Adipsic hypernatremia with a reset osmostat.

D the wide variation in the daily intake of sodium and water, plasma osmolality and sodium are maintained within normal range by vasopressin [arginine vasopressin (AVP) or antidiuretic hormone (ADH)] secretion and thirst. The osmotic threshold for AVP secretion is 283 mOsm/kg and for thirst sense is 293 mOsm/kg. Hence, by the time plasma osmolality reaches the osmotic threshold for thirst, AVP secretion would achieve maximum urine concentration.1 In normal individuals, if water intake is voluntarily increased, the resultant fall in plasma osmolality will cut off AVP secretion allowing excess water to be excreted in urine, thereby maintaining normal plasma osmolality. If the sense of thirst is intact, patients with central diabetes insipidus can almost completely manage to maintain plasma osmolality near normal by increasing water intake. However, if the sense of thirst is lost (adipsia), the intact osmoregulated AVP secretion by itself cannot compensate even at its maximum response as it cannot stimulate water intake.1 A 10-year-old boy, previously healthy, presented to the Pediatric Outpatient Department at the Royal Hospital, Muscat, Sultanate of Oman, with a 3 months history of intermittent headache, lethargy and general weakness with hypernatremia (sodium 178 mmol/L) as an incidental finding. There was no vomiting, visual disturbances, polyuria, polyphagia, excessive thirst or any history of central nervous system infection or head trauma. General examination did not reveal any abnormality. He was prepubertal with height of l48 cm (>97 percentile) and weight 37 kg (>90 percentile). His blood pressure and temperature were normal, and pulse rate was 100/min. There was no clinical evidence of dehydration and skin was not doughy. His visual fields and ocular fundi were normal. The rest of the systemic examination was unremarkable. During his hospital stay, no behavioral, emotional or sleep disturbances were observed. The results of the following investigations were normal: complete blood count, liver function tests, serum calcium, phosphate, alkaline phosphatase and uric acid. Serum electrolytes were: sodium 178 mmol/ L, potassium 4.4 mmol/L, chloride 155 mmol/L, urea