Dipsogenic diabetes insipidus: a newly recognized syndrome caused by a selective defect in the osmoregulation of thirst.

We describe three patients who have polydipsia and polyuria due to an abnormality in the osmoregulation of thirst. The clinical manifestations of the syndrome are similar to those of neurogenic diabetes insipidus. Thus, under basal conditions the patients have thirst, normal to high normal levels of plasma osmolality, and low levels of plasma vasopressin. Moreover, antidiuretic therapy greatly reduces thirst and polydipsia as well as polyuria. The only clinically distinguishing feature of the response is that thirst and water intake decrease less rapidly than water excretion. As a consequence, the patients with this syndrome develop variable degrees of dilutional hyponatremia and hypoosmolemia during treatment. The plasma vasopressin response to osmotic stimulation is relatively normal. In most of the patients, the osmotic threshold for vasopressin release is at the upper limit of normal, but this finding only explains their modest elevation in basal plasma osmolality. Thirst and water intake also change as a function of plasma osmolality. However, the threshold or "set" of the thirst osmostat appears to be abnormally low. The degree of downward resetting varies from patient to patient, but is always sufficient to stimulate thirst and water intake at levels of plasma osmolality below the normal range. This abnormality can account not only for the thirst and polyuria under basal conditions but also for the overhydration that occurs during antidiuretic therapy. The pathogenesis of the osmoregulatory abnormality is unknown but may be due to disruption of one or more of the afferent pathways that regulate the "set" of the thirst and vasopressin osmostats.