Rebound Thymic Hyperplasia after Recovery from Ectopic Cushing Syndrome: A Case Report and Literature Review

Introduction: Rebound thymic hyperplasia (RTH) is a rare benign condition resulting from compensatory proliferation of thymocytes after remission from stressful conditions, including hypercortisolism. Its appearance can sometimes be misleading, making differential diagnosis with thymic epithelial or neuroendocrine tumor challenging. Case presentation: We describe the case of a 31-year-old patient who was referred to our outpatient clinic for worsening hirsutism, acne, and fatigue. Biochemical and dynamic tests showed an ACTH-dependent hypercortisolism of ectopic origin (Ectopic Cushing syndrome, ECS). An enhanced total body CT scan was performed to localize the tumor, revealing a 14 mm peribronchial nodule in the left upper lung that was surgically removed, leading to hypercortisolism resolution and hypoadrenalism. During a follow-up chest-abdomen CT scan, an anterior mediastinal enlargement was found. To better characterize the lesion, the patient underwent an enhanced MRI, which provided a conclusive diagnosis of RTH. This allowed the medical team to avoid unnecessary and invasive surgical thymectomy. Conclusion: The present case highlights the diagnostic importance of recognizing RTH as a benign mimic of thymic tumors in adults recovering from ectopic Cushing syndrome. Misinterpretation may lead to unnecessary surgical intervention. MRI may be useful to characterize RTH in a non-invasive way. Successful treatment requires collaboration among endocrinologists, surgeons, and radiologists.