A knowledge gap may exist when attempting to identify the pathogenetic mechanisms resulting in the syndrome of inappropriate antidiuretic hormone (SIADH) or hypotonic hyponatremia. Ectopic secretion of antidiuretic hormone [ADH] is the classic cause of SIADH. But another form of inappropriate secretion of ADH occurs when interleukin 6 is activated. Hypotonic hyponatremia can also occur in patients with cerebral salt wasting, but the secretion of ADH is appropriate, responding to volume depletion induced by excessive natriuresis. Reset osmostat (RO) is another cause of hypotonic hyponatremia caused by an unknown anomaly in the hypothalamus. This review discusses the pathophysiology of and the identical laboratory findings found in classic ectopic ADH secretion, interleukin 6-mediated ADH secretion, cerebral salt wasting-induced ADH secretion, and RO. This review also discusses potential methods to discern which hypotonic hyponatremic syndrome is present and current recommendations for treatment.