A serum sodium level of 115 mEq/L may be due to:

A serum sodium level of 115 mEq/L indicates hyponatremia, a condition where sodium levels in the blood are abnormally low. This situation often occurs due to overhydration, where excess fluid dilutes the concentration of sodium in the bloodstream. When the body retains too much fluid or takes in excessive water relative to sodium, the sodium concentration drops, leading to potential physiological complications.

Overhydration can occur for several reasons, including excessive fluid intake, conditions that lead to water retention (like heart failure or certain kidney diseases), or inappropriate secretion of antidiuretic hormone (ADH). In contrast, dehydration typically causes elevated sodium levels due to decreased fluid volume in the body, leading to a higher concentration of sodium.

A very high sodium intake would not likely result in hyponatremia, as the body usually retains sodium effectively, thus raising blood sodium levels rather than lowering them. Similarly, an intake of 5.5 grams of sodium per day is generally considered within a normal range for most individuals and would not typically result in such low serum sodium levels. Therefore, overhydration is the correct explanation for a serum sodium level of 115 mEq/L.