Objectives: The objectives of this paper are to review the presentation of hyponatraemia and diagnose its cause in psychiatric inpatients.

Method: Episodes of significant hyponatraemia (sodium <130mmol/L) were identified from computerised biochemical data and a retrospective review of patients' notes was carried out.

Result. The diagnoses of hyponatraemia reached were different for the two groups of patients studied, adults of over 65 years and under 65 years. Carbamazepine, polydipsia and medical conditions were found as causes in the below 65 age group. Diuretics, medical conditions and lofepramine were identified as causes in the over 65 age group. Inpatients with polydipsia below the age of 65 had lower sodium (p = 0.013) and urea levels (p = 0.00059) than other groups and patients with carbamazepine induced hyponatraemia tended to have episodic hypocalcaemia (p = 0.056). These groups were compared using Fisher's exact test. This study also showed the overall frequency of serum sodium of less than 135mmol/L to be 9.5% in psychiatric inpatients.

Conclusion: Hyponatraemia is a not uncommon finding in psychiatric inpatients. Attention to drug therapy and biochemical findings are helpful in its diagnosis. Accurate diagnosis is usually possible and may vary according to age-group.