Plasma cell granuloma of the thyroid.

Plasma cell infiltrates in the thyroid are rare. They may represent several processes, such as localized plasmacytoma, multiple myeloma, or plasma cell granuloma (PCG). It may be difficult to distinguish these disorders on the basis of morphologic features in sections stained with hematoxylin-eosin. All may be composed of mature plasma cells, without admixed macrophages or lymphocytes, that infiltrate the thyroid and adjacent connective tissue. The identification of the uncommon, but benign, PCG is facilitated by immunohistochemical staining to demonstrate an admixture of plasma cells with cytoplasmic kappa and lambda light chains. The polyclonal nature of the plasma cell infiltrate distinguishes PCG from neoplastic plasma cell proliferation.