Bone metastasis of papillary thyroid carcinoma simulating a pulmonary origin. Unusual immunohistochemistry leading to misdiagnosis.

Immunophenotypical features from tumors can be variable and, sometimes, confusing. We herein report a 61 year old woman with two foci of papillary thyroid microcarcinoma who developed a bone lesion four months after total thyroidectomy/central lymphadenectomy (mpT1aN0), with an immunohistochemical pattern suggestive of a pulmonary rather than a thyroid origin (CK7, napsin-A and TTF1 positive; and negative thyroglobulin). Further biomarkers (HBME-1 and PAX8) were performed in order to confirm primary tumor, leading to conclusion of a bone metastasis from thyroid papillary carcinoma. We believe it is always advisable to perform more than one biomarker as part of a panel to get a more reliable diagnosis.