Ectopic Intrathyroidal Thymic Tissue Mimicking Thyroid Nodules in Children

Abstract

Ectopic intrathyroidal thymic tissue is a benign lesion of nonthyroid origin occasionally found in the pediatric thyroid gland. Accurate diagnosis of such lesions is critical to avoid unnecessary biopsy or surgery. Twelve children referred to our center for the concern of thyroid nodules were found to have intrathyroidal thymic tissue. Most of the lesions had a classic sonographic appearance of a hypoechoic mass with sharp margins and multiple focal internal nonshadowing echogenicities identical to thymic tissue. Sonography and, in select cases, fine-needle aspiration can be used to diagnose benign thymic tissue within the thyroid and avoid unnecessary surgery.

Long-Term Follow-Up Ultrasonographic Findings of Intrathyroidal Thymus in Children

“…Some studies have confirmed that the presence of an intrathyroidal thymus through cytology or surgery due to the fear that children with suspicious thyroid lesions are misdiagnosed ( 4 5 12 13 15 16 21 22 ). However, a few studies on the ultrasound characteristics and the prevalence of intrathyroidal thymus were based on US images only, without cytological evaluation, and suggested that US could be used as a common diagnostic tool to identify a benign intrathyroidal thymus ( 11 13 17 23 ). In addition, awareness of the characteristic features of an intrathyroidal thymus and how they differ from the malignant finding and of the follow-up changes will be helpful for accurately diagnosing patients and determining the next procedures to be performed.…”

“…In previously reported studies investigating intrathyroidal thymus follow-up, an initial US was performed in 9 to 22 patients aged of 2.5 weeks to 17.5 years. In these studies, follow-up US was performed in 1 to 13 patients, at interval ranging from 6 to 84 months (mean, 30.7 months) ( 1 6 11 13 15 18 ). In our study, we analyzed the initial US findings of 41 patients with an average age of 8.7 months, with an intrathyroidal thymus.…”

“…When ectopic thymic tissue abuts on the inferior or posterior surface of the thyroid or is enclosed by the thyroid parenchyma, the thymus is described as intrathyroidal ( 4 5 6 7 8 9 10 ). The intrathyroidal thymus has been considered a rare entity, a benign lesion that is unique to the pediatric population and generally requires no treatment ( 13 ). The incidence of ectopic intrathyroidal thymus is reported to be between 1% and 5% on screening ultrasonography (US).…”

“…The incidence of ectopic intrathyroidal thymus is reported to be between 1% and 5% on screening ultrasonography (US). However, because US has become more frequently used in children, the characteristic features of intrathyroidal thymic tissue used to distinguish it from thyroid nodules have been reported more often, and this intrathyroidal thymus is now more frequently found as a common variant ( 8 11 12 13 14 15 ). Until 2015, 59 cases of ectopic thymic tissue mimicking thyroid nodules or neoplasia detected on US had been published and more cases have been reported since ( 12 13 15 16 17 18 ).…”

“…Certain, characteristics leading to malignancy suspicion and misdiagnosis have resulted in children undergoing thyroid surgery for suspected malignancy that proved to be benign thymic tissue on surgical pathologic analysis ( 7 8 12 13 14 19 20 ). A few papers have reported on follow-up US of the intrathyroidal thymus in small numbers of patients ( 1 6 11 13 15 18 ). The purpose of this study was to analyze the ultrasonographic features of the intrathyroidal thymus mimicking thyroid nodules, upon long-term follow-up, a large number of patients.…”