Definition: < 134 mEq/L; severe < 120
Manifestations: anorexia, apathy, restlessness-->confusion, muscle cramping, lethargy, anorexia, nausea --> seizure, coma


1. Check Osmolarity:

2. If hypotonic hyponatremia, check urine osmolality

3. If Hypotonic, Hyperosmolar urine, check volume status:


Quick Workup:



Causes of SIADH


Acute Management

If symptoms have developed acutely (risk of cerebral edema >> risk of rapid correction):

Chronic development

Unknown duration

Volume Depletion



SIADH: free water restrict
do not correct faster than 1 meq/Hr



Osmolality Formulas:

Calculated Osmolality:    2([Na] +[K]) + (Glc/18) + (BUN/2.8))

Effective Osmolality: 2[Na] + (g/c/18)

Osmolal gap: measured osmolality - calculated osmolality

Corrected [Na] = measured Na + 1.6 x (glucose -100)/100

Total Body Water (TBW) = 0.6 (men) or 0.5 (women) x body weight (kg)

Sodium Deficit = TBW x (Desired [Na] - Plasma [Na]