People who have diabetes lack a hormone or the hormone doesn't work properly.

the question is what does this hormone normally control in the body

People with diabetes either lack the hormone insulin or are not able to use insulin effectively. Insulin is made by the pancreas and it allows your body to use the glucose in the food that you eat for energy or to store glucose for later. The hormone insulin helps to keep your blood sugar from getting too high or too low. Insulin also affects other metabolic processes such as the breakdown of fat and protein.