Thymic Hyperplasias in Practice: Clinical Context, Histological Clues, and Management Implications

Thymic hyperplasias constitute a heterogeneous group of primarily non-neoplastic conditions characterized by either enlargement of the thymic parenchyma or the proliferation of lymphoid follicles. Differentiating these entities is crucial, as they exhibit significant differences in their epidemiology, clinical associations, and implications for patient management. This review aims to synthesize the current knowledge on true thymic hyperplasia, rebound thymic hyperplasia, thymic follicular hyperplasia, and thymic hyperplasia with lymphoepithelial sialadenitis-like features, and to delineate their diagnostic boundaries in histology.

A systematic literature review of the Medline database was conducted, encompassing all publications available up to November 2025. The search terms included thymic hyperplasia, true thymic hyperplasia, rebound thymic hyperplasia, thymic follicular hyperplasia, and lymphoepithelial-sialadenitis-like thymic hyperplasia. Studies concerning the histopathological, radiological, and clinical presentations as well as reference works concerning age-related thymus size and weight were included. Limitations stem from the scarcity of systematic investigations and heterogeneity of the available studies.

Thymic hyperplasia encompasses biologically diverse entities unified solely by their benign nature, yet distinguished by distinct histological patterns and clinical scenarios. True thymic hyperplasia and rebound hyperplasia preserve the organ's fundamental architecture and lack association with autoimmunity; however, they differ in age distribution and clinical context. Thymic follicular hyperplasia reflects an immune-mediated process, strongly linked to early-onset myasthenia gravis, and is characterized by germinal-center formation within thymic lobes. Lymphoepithelial-sialadenitis-like thymic hyperplasia, by contrast, represents an epithelial-lymphoid proliferation with pronounced architectural distortion, frequent cystic changes, and notable associations with systemic non-myasthenic autoimmune diseases as well as thymic mucosa-associated-lymphatic-tissue lymphoma.

Overall, clinical history, serological data, imaging, and detailed histopathology remain indispensable for differentiating thymic hyperplasias, some of which necessitate further systemic evaluation due to their association with autoimmune diseases or lymphomas.