The risk is particularly pronounced among older adults and women.

In the large, registry-based cohort study, researchers at the Department of Clinical Research and Education, Södersjukhuset, Karolinska Institutet, analyzed the relationship between outdoor temperature and severe hyponatremia among adults. The study included more than 50,000 episodes in nearly 22,000 individuals, making it one of the largest investigations of its kind.

“We observed a clear increase in severe hyponatremia with rising temperatures, particularly at temperatures above 20 °C. The effect was most pronounced in older individuals and in women,” says Issa Issa, lead author and researcher at Karolinska Institutet.

The results showed that the incidence of severe hyponatremia increased almost linearly with temperature up to about 20 °C, and then rose sharply. Among individuals aged over 80, there were more than 100 episodes of severe hyponatremia per million person-days at temperatures exceeding 22 °C. Women were affected about twice as often as men.

A growing public health concern

Using a predictive model combining projections of climate change and demographic trends, the researchers estimate that the incidence of severe hyponatremia in Stockholm could increase by up to 66 percent by 2050, given a temperature rise of just 1 °C.

About the study

Published in Journal of the American Society of Nephrology (JASN). The work forms part of Issa Issa’s doctoral thesis, From Drugs to Climate: Associations Between External Factors and Profound Hyponatremia, which he successfully defended in December 2025.