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Hyponatraemia is a common cause of hospitalisation in older adults. Adrenal insufficiency (AI) can result in hyponatraemia.The aim of our study was to determine the frequency and characteristics of AI in elderly patients with hyponatraemia.Thirty patients ≥65 years with Na+≤130 mmol/L and 30 age-matched control subjects, all hospitalised, were included in the study. Plasma cortisol levels were determined before and after intravenous administration of 1 μg synthetic adrenocorticotropin hormone. A peak cortisol >550 nmol/L was considered to exclude AI.Sodium levels were 125 ± 5 and 139.8 ± 2 mmol/L in the hyponatremic and control groups respectively. Baseline cortisol <550 nmol/L was found in a half of hyponatremic patients. However, stimulated cortisol levels were compatible with AI in only one case (3%) and none of the controls. The mean cortisol levels were significantly higher in hyponatremic compared with control subjects, both in the basal state (585 ± 215 and 381 ± 135 nmol/L, respectively, P < 0.001) and after stimulation (933 ± 254 and 781 ± 160 nmol/L, P < 0.05). However, the incremental increase in cortisol levels after stimulation was similar in the two groups (361 ± 196 and 403 ± 155 nmol/L)AI is an uncommon cause of hyponatraemia in older age. Based on this small cohort, AI may be present in 3% of elderly patients with hyponatraemia. AI cannot be excluded by baseline cortisol in a significant minority of hyponatremic patients and further testing with adrenocorticotropin hormone stimulation is needed.