Glucose (a type of sugar) is the body's main energy source. Hormones, such as insulin, control the level of glucose in the blood. It's unhealthy if a person's glucose levels get too high or too low. Hyperglycemia occurs when the levels get too high. This can happen to someone who has diabetes but doesn't know it yet. It also can happen to someone whose diabetes is not under control. Signs of hyperglycemia include increased urination (peeing), extreme thirst, and unexplained weight loss.