If your blood sugar levels are too high, it can cause a range of unpleasant symptoms. The most common symptoms include extreme thirst and the constant feeling that you need to urinate. Other symptoms include headaches, feelings of tiredness or fatigue, blurry vision, nagging hunger, or trouble focusing or concentration. Those with extremely high blood sugar levels may experience diabetic coma which requires immediate medical attention. It is very important to monitor glucose levels to try and avoid feeling these symptoms.