Objective: the most common cause of hyponatraemia in hospital settings is the syndrome of inappropriate antidiuretic hormone secretion (SIADH). This study compares the efficacy and safety of urea versus fluid restriction in the treatment of hyponatraemia caused by SIADH.

Methods: an observational cohort study was conducted with 212 patients suffering from hyponatremia (Na+ < 135 mmol/L) due to SIADH at the Complejo Hospitalario Universitario de Pontevedra between January 2015 and May 2022. Of these, 112 patients received urea (15 g/day) and 100 were treated with fluid restriction (1 liter/day). The primary objective was to normalize sodium levels (Na ≥ 135 mmol/L).

Results: urea was significantly more effective than fluid restriction. Sodium levels increased from 126.35 to 133.9 mmol/L with urea, compared to an increase from 126.44 to 130.5 mmol/L with fluid restriction (p < 0.001). Sodium normalization was achieved in an average of 6 days with urea, compared to 8 days with fluid restriction (p = 0.04). At discharge, 59.8 % of patients treated with urea reached normal sodium levels, compared to 42 % in the fluid restriction group (p = 0.007). The 60-day mortality rate was lower in the urea group (16.1 %) compared to the fluid restriction group (32.8 %) (p < 0.007).

Conclusions: urea is more effective than fluid restriction in normalizing sodium levels, with a better safety profile and lower 60-day mortality.