Hashimoto's thyroiditis with a monoclonal antithyroglobulin autoantibody: disappearance of the monoclonal antibody after thyroidectomy

A 46-yr-old man had typical Hashimoto's thyroiditis (HT) and a serum monoclonal immunoglobulin (MIgG kappa). There was no evidence of disseminated lymphoid malignancy. Because of rapid enlargement of the goiter with pressure symptoms, a total thyroidectomy was performed. The surgery was followed by the complete disappearance of the MIgG kappa paralleled by the disappearance of serum antithyroglobulin autoantibodies. Thyroid sections demonstrated HT. Direct immunofluorescence assay showed diffuse infiltration by lymphocytes and plasma cells that were mainly IgG kappa positive. Moreover, the serum MIgG kappa had antithyroglobulin activity, as demonstrated by two different methods, namely labeling of focused serum proteins with radioiodinated human thyroglobulin and immunoadsorption of the monoclonal IgG on a human thyroglobulin affinity column. These results demonstrate that a monoclonal antithyroglobulin autoantibody was produced within the thyroid in a patient with HT. As the production of a MIg is regarded as the last stage preceding the malignant transformation of normal B lymphocytes, this report strongly suggests that a B lymphoma can directly emerge from the intrathyroidal lymphocytes involved in the pathogenesis of Hashimoto's thyroiditis.