Introduction The syndrome of inappropriate antidiuresis (SIAD) is the commonest cause of euvolaemic hyponatraemia in patients admitted to hospital. The mortality after discharge from hospital has not been previously studied in patients with SIAD.

Aims To compare mortality in patients with SIAD and those with kidney injury (KI). To identify underlying diagnoses associated with deaths due to SIAD.

Methods Single-centre retrospective cohort analyses of 804 patients with severe hyponatraemia over a 3-year period. Five-year survival data in patients with SIAD and those with KI were compared. The underlying diagnoses that contributed to SIAD in this cohort were analysed using ICD-10 codes.

Results 202 patients had SIAD using biochemical cut-off parameters; 248 patients had KI. Patient with KI had a statistically significant (log-rank p<0.0001) shorter median survival time (2.24 months (95% CI 1.3 to 4.3)) compared with those with SIAD (31.0 months (95% CI 21.6 to 54.8)). 53.8% (n=78) of patients with hyponatraemia due to SIAD died within the first year after admission; the corresponding figure for those presenting with KI was 74.1% (n=166). Five years after admission, 80.8% (n=117) of those with SIAD had died; the corresponding figure for those with KI was 88.4% (n=200). In those patients with SIAD that died within the first year, malignancy appeared to be the most common cause (25.4%) followed by infection (23.8%).

Conclusions Severe hyponatraemia in SIAD carries a high mortality after discharge, and although this seems often to be attributable to the underlying cause, the extent to which treatment with V2-recptor antagonists may help to correct the hyponatraemia associated with SIAD and influence the medium-to-long-term outcome in such patients is worthy of further study.