This test measures the amount of insulin, the hormone that lets cells take in glucose. Glucose, a sugar that comes from food, is the body's main source of energy. Our bodies break down foods we eat into glucose and other nutrients, which are then absorbed into the bloodstream from the gastrointestinal tract. Glucose levels in the blood rise after meals and trigger the pancreas to make insulin and release it into the blood. Insulin works like a key that opens the doors to cells and allows the glucose in. Without insulin, glucose can't get into the cells and it stays in the bloodstream. For good health, the body must be able to keep insulin and glucose levels in balance. With too little insulin, blood sugar remains higher than normal (a condition known as hyperglycemia) and cells can't get the energy they need. With too much insulin, blood sugar decreases (hypoglycemia), causing symptoms such as sweating, trembling, lightheadedness, and in extreme cases, shock. The most common cause of abnormal fluctuations in blood sugar is diabetes.