Normal Sodium Levels in Adults
The normal serum sodium concentration in adults is 135-145 mmol/L (or mEq/L), which is the standard reference range used universally across clinical practice. 1
Standard Reference Range
- Serum sodium 135-145 mmol/L defines the normal range for adults, with values below 135 mmol/L classified as hyponatremia and values above 145 mmol/L classified as hypernatremia 1, 2
- This reference range applies consistently across adult age groups and is derived from healthy populations 3
Clinical Context and Nuances
Hospital vs. Community Settings
- Hospitalized patients typically have lower sodium levels than outpatients of the same age, even when values fall within the "normal" range 3
- Among hospitalized adults, approximately 76% have normal sodium values (135-145 mmol/L), 22% have hyponatremia, and 3% have hypernatremia 4
Mortality Risk Within "Normal" Range
Even small deviations within the normal range carry prognostic significance:
- Risk of in-hospital mortality begins to rise at sodium levels >139-140 mmol/L, well within the traditionally accepted normal range 3
- Patients with admission sodium of 145 mmol/L have a 3.7-fold higher risk of in-hospital death compared to those with sodium of 140 mmol/L 3
- In elderly adults, each 2 mEq/L higher sodium level within the normal range reduces mortality risk by 14.9% 5
- Sodium levels of 135-138 mmol/L, though technically "normal," are associated with 2.7 times higher mortality risk compared to levels of 138.1-142 mmol/L 5
Age-Related Considerations
- Emergency admissions and elderly patients tend to have lower sodium levels within or below the normal range 3
- The distribution of sodium values in hospitalized elderly patients is broader and lower than the commonly accepted reference range 3
Dietary Sodium Intake
- Average sodium intake in US adults greatly exceeds recommendations, with men consuming approximately 4127 mg/day and women 3002 mg/day 6
- The American Heart Association recommends limiting sodium intake to <2400 mg/day, with further reduction to 1500 mg/day providing even greater cardiovascular benefits 6
- 99.4% of US adults consume more sodium than recommended by the AHA, with commercially processed foods accounting for the vast majority of sodium consumed 6
Common Pitfall
Do not dismiss sodium levels of 135-138 mmol/L as clinically insignificant simply because they fall within the "normal" range - these values are associated with increased mortality risk, particularly in hospitalized and elderly patients 3, 5. Even mild abnormalities warrant investigation of underlying causes and close monitoring 1.