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Lung adenocarcinoma is a form of non-small-cell lung cancer with high mortality in the advanced stages, and is one of the most common histological subtypes of lung cancer in most countries. Prognosis of lung adenocarcinoma is generally poor, with a median survival of 4–13 months. We report a case of unusually prolonged survival of a patient with advanced lung adenocarcinoma complicated by hypothyroidism. A 71-year-old man with stage IV lung adenocarcinoma presented with hypothyroidism. Surprisingly, without any anti-tumor and anti-hypothyroidism therapy, he survived this lung cancer for longer than 2.5 years before his last follow-up visit. Patients with advanced lung adenocarcinoma rarely survive for longer than 2 years, even after therapy. We hypothesize that hypothyroidism is the cause for this discrepancy. Thyroid hormones can promote growth of carcinoma. Therefore, hypothyroidism appears to be beneficial to anti-cancer therapy. We believe that hypothyroidism, as an adverse event commonly occurring in anti-tumor therapy (e.g., an immune checkpoint inhibitor), might not be able to be completely eliminated.