Congenital sinus cyst in a foal

A rhesus monkey developed psychogenic polydipsia after a change in feeding regimen was instituted. Hyposthenuria, hyponatremia, hypochloremia, and an abnormal calculated plasma osmolality were found. Water deprivation testing and administration of antidiuretic hormone indicated that renal medullary solute depletion had resulted from the prolonged polyuria. Restricted water consumption resulted in resolution of the electrolyte abnormalities and restored ability to excrete concentrated urine.