Clinical Biochemistry & Internal Medicine & Family Medicine

Sodium (natremia) — hyponatremia and hypernatremia

Sodium (Na⁺) is the primary cation in the extracellular compartment—accounting for approximately 90% of the osmotically active solutes in plasma—and the major determinant of plasma osmolality (osmolality = 2 × [Na⁺] + [glucose mmol/L] + [urea mmol/L]) and thus of cell volume: a change in serum sodium most often reflects a disturbance in the water-sodium balance (excess free water in hyponatremia → swollen cells + deficit of free water in hypernatremia → shrunken cells) rather than a change in total sodium. Normal serum sodium levels range between 136 and 145 mmol/L in adults and are maintained by two regulatory systems: antidiuretic hormone (ADH—vasopressin), secreted by the posterior pituitary in response to plasma hyperosmolality or hypovolemia → acts on V2 receptors in the renal collecting duct → reabsorption offree water → urine concentration and normalization of serum sodium levels + the thirst mechanism triggered by hypothalamic osmoreceptors → increased fluid intake + and the renin-angiotensin-aldosterone system regulating sodium retention. Both conditions—hyponatremia (< 136 mmol/L) and hypernatremia (> 145 mmol/L) — are among the most common and potentially serious electrolyte abnormalities in clinical practice, particularly in hospitalized elderly patients, due to their direct impact on the volume and function of brain cells (the brain is particularly sensitive to osmotic changes) — and due to the specific danger of too rapid correction: too rapid correction of chronic hyponatremia can cause irreversible centropontine myelinolysis (osmotic demyelination syndrome—ODS), and too rapid correction of hypernatremia can cause fatal cerebral edema.

Reference values and classification

Normal adult natremia: 136 to 145 mmol/L (mEq/L)
Mild hyponatremia 130–135 mmol/L + moderate: 125–129 mmol/L + severe: < 125 mmol/L + very severe with serious symptoms: Less than 120 mmol/L
Mild hypernatremia 146–150 mmol/L + moderate: 151–159 mmol/L + severe: ≥ 160 mmol/L
Pseudohyponatremia Artifactual hyponatremia in massive hypertriglyceridemia + significant paraproteinemia → water is diluted in a plasma rich in lipids or proteins → low natremia due to measurement artifact (flame method) → normal measured plasma osmolality → verify by direct method (potentiometry) or calculate osmolality

Hyponatremia: Causes and Diagnostic Approach

Urine osmolality + Natriuresis	Cause	Mechanism + context
Urine osmolality < 100 mOsm/kg (dilute urine) = psychogenic polydipsia or excessive water intake	Potomania + primary polydipsia + excess beer (beer potomania) + hypoosmolar infusions	Free water intake exceeding renal excretory capacity + normal but water-overloaded kidney + treatment: fluid restriction
Urine osmolality > 100 mOsm/kg (concentrated urine) + Natriuresis < 30 mmol/L (kidney conserves Na)	True hypovolemia (extrarenal Na loss): diarrhea + vomiting + third-spacing + adrenal insufficiency + burns	Fluid loss → ADH release → fluid retention → dilutional hyponatremia + kidneys reabsorb Na (low natriuresis) + treatment: 0.9% NaCl 1–3 times daily
Urinary osmolality > 100 mOsm/kg + Natriuresis > 30 mmol/L (kidney loses Na)	SIADH (Syndrome of Inappropriate Antidiuretic Hormone Secretion) + adrenal insufficiency + hypothyroidism + thiazide diuretics	SIADH = most frequent cause of hyponatremia in hospital → ADH secreted despite normal or low osmolality → water retention + volume expansion + Na loss in urine + causes of SIADH: bronchial cancer + pneumonias + medications (SSRIs + carbamazepine + cyclophosphamide + NSAIDs) + neurological (SAH + meningitis + trauma)
Urinary osmolarity > 100 mOsm/kg + variable urinary Na + hypervolemia (edema)	Heart failure + liver cirrhosis + nephrotic syndrome	Reduced circulating effective volume → ADH activation → water retention + increased total sodium but low serum sodium (dilution) + treatment: sodium and fluid restriction + diuretics

Symptoms and emergencies of hyponatremia

Mild to moderate symptoms: nausea + headaches + mild confusion + fatigue + irritability + muscle cramps
Severe symptoms (hyponatremic encephalopathy — emergency): somnolence + stupor + vomiting + seizures + coma + respiratory arrest + cerebral herniation → usually hyponatremia < 120–125 mmol/L or sudden drop regardless of absolute value
Severe symptomatic hyponatremia – urgent correction: sérum salé hypertonique (NaCl 3 %) 150 mL IV en 20 minutes → objectif : augmentation de la natrémie de 4 à 6 mmol/L dans la première heure pour stopper les symptômes cérébraux → puis correction lente à ne pas dépasser 10 à 12 mmol/L par 24 heures (risque de myélinolyse centropontine si correction trop rapide)
Central Pontine Myelinolysis (CPM — Osmotic Demyelination Syndrome): irreversible demyelination of the brainstem (pons) if correction of chronic hyponatremia is too rapid → locked-in syndrome + quadriplegia + dysphagia + encephalopathy → maximum risk factors: hypokalemia + alcoholism + malnutrition + correction > 12 mmol/L/24 h

SIADH — Diagnostic Criteria and Treatment

Diagnostic Criteria for SIADH (Bartter and Schwartz): Hyponatremialess than 136 mmol/L) + low plasma osmolality (( 100 mOsm/kg → urine not maximally diluted) + natriuresis > 30 mmol/L + clinically normal blood volume (no edema + no dehydration) + normal renal + thyroid + adrenal function
Management of asymptomatic or mildly symptomatic chronic SIADH: Water restriction (500–1,000 mL/day) = first-line treatment + treatment of underlying cause + salt + oral urea (0.25–0.5 g/kg/day — increases free water excretion capacity) + or vaptans (tolvaptan — Samsca® + IV conivaptan) → ADH V2 receptor antagonists → selective aquaresis (free water excretion without Na) → very effective + but risk of overly rapid correction → cautious use under close monitoring of natremia

Hypernatremia - causes and treatment

Mechanism	Main cause	Treatment
Free water loss (hypertonic dehydration)	Central diabetes insipidus (ADH deficiency — hypothalamic tumor + head trauma + post-surgery) or nephrogenic (kidneys insensitive to ADH — lithium + hypercalcemia + kidney disease) + insensible skin losses (fever + sweating + burns) + osmotic diarrhea	Oral free water if conscious + or 0.45% NaCl 1–3 L IV or SG 5 1–3 L (IV free water) + conservative correction: do not exceed a 10 mmol/L decrease in serum sodium per 24 hours + calculation of free water deficit: Deficit (L) = 0.6 × weight × (serum sodium/140 − 1)
Excessive sodium intake	Hypertonic NaCl infusions + IV sodium bicarbonate + inappropriate oral NaCl + hyperosmolar enteral feeding	Stop sodium intake + free water compensation + furosemide if hypervolemia
Mixed water + sodium loss (water > sodium)	Osmotic diarrhea + significant renal losses (loop diuretics + hyperglycemia + mannitol)	Progressive replacement with hypo-osmolar solutions

ℙ️ The 10 mmol/L per 24-hour rule applies to both conditions: do not correct hyponatremia by more than 10 to 12 mmol/L per 24 hours (risk of ODS) + do not correct hypernatremia by more than 10 mmol/L per 24 hours (risk of cerebral edema during cellular rehydration). Exception for severe symptomatic hyponatremia: an initial rapid correction of 4 to 6 mmol/L within 1 hour using 3% NaCl is justified to stop seizures or coma—followed by mandatory slowing of the rate.

Medical emergencies

Call 911 or go immediately to the emergency room if severe hyponatremia (< 125 mmol/L) is accompanied by seizures + deep drowsiness + coma + or repeated vomiting—IV NaCl 3 % must be started immediately to prevent cerebral herniation. Also seek immediate medical attention if severe hypernatremia (> 160 mmol/L) is detected in an elderly person or an infant—intracellular dehydration can cause cerebral thrombosis and intracranial hemorrhage. Serum sodium levels must be measured promptly in any case of acute mental confusion or unexplained seizures.

For electrolyte balance including natremia and urinary osmolality, interpretation of hyponatremia or hypernatremia, and referral to the emergency room if necessary, Clinique Omicron offers consultations at its service points in Quebec and via telemedicine. To make an appointment, visit cliniqueomicron.ca.

Consult at Clinique Omicron

Clinique Omicron's doctors and nurse practitioners (NPs) prescribe and interpret natremia with plasma and urine osmolality and natriuresis to guide the etiology of dysnatremias. They differentiate hyponatremias due to excess water (SIADH + potomania + heart failure) from hyponatremias due to sodium loss (digestive losses + diuretics). They initiate corrective measures (water restriction + NaCl if hypovolemic) according to safety correction rules, and refer severe or symptomatic abnormalities to the emergency department. Consultations are available at several service points in Quebec and via telemedicine. To make an appointment, visit cliniqueomicron.ca.

The content of this page is for informational purposes only and does not substitute medical advice. Dynatremias—particularly severe hyponatremia and hypernatremia—can be life-threatening and require careful, controlled correction under close medical supervision. Overly rapid correction of hyponatremia can cause irreversible central pontine myelinolysis.

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