Severe hyponatremia due to paraneoplastic syndrome of inappropriate antidiuretic hormone secretion in non-small cell lung carcinoma transforming to small cell lung carcinoma during treatment with immune checkpoint inhibitor

This is the first case described of transformation from EGFR wild-type non-small cell lung cancer (NSCLC) to small cell lung cancer (SCLC) during immunotherapy presenting with severe hyponatremia due to paraneoplastic syndrome of inappropriate antidiuretic hormone secretion.