Thyroglobulin

Thyroglobulin is a glycoprotein expressed by thyroid follicular cells.  It is a specific marker for thyroid derivation, and may be useful in the work-up of carcinomas of unknown primary.  It may be less sensitive in less differentiated thyroid tumors, as anapestic thyroid carcinoma is notorious for not staining with about anything.
 
Pitfalls
  • Medullary thyroid carcinoma does not express thyroglobulin.
  • Thyroglobulin can have a lot of artifactual staining, which may cause difficultly diagnosing tumors like medullary carcinoma (expected to be thyroglobulin negative).
  • Dedifferentiated thyroid tumors may not express thyroglobulin.
Photomicrographs
Thyroglobulin - Follicular Carcinoma of Thyroid
Thyroglobulin – Follicular Carcinoma of Thyroid
Thyroglobulin - Follicular Carcinoma of Thyroid
Thyroglobulin – Follicular Carcinoma of Thyroid

References:
 
Fischer, S., & Asa, S. L. (2008). Application of immunohistochemistry to thyroid neoplasms. Archives of Pathology & Laboratory Medicine, 132(3), 359–372.