Unstable Osmoreceptors and Defective Thirst in Hypothalamic Hypopituitarism

An 8-year-old girl with hypothalamic hypopituitarism is described. The clinical course was characterized by fluctuation between diabetes insipidus and water intoxication. In an attempt to find a physiological explanation she underwent two sets of dehydration and osmotic threshold tests. The presence of endogenous vasopressin, and the normally functioning volume receptors, was demonstrated by the normal urine osmolality during dehydration. The osmotic threshold was 263 mosm/kg on one test and 300 mosm/kg on the other. More extreme values might be suspected based on simultaneous urine and plasma osmolality obtained during acute episodes of water intoxication and severe dehydration. With plasma osmolality as high as 307 mosm/kg, the child denied thirst. The data appear to indicate an instability of the osmoreceptor mechanism and a deficiency of the thirst mechanism with intact volume receptors.