Thymic hyperplasia

Thymic hyperplasia is a disorder whereby there is hyperplasia of the thymus gland. 

Pathology

Thymus hyperplasia can be subdivided into two forms: 

  • true thymic hyperplasia
  • lymphoid hyperplasia

Both true thymic hyperplasia and lymphoid hyperplasia manifest as diffuse symmetric enlargement of the thymus, so that it is difficult to distinguish between the two types on the basis of imaging findings alone.

It is important that radiologists be able to distinguish thymic hyperplasia from neoplasm, which tends to present as a focal mass. 

True thymic hyperplasia

True thymic hyperplasia associations include:

  • rebound hyperplasia to chemotherapy / steroids (see: thymic rebound hyperplasia)
  • radiation therapy
  • burns
  • other severe systemic stresses
Lymphoid hyperplasia

This is also known as lymphoid follicular thymic hyperplasia or autoimmune thymitis. Lymphoid hyperplasia associations include

Radiographic features

Diffuse symmetric enlargement of the gland is the key morphologic feature of hyperplasia (neoplasm tends to manifest as a focal mass).

MRI

Chemical shift artifact may be useful in differentiating from other tumours 3-5.

Differentiating normal from hyperplastic thymus can be difficult and guidelines for making this distinction and verifying the presence of normal thymus include 6:

  • presence of rounded soft-tissue masses > 7 mm
  • presence of a convex contour of the thymus beyond 19 years of age
  • presence of soft-tissue lobulation
  • presence of increased thymic thickness (should be ≤1.3 cm beyond age 20 years)
  • presence of a diagnosis associated with thymic enlargement or hyperplasia, e.g. Graves disease
Nuclear medicine

FDG PET is often performed in patients with malignancy; however, differentiation between thymic hyperplasia and thymic involvement by malignancy is difficult because the thymus demonstrates normal physiologic uptake.

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Article Information

rID: 15613
Section: Pathology
Synonyms or Alternate Spellings:
  • Hyperplasia of the thymus
  • Hyperplastic thymus
  • Lymphoid follicular thymic hyperplasia
  • Autoimmune thymitis

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