I, too had and continue to have a problem with low sodium. It kept me in the hospital for a few days longer, when I was first diagnosed. As it was explained to me, it is caused by a hormone which is released by the cancer and effects your electrolytes, which sodium is one. The danger of low sodium is that it can effect kidney function, and ultimately lead to kidney failure. During my remission period, not surprisingly, my sodium was fine. Now I have another active tumor and have been taking thermotabs ( salt pills) to keep my sodium level up. It's a sticky wicket and something they will monitor, but it's not an insurmountable issue. Keep your chin up!!