To clarify the cellular composition of subacute thyroiditis, histologic and 
immunohistochemical studies were performed. Histologically, the lesion presented 
a patchy distribution of non-caseous granulomas comprising colloid, small 
lymphocytes, neutrophils, macrophages with or without epithelioid features, and 
multinucleated giant cells of foreign body type. In addition, numerous 
plasmacytoid monocytes were closely associated with the granulomas. The giant 
cells were CD68+, thyroglobulin- and cytokeratin-. Usually, small lymphocytes in 
the granulomas are CD3+, CD8+, CD45RO+ cytotoxic T-cells. In the 
non-granulomatous lesion, the follicles were often infiltrated by CD8+ 
T-lymphocytes, plasmacytoid monocytes and histiocytes, resulting in disrupted 
basement membrane and rupture of the follicles. Lymphoid follicles with or 
without active germinal centers were not observed. Moreover, no residual 
follicular dendritic cell networks were detected by CD23 and CAN.42 
immunostains. In the interfollicular area, scattered plasma cells were observed 
among infiltrating cells. Neither human herpes virus 8 nor EBER-positive cells 
were detected in the six patients. The findings of our study suggest that 
cellular immune response may play an important role in the pathogenesis of 
subacute thyroiditis.