Endocrine Abstracts

Introduction: A recent study suggests that copeptin, a surrogate measure of the hormone vasopressin and a known risk marker of cardiometabolic disease and decreased hydration, is non-linearly related to outdoor temperature with increased levels in both hot and cold temperatures. The same u-shaped relationship has been suggested between outdoor temperature and other hydration markers. In this study, we investigated the effect of apparent outdoor temperature on plasma sodium, the main contributor to increased plasma osmolality and vasopressin secretion.

Method: Data on plasma sodium from all adult patients visiting one of eight emergency departments (n=331,311) in 2017- 2018 in the southernmost region in Sweden (approximately 1.3 million inhabitants) were used. To investigate the association between apparent outdoor temperature (mean value the day of blood sampling) and plasma sodium, a cubic spline regression model with knots at the 25th, 50th, 75th and 90th temperature percentile was used. To investigate the relationship between apparent temperature and odds of hypo- and hypernatremia (P-Sodium <135 mmol/l and >145 mmol/l, respectively) among patients, a logistic regression model with the same knots was used. All of the analyses were adjusted for age, sex, day of the week, glucose, potassium and eGFR in a first model and ambulance arrival, hospital admission and current medication (diuretics, renin-angiotensin blocking agents, selective serotonin reuptake inhibitors and antidiabetics) on top of model one in a second model.

Results: Plasma sodium among patients was significantly and non-linearly associated with apparent temperature (P<0.001 for both). The highest plasma sodium concentration was observed at -0.2 degrees C mean outdoor temperature. The associations between apparent temperature and hyper- and hyponatremia, respectively, were significant and non-linear (P<0.001 for all). The odds of hyponatremia decreased with around 15% at mean temperatures between 0 to 10 degrees C, and increased at temperatures below -5 and above 19 degrees C. Hypernatremia increased with around 15% at temperatures around 0 degrees C and at temperatures above 20 degrees C.

Conclusion: Among patients seeking acute care, plasma sodium is related to outdoor temperature and reaches its highest concentration during cold temperatures. Our results indicate that hypohydration is more common during winter and could help explain the winter peak in copeptin concentration observed in previous studies. Given the pleiotropic hormonal effects of vasopressin, moderately increased water intake to lower vasopressin might mitigate adverse health effects of outdoor temperatures.