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Multiple primary malignant tumors (MPMTs) are defined as 2 or more independent primary malignancies of different histologies/origins in the same individual. Although the incidence of MPMTs is being increasing, second primary cancer (SPC) is still rare and difficult to distinguish from metastasis. Here, we present a case of thyroid carcinoma with lung metastasis and primary lung cancer.The patient was a 66-year-old man diagnosed of papillary thyroid carcinoma (PTC) with lymph nodes, lung, and skeletons metastases. One of the pulmonary nodules had contrary manifestations, such as, noniodine avid, high uptake of 18F-fluorodexyglucose (18F-FDG) and progress after iodine-131 radioiodine therapy.CT guided biopsy and I-125 seed brachytherapy of nodule in right upper lobe were performed.The patient was diagnosed of PTC with lymph nodes, lung, and skeletons metastases, accompanied by primary lung adenocarcinoma.After 2 years of follow-up, nodules of inferior lobes almost disappeared and the nodule of right upper lobe shrank to only 0.7 cm.Physicians should be aware of SPC in clinical work, and improve the early diagnosis rate with a variety of examination methods and select the best treatment to improve the prognosis of patients.