What is the normal serum sodium level?

• A. 120-130
• B. 125-135
• C. 135-145
• D. 150-160

Answer: (C)

Normal serum sodium is about 135 to 145 mEq/L. This range is the standard reference interval used in clinical labs because sodium is the main extracellular cation that helps regulate extracellular fluid volume and osmolarity, which affects cell hydration and nerve/muscle function. The kidneys, along with hormones like aldosterone and antidiuretic hormone, keep this balance tight. Values below 135 indicate hyponatremia, values above 145 indicate hypernatremia, with potential symptoms tied to brain function and fluid balance. Minor variations between laboratories can occur, but the commonly accepted normal range remains 135–145 mEq/L.