Localized primary amyloid tumor of the thyroid developing in the course of Hashimoto's thyroiditis

A case of localized primary amyloid tumor of the thyroid gland developing in the course of Hashimoto's thyroiditis was studied using histochemistry, immunohistochemistry and electron microscopy. The patient was diagnosed as having Hashimoto's thyroiditis by histological examination of the thyroid and by the presence of a high titer of serum thyroglobulin and thyroid microsomal antibodies. In addition, the thyroid gland exhibited multiple nodular deposits of amyloid which were resistant to prior incubation with potassium permanganate. The amyloid deposits were surrounded by numerous histiocytes and multinucleated giant cells which contained small amyloid droplets in their cytoplasm. However, no amyloid deposits were observed in the walls of blood vessels. Immunohistochemistry showed that the amyloid was strongly positive for amyloid P component, IgG and kappa light chains. Ultrastructurally, the amyloid was composed of straight fibrils with a diameter of 7 to 10 nm. Histiocytes extended slender cytoplasmic processes in a radial fashion into amyloid fibrils, which exhibited a highly organized star-like pattern. This was considered to be an extremely rare case of localized primary amyloidosis of the thyroid, in which IgG, especially kappa light chains (AL), was present as a precursor protein.