My mom was diagnosed with lung cancer in February. She went to her family physician feeling nauseous. When she actually got her doctor she was a bit "loopy" and not very lucid. The doctor ordered blood test specifically looking for low sodium. The tests came back and her sodium level was 116. Normal is around 155. The doctor ordered her to the hospital where initially they took a chest x-ray. Her lungs didn't look normal so they ordered a cat scan and eventually a biopsy where they found lung cancer. She wasn't ever in any pain. Had no shortness of breath. Had somewhat of a cough that she attributed to allergies (in hindsight, it probably wasn't allergies at all). If the doctor had not ordered the blood test to look at the possibility of low sodium it might have been a while longer before the lung cancer was ever found. She has had 2 cycles of chemo and is currently in the hospital again with a very low sodium level. Left untreated low sodium can cause seizures and periods of delusion and even death. So, Yes low sodium can without question be a sign of lung cancer. Sometimes, it's your only sign.