Pseudolymphoma of the thyroid gland. A case report.

An unusual case of pseudolymphoma of the thyroid gland is presented. A well-demarcated whitish mass measuring 1.0 cm in diameter was found in the upper center of the right lobe of the thyroid gland. Microscopically, the lesion was composed of hyperplastic lymphoid tissues with many follicular centers and mixed infiltration of plasma cells and macrophages. The immunostain revealed a similar distribution of T-and B-lymphocytes to reactive lymph node and a polyclonal nature of the plasma cell infiltrates. No lymphoepithelial lesion was associated. The adjacent thyroid tissues showed chronic lymphocytic thyroiditis. This finding suggests that the lesion is a pseudolymphoma of the thyroid gland. This condition is rare, but it should be considered during the differential diagnosis of lymphoproliferative lesions occurring in the thyroid gland, especially with low-grade malignant lymphoma of the thyroid gland.