What is the normal range for adult serum sodium?

Normal serum sodium is 135-145 mEq/L. Sodium is the major extracellular cation, essential for fluid balance and neuromuscular function. Hyponatremia (below 135): symptoms based on rate of change — acute hyponatremia is more dangerous than chronic. Symptoms include nausea, headache, confusion, lethargy, seizures (especially below 120), coma. Causes: SIADH, diuretics, vomiting/diarrhea, heart failure, kidney failure, excessive water intake. Correction must be slow (no more than 8-10 mEq/L in 24 hours) to avoid osmotic demyelination syndrome. Hypernatremia (above 145): symptoms include thirst, dry mucous membranes, restlessness, irritability, weakness, seizures, coma. Causes: dehydration, diabetes insipidus, excessive sodium intake, certain medications. Treatment: gradual rehydration with hypotonic fluids. Both extremes require careful, controlled correction to prevent brain injury from rapid fluid shifts.