​Symptoms and Signs – Differential Diagnosis of Hyponatremia
HYPOTONIC HYPONATREMIA
Isovolemic Hyponatremia
• Syndrome of inappropriate antidiuretic hormone (SIADH)
• Water intoxication (e.g., schizophrenic patients, primary polydipsia, sodium-free irrigant solutions, multiple tap-water enemas, dilute infant formulas). These entities are rare and often associated with a deranged antidiuretic hormone (ADH) axis.
• Renal failure
• Reset osmostat (e.g., chronic active TB, carcinomatosis)
• Glucocorticoid deficiency (hypopituitarism)
• Hypothyroidism
• Thiazide diuretics, nonsteroidal anti-inflammatory drugs (NSAIDs), carbamazepine, amitriptyline, thioridazine, vincristine, cyclophosphamide, colchicine, tolbutamide, chlorpropamide, angiotensin-converting enzyme (ACE) inhibitors, clofibrate, oxytocin, selective serotonin reuptake inhibitors (SSRIs), amiodarone. With these medications, various drug-induced mechanisms are involved.

Hypovolemic Hyponatremia
• Renal losses (diuretics, partial urinary tract obstruction, salt-losing renal disease)
• Extrarenal losses: GI (vomiting, diarrhea), extensive burns, third spacing (peritonitis, pancreatitis)
• Adrenal insufficiency
Hypervolemic Hyponatremia
• Congestive heart failure (CHF)
• Nephrotic syndrome
• Cirrhosis
• Pregnancy

ISOTONIC HYPONATREMIA
Normal Serum Osmolality
• Pseudohyponatremia (increased serum lipids and serum proteins). Newer sodium assays eliminate this problem.
• Isotonic infusion (e.g., glucose, mannitol)

HYPERTONIC HYPONATREMIA
Increased Serum Osmolality
• Hyperglycemia: each 100 mg/dL increment in blood glucose level above normal decreases plasma sodium concentration by 1.6 mEq/L
• Hypertonic infusions (e.g., glucose, mannitol)