Diabetes insipidus is a hormonal disorder caused by the lack of the hormone vasopressin that regulates urine production. It can also be due the kidneys being unresponsive to vassopressin. Left to their own devices, the kidneys will produce urine continuously, so a hormone restricts this production when hydration levels in the body are low to prevent dehydration.
However, when this hormone is missing (primarily due to an injury to the pituitary gland, which produces the hormone) or not being responded to, the kidneys produce urine continuously. This results in dehydration, which causes the patient to drink water almost continuously, day and night, in order to replace fluids.
However, the disease is difficult to diagnose. It can be mistaken for diabetes mellitus, which also causes frequent urination and severe thirst. However, the urine of insipidus patients does not contain high levels of glucose. It can also be mistaken for certain psychological disorders that cause constant water drinking. However, insipidus patients generally show to be slightly dehydrated rather than fully hydrated. In Top Secret, it was misdiagnosed as Obsessive-Compulsive Disorder, but as House pointed out, OCD patients don't get thirsty in their sleep.