Glucose (a type of sugar) is the body's main energy source, and hormones (such as insulin and glucagon) control the level of glucose in the blood. Hyperglycemia occurs when the level of glucose in the blood is higher than it should be.
Although there are other things that can lead to hyperglycemia, the most common cause is diabetes. In diabetes, the body does not make enough insulin or it can't respond normally to the insulin that is made. This causes the level of glucose in the blood to rise, leading to symptoms such as increased urination, extreme thirst, and unexplained weight loss.