A case of thymic hyperplasia associated with hyperthyroidism

Abstract

A 30-year-old woman complaining of sternum and back pain showed an abnormal shadow on a chest radiograph at a neighboring clinic. A swollen thyroid and mediastinal mass were detected on chest CT at our hospital. She was diagnosed with hyperthyroidism based on a thyroid function test. The mediastinal mass was homogeneous on both T1- and T2-weighted images of chest MRI, identified as a diffusely enlarged thymus. In addition, the mass showed a lower signal intensity on an out of phase T1-weighted image compared to an in-phase T1-weighted image on chemical shift MR imaging. This phenomenon reveals minimal fat in the mediastinal mass, and so this mass was diagnosed as thymic hyperplasia associated with hyperthyroidism. Ten months after the start of treatment with an antithyroid agent for hyperthyroidism, the size of the thymic hyperplasia clearly decreased. Chemical shift MR imaging can be used to differentiate thymic hyperplasia from thymoma. Thymic hyperplasia associated with hyperthyroidism should be treated conservatively using an antithyroid agent, and unnecessary thymectomy should be avoided.