Thymic hyperplasia secondary to thyrotoxicosis

Clinical History

We present a case of a 56-year-old female patient with a history of thyrotoxicosis a month before, who underwent a CT examination because of constitutional symptoms.

Imaging Findings

CT showed an incidental finding of a diffuse symmetric enlargement of the thymus, with preservation of normal shape (figures 1, 2 and 3). These findings were suggestive of thymic hyperplasia. There were no other abnormal findings.
One month before, the patient had presented a hypermetabolic clinical syndrome resulting from serum elevations in thyroid hormone levels, and she was diagnosed with thyrotoxicosis.

Discussion

There are two distinct histologic types of thymic hyperplasia: true thymic hyperplasia and follicular hyperplasia. Both types manifest as diffuse symmetric enlargement of the thymus [1].
The true thymic hyperplasia is defined as an increase of both size and weight of the gland while it maintains normal microscopic architecture. Enlargement of the thymus gland is reported as a form of rebound phenomenon in a number of conditions like recovery from severe stress situations, after administration of steroids, and after treatment of malignant tumours [2].
Follicular or lymphoid hyperplasia of the thymus refers to the presence of an increased number of lymphoid follicles. This condition is frequently found in association with autoimmune disorders, especially in patients with myasthenia gravis, being seen in up to 65% of cases [1]. Lymphoid hyperplasia of the thymus is observed in a number of immunologically mediated disorders, including SLE, rheumatoid arthritis, scleroderma, vasculitis, thyrotoxicosis and Graves disease.
Although benign thymic hyperplasia is a known feature of hyperthyroidism, this is infrequently appreciated by clinicians.
Approximately one third of patients with thyrotoxicosis will have microscopic abnormalities of the thymus gland with the presence of medullary lymphoid follicles upon biopsy. However, massive enlargement of the thymus detectable radiologically is uncommon and has been infrequently reported [3].
Thymic hyperplasia appears to be induced as a consequence of immunological and hormonal reactions in hyperthyroidism.
Treatment with appropriate antithyroid therapy results in regression of the thymus.

Differential Diagnosis List

Final Diagnosis

Lymphoid thymic hyperplasia secondary to thyrotoxicosis

URL:	https://eurorad.org/case/9307
DOI:	10.1594/EURORAD/CASE.9307
ISSN:	1563-4086