A case report of large thymic hyperplasia associated with hyperthyroidism.

A 32-year-old female case of large thymic hyperplasia with hyperthyroidism is reported. A computed tomography (CT) examination disclosed a large mediastinal mass (16 x 11 cm) with a heterogeneous internal structure containing both soft tissue density areas and fat density areas. The mass was histologically diagnosed as thymic lymphoid hyperplasia. The thymic mass enlarged during hyperthyroidism and then regressed markedly after treatment with antithyroid drugs. After the thymic mass decreased by about one third of its maximum volume, the mass stopped regressing and has remained the same size for more than 6 years. A CT scan showed a decrease in the soft tissue density area and predominance of the fat density area. The potential response to antithyroid therapy must be considered before recommending resection of thymic tumors diagnosed as hyperthyroidism-related thymic hyperplasia.