to rise as the level of blood glucose increases (regardless of age, weight, blood pressure, cholesterol and smoking status).