Metastatic recurrence of lung adenocarcinoma in thyroid clinically masquerading as primary thyroid cancer and immunohistochemically positive for thyroglobulin.

We report a case of a man in his 60s with metastatic lung adenocarcinoma in the thyroid and cervical lymph nodes. The lung cancer was resected 5 years prior to presentation. The metastasis mimicked primary thyroid cancer by clinical examination and CT. However, fine-needle aspiration cytology of the thyroid and the lymph node lesions favoured lung cancer metastasis rather than thyroid cancer. Left thyroid lobectomy and lymphadenectomy were performed. Pathology confirmed the presence of an adenocarcinoma in the thyroid and two lymph nodes, which was similar to the previous lung cancer. The tumour cells in the thyroid were immunohistochemically positive for TTF1 and thyroglobulin and negative for PAX8. This is the second reported case of metastatic lung cancer in the thyroid that was focally positive for thyroglobulin. This can be a pitfall in differentiating between primary thyroid tumour and metastatic lung adenocarcinoma by pathological and cytological examination.