I just got out of the hospital for 3 days for low sodium in blood. My Pulmonologist sent a sputum sample to Mayo in Rochester after my 1st time with low sodium in August ( November low sodium was my second incident) and it came back negative for MAC. I have BE and Pulmonary Fibrosis. I was drinking a lot of water to help keep the mucus thinned, and the hospital had a nephrologist (Kidney Doctor) see me who determined that because of my lung diseases it triggered the release of a hormone in my body to have my kidney retain as much water as possible to help the injured organ (Lungs) I think he called it SGIH hormone or something like that. I was put on fluids restriction of 60Oz max per day, salt tablets twice per day and furosemide 20 mg once per day. When I went into the hospital my sodium was 122. (Minimum normal is 136) after 3 days I was up to 133, and discharged on the 13th. Keeping the same treatment at home my blood test on the 25th sodium was at 137. So I believe the Kidney Dr. was right on with his diagnosis. Good luck with your situation.