Hashimoto's thyroiditis as a risk factor of thyroid lymphoma

Hashimoto's thyroiditis (struma lymphomatosa), first described by Hashimoto in 1912, is an autoimmune inflammation of the thyroid commonly affecting middle-aged women. Histologic features of Hashimoto's thyroiditis (HT) include diffuse infiltration of lymphoid cells usually with formation of lymphoid follicles, varying degrees of fibrosis, oxyphilic change or squamous metaplasia in the epithelial cells. When the presence of focal lymphocytic infiltration is assumed to be an adequate criterion for diagnosis of autoimmune thyroiditis, the incidence appears to be as high as 16-23% in elderly females. An etiologically important role of HT in the development of thyroid lymphoma had been postulated, and recently, this was confirmed by epidemiological studies. In this article, a brief review of HT is given, together with results of our studies on thyroid lymphoma, and a discussion in the light of the pertinent literature. Immunologic and immunohistologic studies revealed that all of the thyroid lymphomas were of the B-cell type. Malignant lymphomas developing in patients with other autoimmune diseases such as Sjögren syndrome and rheumatoid arthritis have also been reported to be B-cell derived. Therefore it is suggested that immune deficiency is a causal factor for B-cell lymphoma.