Ectopic intrathyroidal thymus misdiagnosed as a thyroid nodule: Sonographic appearance

Abstract

An 11-year-old boy under suppression therapy for a solitary thyroid nodule was referred for sonographic examination. The diagnosis had been made at another institution, based only on sonography without cytopathologic verification. A small fusiform lesion, which was homogeneously hypoechoic with diffuse bright internal echoes, was demonstrated in the right lower pole of the thyroid. A normal elongated thymus with a cervical component was then found connected to the thyroid with an accessory lobe, which was embedded in the lower thyroid pole mimicking a solitary nodule. We discuss the developmental abnormalities of the thymus and their clinical significance in childhood with a brief review of the literature.

Intrathyroidal ectopic thymus in children: a sonographic survey

“…Of special note is the fact that none of the nodules were located anteriorly or superiorly. This is similar to other published results [7,9,17]. However, in the study by Yildiz et al the authors report all but one IET in the mid portion of the gland [4].…”

“…However, our patient was a 5 yearold, which would be too early for pubertal regression of thymic tissue; therefore, it seems such changes might be encountered earlier than previously reported. Another point to note would be that ectopic thymic tissue has been reported to enlarge with infection or vaccination, which resolves with antibiotic or steroid treatment [9]. To our knowledge, there are no reported cases of such an enlargement in an IET and we did not encounter such a case in our patients.…”

“…Thymus arises bilaterally from the 3 rd , and to a lesser extent 4 th branchial pouches starting from the fifth or sixth gestational week [4,9]. During the seventh week thymic primordia located in the pharynx start their descent medially and caudally.…”

“…In the eighth week, thymic tissue reaches anterior mediastinum and left and right primordia fuse at their lower poles. At this point, thymus consists only of epithelial cells, but from week 9 to 14-16, lymphoid cells from the liver and bone marrow migrate to the thymus, which then differentiates into cortex and medulla [9]. Thymus may fail to descend, or even when it descends to anterior mediastinum, there may be thymic remnants at any point along its path [4,8].…”

“…The thymus has the greatest relative size at birth, and continues to grow until puberty, when it reaches its greatest absolute size [9]. Since IET is essentially normal thymic tissue in an abnormal location, it would be logical to expect it to follow the same growth trend.…”