Sodium: Sodium is an electrolyte essential for fluid balance, kidney and heart function, and nerve signaling. Most sodium is found in fluids around your cells. High levels can occur with dehydration, hypertension, fluid retention, or kidney/heart issues. Low levels may result from diuretics, excessive sweating, diarrhea, or overhydration. | Patient Information Leaflet (PIL): www.askdrskip.com/hydration