---according to this study.
Death was far more often due to associated conditions than to neurologic damage attributable to the hyponatremia. There was also a U shaped curve, with mortality increasing as sodium levels decreased, but the trend reversed at sodium levels below 120. This apparently means that underlying medical conditions are generally associated with levels of 120-134. Levels lower than that are more likely to be caused by external factors (medications, etc) rather than severe underlying medical illnesses.
Via Uremic Frost.