Hyponatremia workup
Formal Definition
A structured diagnostic algorithm for evaluating hyponatremia (serum sodium <135 mEq/L) — the most common electrolyte abnormality in hospitalized patients; the standard stepwise approach uses volume status (hypovolemic vs euvolemic vs hypervolemic), serum osmolality (true hyponatremia vs pseudohyponatremia vs translocational), and urine osmolality (>100 suggests ADH activity, <100 suggests primary polydipsia or low solute intake) — to identify the underlying etiology; euvolemic hyponatremia with concentrated urine classically suggests SIADH in the appropriate clinical context; hypovolemic hyponatremia with concentrated urine suggests extrarenal volume loss; hypervolemic hyponatremia suggests heart failure, cirrhosis, or nephrotic syndrome.
How It's Used on the Ward
"Hyponatremia workup" or "Na down" — when the patient's sodium is low, the team works through volume status, urine studies, and clinical context to identify the cause; SIADH, hypovolemic Na losses, diuretic-induced, and water overload from CHF/cirrhosis are the most common; each has different treatment so getting the diagnosis right matters.
Example
""68-year-old nursing home resident with new confusion. Sodium 118 mEq/L. Exam: euvolemic (no edema, no orthostasis, lungs clear). Labs: serum osm 252 (low), urine osm 380 (concentrated despite dilute serum — inappropriate ADH activity), urine Na 64 (elevated). TSH normal. Cortisol normal. Diagnosis: SIADH after exclusion of hypovolemia, adrenal insufficiency, hypothyroidism. Further workup to identify cause: review medications (started sertraline 3 weeks ago), CT chest (small right hilar mass — possible underlying lung cancer), MRI brain if indicated. Fluid restriction initiated.""
Clinical Context
Hypovolemic hyponatremia — patient has lost sodium AND water (with net sodium loss); orthostatic, dry mucous membranes, low JVP. Causes: extrarenal (vomiting, diarrhea, third-spacing), renal (diuretics, salt-wasting nephropathy, mineralocorticoid deficiency, cerebral salt wasting). Euvolemic hyponatremia — no obvious volume disturbance. Causes: SIADH (most common), primary polydipsia, beer potomania, low solute intake (tea and toast), reset osmostat, hypothyroidism, adrenal insufficiency. Hypervolemic hyponatremia — patient has fluid overload AND hyponatremia. Causes: heart failure (low cardiac output triggers ADH), cirrhosis (splanchnic vasodilation), nephrotic syndrome. Severe symptomatic hyponatremia (Na <120 with seizures, severe altered mental status, coma): 3% hypertonic saline 100 mL bolus, repeat up to 3 times — target 4-6 mEq/L rise in first hour, do NOT exceed 8-10 mEq/L in 24h (risk of osmotic demyelination syndrome at >10-12 mEq/L in 24h). SIADH treatment: fluid restriction (typically 800-1000 mL/day), vaptans (tolvaptan for refractory), salt tablets, urea. Risk of correction: osmotic demyelination syndrome (central pontine myelinolysis) — historically seen with rapid sodium correction, especially in chronic alcoholics, malnutrition, hypokalemic patients — must correct slowly.