The syndrome of inappropriate ADH secretion is characterized by the following:
In SIADH, the excessive retention of water expands extracellular and intracellular volume. This explains Dilutional Hyponatremia. Since the urine is concentrated hence urine sodium mEq/unit of time can be explained. Another plausible explanation is that expanded volume leads to release of ANF and resultant natriuresis explains the urine sodium> 20mEq/L. Hyponatremia sub-classified according to volume status Clinical example Sodium levels TBW Hypovolemic Hyponatremia Diarrhea Cerebral salt wasting Low Low Euvolemic Hyponatremia SIADH Normal Increased Hypervolemic Hyponatremia CHF Cirrhosis Nephrotic Increase Increased
Hyponatremia and urine sodium excretion > 20 mEq/L
Hypernatremia and urine sodium excretion > 20 mEq/L
Hyponatremia and hyperkalemia
Hypernatremia and hyperkalemia
Disorders of pituitary gland