Diseases of the thymus
- Lesion
The thymus can fail to form, grow lymphoid follicles in autoimmune disease, or give rise to thymoma, an epithelial tumour linked to myasthenia.
What it is
Thymic disease is limited but important. Hypoplasia or aplasia occurs in DiGeorge (22q11.2 deletion) syndrome with loss of T cells. Cysts are usually incidental but can hide a tumour. Thymic follicular hyperplasia, with B-cell germinal centres in the medulla, is found in most patients with myasthenia gravis and in other autoimmune diseases. Thymoma, a tumour of thymic epithelial cells with admixed immature T cells, is the commonest thymic neoplasm; its behaviour depends mainly on invasion, and it is tied to paraneoplastic autoimmunity. Thymic carcinoma is cytologically malignant.
Structure
- Thymoma gross: lobulated, firm grey-white encapsulated mass; some invade the capsule and surrounding structures.
- Medullary-type thymoma: spindle-shaped bland epithelial cells with few thymocytes, usually noninvasive.
- Cortical-type thymoma: plump polygonal epithelial cells with many immature T cells; more often invasive.
- Thymic carcinoma: overtly malignant squamous or other carcinoma cells.
Step by step
- Thymic epithelial cells normally teach developing T cells to tolerate self, partly through AIRE.
- In thymoma, neoplastic epithelium supports T-cell development but selects them poorly.
- Autoreactive T cells leave the tumour and help B cells make autoantibodies (for example against the acetylcholine receptor).
- Myasthenia gravis, red cell aplasia or other autoimmunity follows.
- Local growth compresses mediastinal structures; invasive thymomas spread to pleura and pericardium.
Thymic lesions
| Lesion | Main features | Association |
|---|---|---|
| Hypoplasia or aplasia | Absent or tiny thymus, T-cell deficiency | DiGeorge (22q11.2 deletion) syndrome |
| Thymic cyst | Serous or mucinous cyst, often incidental | May coexist with lymphoma or thymoma |
| Thymic follicular hyperplasia | B-cell germinal centres in the medulla | Myasthenia gravis, Graves disease, lupus, rheumatoid arthritis |
| Thymoma | Bland epithelial cells with thymocytes; invasive or not | Myasthenia gravis, pure red cell aplasia, hypogammaglobulinaemia |
| Thymic carcinoma | Cytologically malignant, often squamous | Aggressive, poor prognosis |
Clinical notes
- Anterior mediastinal masses: thymoma, lymphoma (Hodgkin, T-lymphoblastic, primary mediastinal B), germ cell tumours and retrosternal thyroid.
- Thymectomy helps many patients with myasthenia gravis, with or without thymoma.
Test yourself
Think of the answer, then tap the question to see it.
Which autoimmune disease is most often associated with thymoma?
Myasthenia gravis.
What is the most important prognostic feature of a thymoma?
Whether it has invaded through its capsule into surrounding structures (stage).
Which thymic change is seen in most patients with myasthenia gravis without a tumour?
Thymic follicular hyperplasia, with B-cell germinal centres in the medulla.
Elsewhere in DurgiPedia
- Thymus and tonsils under the microscope
- Thymus: where T cells learn
- Pharyngeal pouches: tonsil, thymus, parathyroids and C cells
- T-cell and combined deficiencies: DiGeorge, SCID, Wiskott–Aldrich, ataxia-telangiectasia
Connections
Educational content on the mechanisms of disease; not a diagnostic or treatment guide and no substitute for a health professional's advice. No doses are given.
Sources: Thymoma and Thymic Carcinoma (National Cancer Institute (NCI), 2026).
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Representative model, for educational purposes only.