An analysis of the relationship between serum cortisol and serum sodium in routine clinical patients

Adrenal insufficiency is an uncommon cause of hyponatraemia that should not be overlooked due to the severe consequences of an Addisonian crisis. Using the laboratory database of a large teaching hospital, we have explored the relationship between serum sodium and serum cortisol, and have estimated the frequency of hypoadrenalism in severely hyponatraemic patients.

Data were gathered over a 23 month period from the Laboratory Information Management System at the Leeds Teaching Hospitals NHS Trust for instances where serum sodium and cortisol had been measured on a single sample. Data were also gathered over the same time period for all patients with severe hyponatraemia (serum sodium ≤120 mmol/L) in order to determine the frequency of cortisol requesting and the incidence of adrenal insufficiency.

Analysis of the data (n=3268 patients) revealed a trend showing higher cortisol concentrations in patients who were severely hypo- or hypernatraemic. The median cortisol concentration for patients with sodium ≤110 mmol/L was 856 nmol/L, and there was a gradual decrease in cortisol over the sodium range ≤110-150 mmol/L (Rs =-0.323, p<0.0001). Patients with sodium ≥151 mmol/L had a median cortisol of 725 nmol/L. 42% of the 978 patients with serum sodium ≤120 mmol/L had serum cortisol measured within two weeks, of whom 1.7% were diagnosed with adrenal insufficiency.

This dataset shows rising cortisol in response to hypo- or hypernatraemia, in keeping with the stress response to illness. The data show that adrenal insufficiency is a rare cause of hyponatraemia which may be overlooked.