Need help is solving a medical issue..Does anyone out there know why the sodium levels are severly depleted af?

Father is 80 yr.s old an the doctors can not come up with a reason for this..? The effects are the devastating,seem each time he has surgery his levels drop way below 120..if anyone has had this happen to a loved one,friend,or something you have heard please let me know. Mostly undiscovered and new to the medical profession,it does look like the start of loosing ones marbles with in a period of 12 hours,i can say that Dr's. are trying but they are at a loss also !! My Father is in good health an i would like to keep it that way..any feed back would be appreciated.