Endocrine disorder causing extreme thirst and excessive production of very dilute urine, apparently due to lack of antidiuretic hormone (vasopressin, which regulates the kidney's water conservation and urine production) or failure of the kidney tubules to respond to it.
Injections of synthetic vasopressin are effective if the hormone is lacking but not if the response is absent. Disorders of the hypothalamus are one cause of diabetes insipidus.