Enlarged Thymus Gland: Causes, Evaluation, and When to Worry About Cancer

The thymus, located in the anterior superior mediastinum, is largest in childhood and involutes with age. Enlargement (thymomegaly) has several causes:

Benign/Reactive Causes (Most Common)

• Thymic hyperplasia: Rebound after chemotherapy, steroids, stress, or in myasthenia gravis (MG). Common in children/young adults; often resolves.
• Thymic cysts: Congenital or acquired; thin-walled, fluid-filled.
• Lymphoid hyperplasia: In Graves' disease, SLE, or post-transplant.

Neoplastic Causes

• Thymoma: Epithelial tumor; 80% benign (WHO types A, AB, B1, B2). Associated with MG (30–50%), pure red cell aplasia, hypogammaglobulinemia. Slow-growing; staged by Masaoka-Koga.
• Thymic carcinoma: Rare, aggressive (type C); no MG association.
• Germ cell tumors: Teratoma, seminoma (mediastinal).
• Lymphoma: Primary mediastinal large B-cell lymphoma or Hodgkin's.

Evaluation

• Chest CT with contrast (size, shape, invasion, nodal spread).
• MRI for vascular/neural involvement.
• Biopsy: CT-guided core needle (thymoma) or Chamberlain procedure (mediastinoscopy) if needed. Avoid FNA—insufficient for subtyping.
• Acetylcholine receptor antibodies (MG workup).

Management

• Asymptomatic hyperplasia/cysts: Monitor with imaging.
• Thymoma: Complete surgical resection (thymectomy) ± adjuvant radiation for invasive stages.
• MG with thymoma: Thymectomy improves remission rates.
• Carcinoma/lymphoma: Multimodal (chemo ± radiation ± surgery).

Bottom line: Most enlarged thymuses in young people are benign. In adults >40, new thymic mass warrants thorough neoplasm workup.