Pitfall in immunohistochemical staining for thyroglobulin in case of thyroid metastasis from lung carcinoma

A 73-year-old female presented with an enlarging neck mass over the last 2 years. She then started experiencing hoarseness, dysphagia, and weight loss for 5 months. Physical examination showed an irregular and hard consistency mass with a diameter of 4 cm occupying the left lobe and isthmus of the thyroid gland. Fine-needle aspiration biopsy (FNAB) was performed and the cytology specimen revealed sheets of neoplastic cells with enlarged, hyperchromatic, slightly pleomorphic nuclei, prominent nucleoli, and brownish pigment inside the tumor cells [Figure 1a]. The definitive diagnosis could not be made from cytology alone. Immunohistochemical staining was requested with positive results for thyroglobulin [Figure 1b], thyroid transcription factor 1 (TTF-1) [Figure 1c], but negative for calcitonin. Figure 1 (a) Fine-needle aspiration thyroid showed a background of cystic lesion (foamy cells) with few of neoplastic cells and a sheet of degenerative follicular cells (Papanicolaou stain, ×400), (b) cell block preparation showed the immunoreactivity ...