Sodium (Na⁺)

The main electrolyte governing fluid balance.

What it is

Sodium is the principal extracellular cation and is the primary determinant of blood osmolality and fluid distribution between compartments. Small deviations from normal have significant clinical effects.

Why it matters

Hyponatraemia (low sodium) causes symptoms ranging from fatigue and confusion to seizures. It can occur with excessive fluid intake, certain medications, or inappropriate antidiuretic hormone secretion — a known paraneoplastic phenomenon in some cancers. Hypernatraemia typically reflects severe dehydration.

How to test

Standard electrolyte panel.

This information is for educational purposes only. Always discuss testing and interpretation of results with your care team.

Optimal range

Below 136 mmol/L

hyponatraemia — investigate cause

136–145 mmol/L

normal

Above 145 mmol/L

hypernatraemia — usually dehydration

How often

Each oncology blood panel.