We report an unusual case of malignant pseudothyroiditis (MPT) occurred in a 35-year-old female patient presenting with symptoms and signs of thyroiditis. Fine-needle aspiration biopsy of the thyroid demonstrated the presence of a MPT induced by a metastatic extrathyroidal carcinoma. F-FDG-PET/CT showed increased radiopharmaceutical uptake in the thyroid, in multiple cervical and mediastinal lymph nodes, and in a right pulmonary nodule. Histological examination demonstrated the presence of a metastatic non-small cell lung cancer (NSCLC). In this unusual case of MPT F-FDG-PET/CT has been useful in staging the disease and in detecting the primary tumor site.