Metastatic thyroid carcinoma presenting as malignant pleural effusion: A cytologic review of 5 cases.

Malignant pleural effusion can be a manifestation of many malignancies. Involvement of pleural fluid by metatstatic thyroid carcinoma, though reported, is relatively rare. We present 5 cases of metastatic thyroid carcinoma involving the pleural fluid. The diagnosis of thyroid carcinoma in pleural fluid can be particularly challenging as thyroid transcription factor -1 (TTF-1) which is a marker for carcinoma of thyroid origin is also positive in lung adenocarcinomas (which are more frequently associated with pleural effusions) and thyroglobulin (TG) can often be negative in poorly differentiated/analplastic thyroid carcinomas. In our experience, PAX8 is a particularly useful marker in making the distinction. The diagnosis of metastatic thyroid carcinoma in pleural fluid can be challenging and knowledge of the clinical context and supporting immunohistochemical stains is essential for making the right diagnosis. Diagn. Cytopathol. 2016;44:1085-1089.