The serum sodium falls by around 1 mmol/L each decade.4 Hyponatraemia has many causes and some of the more common ones are listed in Table 2. Hyponatraemia is prevalent in hospitalised elderly patients and is associated with increased mortality.20 In the study by Sunderam et al., an iatrogenic origin was thought to explain the low sodium levels in 73% of inpatients and was attributed mainly to intravenous fluids and diuretic use.20 Hyponatraemia is also common in the community, especially in patients with heart failure. In a recent study from Denmark, 2.1% of people aged 55 to 75 years with no apparent heart disease had a serum sodium of 134 mEq/L or less.21 They had a poorer prognosis than individuals with normal sodium levels, especially when it was not associated with diuretic use. When hyponatraemia becomes severe (serum sodium below 125 mmol/L), patients are more likely to develop symptoms, such as confusion and somnolence.