Rebound thymic hyperplasia after treatment of Cushing's syndrome.

Rebound thymic hyperplasia has been described in children and young adults after recovery from stress. Similar thymic enlargement has been observed in patients after remission of Cushing's syndrome. In one patient successfully treated for ectopic adrenocorticotropic hormone (ACTH) syndrome by resection of a bronchial carcinoid with a hilar metastasis, the postoperative enlarging mediastinal mass suggested recurrent tumor. In another patient treated for an undetected ectopic ACTH source by a cortisol antagonist, the enlarging thymus could be confused with a thymic carcinoid. The typical thymic appearance on CT and the chronologic relation to declining cortisol levels should prevent such diagnostic errors.