Primary Polydipsia	

Primary polydipsia may occur for no apparent reason, but more often it is related to some form of psychiatric illness. Mouth dryness caused by drugs (e.g. monoamine oxidase inhibitors) or mouth-breathing must be distinguished from true polydipsia. Vasopressin secretion is normally suppressed by the low plasma osmolality in primary polydipsia, but its renal action may also be impaired if excessive fluid intake is prolonged, since the solute concentration within the renal interstitial medulla is reduced by a ‘washout’ effect.