Thyroid lymphoma in a patient with hyperthyroidism.

A patient presenting with hyperthyroidism had a rapidly enlarging thyroid mass that histopathologically was a diffuse histiocytic lymphoma arising in a gland with Hashimoto's thyroiditis. The concurrent development of both hyperthyroidism and lymphoma may have resulted from similar immunologic abnormalities. Appreciation of the relationship between thyroid lymphoma and Hashimoto's thyroiditis, and their presentation with either hypothyroidism or hyperthyroidism should lead to an earlier diagnosis of lymphoma and improved survival.