True Thymic Hyperplasia: An Extremely Rare Incidental Finding

Thymic hyperplasia is a rare and benign cause of mass in the anterior mediastinum in adults. This pathology is characterized by an increase in the size and weight of the thymus while preserving its architecture and histology. In most cases, it is asymptomatic, and routine imaging exams are used to make the diagnosis. Thymic hyperplasia can be divided into reactive and true hyperplasia based on morphological characteristics. The main differential diagnosis of this pathology is thymoma, an epithelial neoplasm of the thymus. True thymic hyperplasia is the rarest subtype, and, unlike reactive or lymphofollicular, it does not present histologically with lymphoid follicles and is not associated with autoimmune diseases, such as myasthenia gravis and Graves' disease. Here we present the case of an adult patient with thymic hyperplasia incidentally diagnosed in imaging exams. The established treatment was complete surgical resection, which is indicated as the first-line treatment. Subsequent histopathological analysis confirmed a diagnosis of true thymic hyperplasia.