Our study aimed to determine the frequency of sick euthyroid syndrome (SES) among patients diagnosed as non-small cell lung cancer (NSCLC) and its association with the stage of the disease, Karnofsky index (KI), and nutritional parameters. We enrolled 80 consecutive patients with newly diagnosed NSCLC. Cases with NSCLC were staged by using TNM system. The cases were examined for thyroid function tests, KI and nutritional evaluation before treatment. Moreover, cases were investigated for their overall survival ratio at sixth, twelfth, and eighteenth months. Out of 80 patients, sick euthyroid syndrome (SES) were identified in 28 (35%). SES was more frequent among stage III (26%) and stage IV (62%) cases. The body mass index (BMI), KI and serum albumin level were detected to be significantly low in cases with SES when compared to cases without SES. SES was found to be negatively correlated with BMI, KI and serum albumin level, and it was positively correlated with disease stage. SES was frequently seen in cases with NSCLC. SES can be used as a predictor of poor prognosis.