Unexpected decline of pulmonary function in a patient with thymoma mimicking obstructive bronchiolitis.

Obstructive bronchiolitis (OB) is a rare disorder characterized by progressive airflow limitation and is most commonly associated with autoimmune diseases, inhalation of toxins, lung transplantation, and hematopoietic stem cell transplantation. Thymoma is a rare mediastinal tumor frequently accompanied by autoimmune diseases, such as myasthenia gravis. We report a male patient with mild chronic obstructive pulmonary disease who developed thymoma and severe respiratory dysfunction mimicking OB. Postmortem autopsy revealed histopathological findings resembling systemic graft-versus-host disease with OB-like bronchiolar lesions. This case highlights the potential link between thymoma and severe small-airway disease and underscores the diagnostic challenges in identifying OB-like pathology.