Urine sodium concentrations are predictive of hypoadrenocorticism in hyponatraemic dogs: a retrospective pilot study

Objectives To determine if a urine sodium concentration could be used to rule out hypoadrenocorticism in hyponatraemic dogs. Materials and Methods Medical records were reviewed for hyponatraemic dogs (serum sodium<135 mmol/L) that had recorded urine sodium concentrations. Twenty hyponatraemic dogs were included: 11 diagnosed with classical hypoadrenocorticism and nine with non‐adrenal causes of hyponatraemia. A Wilcoxon rank‐sum test was used to compare results between groups. Results No dog with hypoadrenocorticism had a urine sodium concentration less than 30 mmol/L. Urine sodium concentration in dogs with hypoadrenocorticism was significantly higher (median 103 mmol/L, range: 41 to 225) than in dogs with non‐adrenal illness (median 10 mmol/L, range: 2 to 86) (P<0·0005). Serum sodium concentrations were not significantly different between dogs with hypoadrenocorticism and dogs with non‐adrenal illness. Clinical Significance These results suggest that urine sodium concentrations can be used to prioritise a differential diagnosis of hypoadrenocorticism in hyponatraemic dogs. A urine sodium concentration less than 30 mmol/L in a hyponatraemic dog makes classical hypoadrenocorticism an unlikely cause of the hyponatraemia. Nevertheless, because of the small sample size our results should be interpreted with caution and a larger follow‐up study would be valuable.