Excessive passing of urine (very large quantities) and excessive thirst may be indications of diabetes insipidus. The thirst comes from the need to replace the passed water, however if the thirst is not quenched, severe dehydration may occur. This could lead to confusion, stupor, and even coma.
It is usually due to the failure of the pituitary gland to secrete a certain hormone, which is usually used to regulate the amount of water passed. It may occur due to a disease of the pituitary gland, or after brain surgery in a temporary form. Another form of the disease is due to the kidneys failing to respond to the hormone.