Ectopic Intrathyroidal Thymus in Childhood: A Sonographic Finding Leading to Misdiagnosis

Abstract

Background: During gestation, the primordial thymus migrates from the pharynx to the anterior mediastinum, thus thymic tissue can remain at any point along this path. Intrathyroidal thymic remnants are rare, and their sonographic patterns have only recently been described. This retrospective study presents the sonographic appearance of ectopic intrathyroidal thymus and emphasizes the role of sonography in order to avoid misdiagnosis. Methods: The population consisted of 42 children, 3.5-14 years old, who had a thyroid sonogram performed due to a positive family history or symptoms indicative of thyroid disease, and ectopic intrathyroidal thymus was recognized. Results: In all patients, the same pattern was revealed: a fusiform intrathyroidal lesion, with no mass effect, homogeneously hypoechoic, with diffuse bright internal echoes. The similarity to the characteristic sonographic pattern of the normal mediastinal thymus was crucial for the diagnosis of ectopic intrathyroidal thymic tissue. In 8 cases, a normal elongated thymus was found connected to the thyroid with an accessory lobe embedded in the lower thyroid pole. The above sonographic appearances mimicked a thyroid nodule. Conclusions: Awareness of the sonographic patterns of the ectopic intrathyroidal thymus is mandatory to avoid misdiagnosis. In most cases, further investigation is unnecessary, but sonographic follow-up should be recommended.

Intrathyroidal ectopic thymus: A case series

“…Follow‐up US of intrathyroidal ectopic thymus shows stability of size, shape and sonographic appearance after 18 months, although there are some case reports of decrease in size, which support its benign nature 1,2,5 …”

“…Embryologically, it originates from the third and fourth pharyngeal pouches bilaterally during the sixth week of gestation. In the seventh week, the budlike thymic primordia on each side migrate in an inferomedial direction to the anterior mediastinum and during the eighth week the lower lobes are fused to form the thymus 1–3,5,6 …”

“…In the paediatric population, thyroid nodules are less frequent than in adults, with a prevalence of 0.2–1.5%; however, the malignancy rate is higher in children than in adults (10–50% vs. 5–10%). Ectopic thymus represents a small percentage of the nodular thyroid lesions, but an accurate differentiation is necessary in order to avoid invasive procedures 1,2,8 …”

“…During puberty, thymus may also show a hyperechoic appearance, related to the physiological replacement of fatty tissue 1,2,7 . In addition, all these lesions are located in the mid to low portion of the thyroid and although they can be either unilateral or bilateral, most series describe unilateral left‐sided lesions as the most frequent 1,2 . Also, the thymus does not compress or displace adjacent structures due to its pliability, in contrast to solid nodules 7 …”

Acoustic Radiation Force Impulse Imaging in the Differential Diagnosis of an Ectopic Intrathyroidal Thymus

“…If misdiagnosed as a thyroid nodule, unnecessary interventions and inappropriate treatment may cause unwanted results [2]. Acoustic radiation force impulse-imaging (ARFI) is a quantitative method in determining tissue elasticity and had been used in differentiation of benign and malign thyroid nodules [3].…”