If someone's diabetes isn't under control, he or she might be really thirsty and drink a lot of fluids. Polydipsia is the medical word for this. In diabetes, polydipsia happens because the person's blood sugar levels are high and the kidneys are working overtime to get rid of the sugar. To do this, the kidneys produce more urine (pee). A lot of fluid leaves the body in pee so the person feels really thirsty and tries to replace the fluid by drinking a lot.