What is Sodium?
Sodium (Na⁺) is the most abundant cation in extracellular fluid and the primary determinant of plasma osmolality and fluid distribution throughout the body. The normal serum sodium concentration in adults is 136–145 mmol/L. This narrow reference window is maintained by two complementary systems: antidiuretic hormone (ADH, also called vasopressin), which determines how much water the kidney retains, and the aldosterone-renin-angiotensin system, which governs sodium retention and excretion. Together these systems keep sodium concentration nearly constant despite large swings in salt and fluid intake. Low sodium (hyponatraemia, < 135 mmol/L) is the most common electrolyte disorder in clinical practice. Causes are diverse: excessive ADH secretion (SIADH — syndrome of inappropriate ADH secretion, occurring with lung disease, brain pathology, or certain medications), thiazide diuretics, heart failure, liver cirrhosis, nephrotic syndrome, hypothyroidism, and in otherwise healthy people excessive water intake without salt compensation. Symptoms range from fatigue, nausea, and headache in mild cases to confusion, seizures, and coma in severe or rapidly developing cases. High sodium (hypernatraemia, > 145 mmol/L) is less common and arises almost invariably from insufficient free water intake or excessive water loss: dehydration, heat stress, diabetes insipidus (central or nephrogenic), or rarely excessive salt loading. Hypernatraemia is associated with increased mortality in older hospitalised patients and manifests clinically as confusion, irritability, and muscle weakness.
Why is Sodium relevant?
Sodium is clinically relevant because even mild deviations directly affect central nervous system function. Neurons are particularly sensitive to osmotic shifts: in hyponatraemia, brain cells swell as water flows inward; in hypernatraemia, they shrink. This explains why sodium disturbances produce a spectrum ranging from mild cognitive complaints and coordination problems to severe encephalopathy and seizures, depending on severity and rate of onset. For preventive monitoring in healthy people, sodium is rarely the marker that demands immediate action, but there are situations where it adds relevant information: people using loop diuretics or thiazides (classically increased risk of hyponatraemia), people with kidney problems (impaired regulation of sodium and water balance), athletes drinking large volumes of water without salt replacement (exercise-associated hyponatraemia), and people with persistent complaints of dizziness, headaches, or confusion. When investigating thyroid, kidney, or adrenal function, sodium is part of the broader laboratory picture. With persistently low sodium, SIADH is always considered, for which additional tests — urine osmolality, urine sodium, thyroid function, cortisol — are indicated.
Sodium high or low — what it means
Always interpret sodium alongside potassium, creatinine, and eGFR, and in the context of hydration, medication, and the clinical situation. The first question with an abnormal sodium is whether the disturbance sits in the water balance or the sodium balance: hyponatraemia from water excess (dilutional hyponatraemia) is clinically different from hyponatraemia from sodium loss. Plasma osmolality, urine osmolality, and urine sodium help localise the cause — this is diagnostic territory that belongs with a clinician. With borderline or mild hyponatraemia (130–135 mmol/L) in someone without symptoms, the first question is whether there are confounding factors. Markedly elevated blood glucose raises osmolality and 'dilutes' measured sodium: for every 10 mmol/L rise in glucose, measured sodium falls by roughly 1.6 mmol/L. Markedly elevated triglycerides or proteins can give a falsely low sodium reading with older laboratory techniques (pseudohyponatraemia) — with modern ion-selective electrode methods this is rarely an issue. A repeat test under comparable conditions gives a more reliable picture than a single abnormal value. Persistent, unexplained abnormalities call for additional investigation into the underlying regulatory cause.
Educational information only — not medical advice. Consult a healthcare professional for clinical decisions.
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