Sodium

Serum Sodium is the main cation in extracellular fluid and crucial for maintaining fluid balance, nerve impulse transmission, and muscle function. Levels are tightly regulated by the kidneys and hormones. Abnormal levels (hyponatremia or hypernatremia) usually reflect disturbances in water balance rather than sodium intake, often related to kidney function, hormonal issues (like ADH), or excessive fluid loss/gain.