Someone with high blood sugar has a higher-than-normal amount of glucose in the blood, states the American Diabetes Association. Blood sugar increases when someone does not make enough insulin or does not have a normal response to insulin.
High blood sugar causes increased thirst, frequent urination and sugar in the urine, according to the American Diabetes Association. The medical term for this condition is hyperglycemia. Type 1 diabetics sometimes develop hyperglycemia if they do not take enough insulin. Patients with unmanaged type 2 diabetes can also develop hyperglycemia. Without treatment, hyperglycemia may lead to ketoacidosis, a serious condition that causes nausea, vomiting and shortness of breath; it can lead to a coma.