Systematic Approach to Hyponatremia

Use volume status, urine osmolality, and urine sodium to narrow the differential diagnosis systematically.

Step 1: Assess Volume Status

HYPOVOLEMIC (Dehydrated):

• Urine Na <20 mEq/L: Extrarenal losses (vomiting, diarrhea, third-spacing)
• Urine Na >40 mEq/L: Renal losses (diuretics, salt-wasting nephropathy, adrenal insufficiency, cerebral salt wasting)

EUVOLEMIC (Normal Volume):

• SIADH: Urine osm >100, urine Na >40, normal volume status
• Hypothyroidism (Severe): Low TSH + low free T4
• Adrenal Insufficiency: Low cortisol
• Psychogenic Polydipsia: Urine osm <100 (dilute urine)
• Beer Potomania: Low solute intake + high fluid intake

HYPERVOLEMIC (Edema, Ascites):

• Heart Failure, Cirrhosis, Nephrotic Syndrome, CKD
• Urine Na typically <20 mEq/L

Step 2: Urine Osmolality

• >100 mOsm/kg: ADH present (appropriate or inappropriate)
• <100 mOsm/kg: Suppressed ADH (psychogenic polydipsia, beer potomania)

Common Endocrine Causes:

• SIADH: Cancer (especially SCLC), CNS disorders, lung disease, medications (SSRIs, carbamazepine, cyclophosphamide)
• Hypothyroidism: Check TSH
• Adrenal Insufficiency: Check AM cortisol or cosyntropin stimulation test

Key Diagnostic Tests:

• Serum sodium, osmolality
• Urine sodium, osmolality
• Volume status (physical exam)
• TSH, free T4
• AM cortisol
• Blood glucose (rule out pseudohyponatremia)

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