Lung carcinoma metastatic to microfollicular adenoma of the thyroid. A case report.

A 75-year-old woman with lung adenocarcinoma in whom autopsy revealed metastasis to microfollicular adenoma of the thyroid is described. A thyroid tumor specimen from this patient was initially interpreted as representing primary encapsulated follicular carcinoma of the thyroid associated with a component of poorly differentiated carcinoma of follicular cell origin. The widespread involvement of the lungs and other sites was also interpreted as metastases from the poorly differentiated carcinoma component within the encapsulated follicular carcinoma of the thyroid. Subsequent meticulous histologic examination revealed that the thyroid tumor was a microfollicular adenoma, and that the component of poorly differentiated carcinoma within it was the same as the lung carcinoma. Immunohistochemical investigation revealed that the poorly differentiated carcinoma cells within the thyroid tumor were stained positively with polyclonal and monoclonal antibodies against carcinoembryonic antigen (CEA) and negative for thyroglobulin and calcitonin. The carcinoma cells from the lungs and other sites also showed positive staining for CEA. These findings support the view that the component of poorly differentiated carcinoma was a metastatic lesion from the primary lung carcinoma to the microfollicular adenoma of the thyroid. The present study emphasizes that attention should be directed to thyroid metastasis, even if it is within a thyroid primary neoplasm.