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Synchronous tumours are defined as two or more independent primary neoplasms of different origins diagnosed at the same time in 1 individual. Although rare, its incidence is increasing and the proper diagnosis and staging of each tumour is crucial in defining the patient prognosis and the best therapeutic choice. We present a case of a 56-year-old woman presenting with a lung adenocarcinoma and pulmonary metastases initially diagnosed as stage IV and who was started on a tyrosine kinase inhibitor (erlotinib). In the meantime, she was also diagnosed with papillary thyroid carcinoma and was submitted to complete thyroidectomy. After 6 cycles of erlotinib, thoracic CT showed a decrease in the dimensions of the primary pulmonary tumour, but an increase in the size and number of pulmonary metastases while blood tests showed elevated thyroglobulin. This therefore raised the possibility that the metastases could have originated from the thyroid carcinoma. Anatomo-pathological examination of the lung metastases confirmed this hypothesis. In conclusion, it is important to confirm the origin of metastases in synchronous tumours given this can lead to a re-staging of tumours and a different prognosis, along with other therapeutic options. A multidisciplinary team meeting is crucial to define management and therapeutic approaches for these patients.