Causes of Low Sodium (Hyponatremia)
Hyponatremia results from an imbalance in free water relative to sodium, and the causes are best understood by categorizing patients based on their extracellular fluid (ECF) volume status: hypovolemic (decreased volume), euvolemic (normal volume), or hypervolemic (increased volume). 1
Primary Classification by Volume Status
The most clinically useful approach to identifying causes involves assessing the patient's volume status through physical examination (looking for edema, ascites, jugular venous distension, orthostatic hypotension, dry mucous membranes, and skin turgor) 2, 3.
Euvolemic Hyponatremia (Normal ECF Volume)
This is the most common presentation in clinical practice:
Syndrome of Inappropriate Antidiuresis (SIAD): The leading cause overall, accounting for 35.8% of emergency department cases 4. Results from nonosmotic vasopressin release despite low serum osmolality.
Conditions mimicking SIAD that require careful differentiation:
Endocrine disorders: Hypothyroidism, adrenal insufficiency 5, 6
Medications: Particularly thiazide diuretics, SSRIs, carbamazepine, and other drugs that enhance vasopressin action 7
Postoperative states and trauma 3
Hypovolemic Hyponatremia (Decreased ECF Volume)
Caused by loss of both sodium and water, with relatively greater sodium loss:
- Gastrointestinal losses: Recurrent diarrhea, vomiting 7
- Renal losses: Thiazide diuretics (most common medication cause), osmotic diuresis, salt-wasting nephropathy 7
- Third-space losses: Burns, pancreatitis
- Hemorrhage: Significant blood loss 7
- Excessive sweating without adequate sodium replacement
Hypervolemic Hyponatremia (Increased ECF Volume)
Results from total body sodium excess with even greater water excess:
- Congestive heart failure 2, 3
- Cirrhosis with ascites: In cirrhotic patients, hyponatremia signals worsening hemodynamic status and portal hypertension 5
- Nephrotic syndrome 7
- Advanced chronic kidney disease/renal failure 7
Important Diagnostic Considerations
Pseudohyponatremia
A laboratory artifact occurring with severe hypertriglyceridemia or paraproteinemia. Key distinguishing feature: serum osmolality is NOT low (>280 mOsm/L) 5. This should always be excluded first.
Age-Related Factors
For every 1-year increase in age, serum sodium decreases by 0.14 mmol/L in hyponatremic patients, making elderly patients particularly vulnerable 4.
Acute Precipitants in Cirrhosis
When hyponatremia develops in cirrhotic patients, always evaluate for:
- Infection (perform diagnostic paracentesis with cultures) 5
- Thyroid dysfunction
- Cardiac dysfunction
- Renal impairment
- Adrenal insufficiency
Water Intoxication
Excessive free water intake overwhelming renal excretion capacity, seen in psychogenic polydipsia or after endurance events (though marathon runners typically develop hypernatremia from dehydration) 7.
Critical Clinical Pitfalls
The serum uric acid level provides diagnostic clues: For every 1 mg/dL increase in serum uric acid, serum sodium increases by 0.125 mmol/L 4. Low uric acid suggests SIAD, while normal/high levels point toward other causes.
Many patients initially labeled as SIAD actually have alternative diagnoses upon thorough evaluation 4. Measure urine sodium, urine osmolality, and fractional excretion values to differentiate accurately, as treatment strategies differ significantly between these conditions.
In cirrhotic patients, hyponatremia <130 mEq/L warrants systematic evaluation for non-hepatic causes (alcohol use, thyroid disease, adrenal insufficiency, cardiac disease, renal disease, infection) before attributing it solely to liver disease 5.