Sodium is the most abundant electrolyte in the blood and the fluid surrounding your cells. It is central to maintaining fluid balance, blood pressure regulation, nerve impulse transmission, and muscle contraction. The kidneys are the primary regulators of sodium, constantly adjusting how much is retained or excreted to keep blood sodium within a tight range of 136 to 145 mmol/L.
Because the kidneys are so effective at regulating sodium, blood sodium levels do not directly reflect how much salt you eat. You can consume a high-salt diet and still have a normal blood sodium level, because healthy kidneys simply excrete the excess. Blood sodium is better understood as a reflection of the balance between sodium and water in your body rather than dietary salt intake.
Low sodium (hyponatraemia) is one of the most common electrolyte abnormalities seen in clinical practice. It can result from dilution (too much water relative to sodium), as occurs with excessive water intake, SIADH (syndrome of inappropriate antidiuretic hormone), or heart and liver failure. It can also result from sodium loss through diuretic use, vomiting, diarrhoea, or adrenal insufficiency. Symptoms range from mild (nausea, headache, confusion) to severe (seizures, loss of consciousness) depending on how quickly levels drop.
High sodium (hypernatraemia) is less common and usually reflects inadequate water intake or excessive water loss. It is more frequently seen in elderly patients, those with limited access to water, or in conditions like diabetes insipidus where the kidneys cannot concentrate urine properly.
Sodium is most informative when measured alongside potassium, chloride, and bicarbonate as part of an electrolyte panel. The pattern of abnormalities across these four markers helps identify the underlying cause and guide your practitioner's assessment.
To assess hydration status and fluid balance, investigate symptoms of electrolyte disturbance, screen for kidney or adrenal dysfunction, and monitor the effects of medications that affect sodium (including diuretics, corticosteroids, and some antidepressants).
