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