SIADH leads to retention of water and hyponatraemia (low blood sodium levels). Even though in some situations the hyponatraemia may be very severe, there may be no clinical symptoms. The reason that symptoms may be absent is that the type of hyponatraemia found in SIADH is due to a dilutional effect, not one of sodium depletion. The general symptoms that patients may complain of in SIADH are listed below and are experienced when sodium levels fall below 125mmol/L.
If left untreated the sodium levels can fall to less than 110mmol/L that can ultimately lead to fits and coma. In SIADH, there is no oedema.