Recurrent follicular thyroid carcinoma metastatic to axillary lymph nodes mimicking pulmonary adenocarcinoma

Differentiated thyroid cancers generally have favorable prognoses, though follicular thyroid cancer is overall associated with a worse prognosis due in part to increased incidence of distant metastasis. We report a case of a 51-year-old woman with a history of widely invasive follicular thyroid carcinoma treated with a total thyroidectomy, radioactive iodine and external beam radiation. Five and a half years following her surgery, she was found to have an axillary lymph node mass, multiple lung masses, and a hilar mass in the setting of declining thyroglobulin (Tg) antibodies. Her metastases were initially thought to be due to a primary lung adenocarcinoma given a neoplastic cell immunophenotype that included an absence of Tg expression and co-expression of TTF-1 and Napsin A. However, PAX8 expression demonstrated that the axillary and hilar metastases were actually thyroid in origin rather than lung. Axillary metastases in differentiated thyroid carcinoma are exceedingly rare and previous reports have typically involved widely disseminated disease with extensive neck lymphadenopathy. With a decline in Tg antibodies levels in high-risk patients, one should consider progression and loss of differentiation of thyroid carcinoma rather than a response to treatment. Learning points Axillary metastases in differentiated thyroid carcinoma are uncommon. In patients with high-risk thyroid carcinomas, a decline in thyroglobulin antibody may not signal disease improvement, but rather a progression to a poorly differentiated form of cancer. PAX8 staining can be used to differentiate thyroid carcinomas from lung adenocarcinomas.