Presentations

Abstract

Introduction:

We report a case of incidental bilateral intrathyroidal ectopic thymus (IET) in a 3-year-old male, which was discovered during evaluation of congenital hypothyroidism on ultrasound scan. Ultrasound of the thyroid showed bilateral upper pole lesions with heterogeneous echotexture and irregular, ill-defined margins. While this appearance can be seen in papillary carcinoma, the patient's age made it much less likely and a diagnosis of intrathyroidal ectopic thymus was established, avoiding any further unnecessary testing.

Case Information:

We report on a 3-year-old male with a medical history of congenital hypothyroidism without goiter discovered by abnormal thyroid function test (TFT) on newborn screen. His hypothyroidism was well-controlled on low dose thyroid replacement with levothyroxine and he was able to stop his medications at age 3 with a normal TFT. At this time, he underwent a thyroid ultrasound (US) to assess for thyroid dysgenesis. US showed a thyroid with heterogeneous echotexture along with bilateral lesions on the right and left upper poles. The right pole contained a solid hypoechoic lesion with irregular margins and punctate echogenic foci measuring 0.4 x 0.6 x 0.9 cm. The left pole contained a mostly solid hypoechoic ill-defined lesion with microcalcifications measuring 0.3 x 0.3 x 0.4 cm. Based on the ultrasound findings and this patient's age, he was diagnosed with bilateral intrathyroidal ectopic thymus and no further biopsy or treatment was required.

Discussion:

IET is a condition in which ectopic thymus tissue is located within the thyroid, typically due to a defect in the embryologic migration of the thymus from the mandibular angle to the superior mediastinum. It is benign and requires no treatment. A recent (2017) cross-sectional study with 37,816 children in Japan found the prevalence to be 0.99% in the general population. Of the 375 cases of IET that were diagnosed during this study only 17 (~4.5%) were bilateral. Given that IET is relatively uncommon, it can be misdiagnosed as a thyroid tumor, potentially resulting in unnecessary treatment and surgery. Thyroid nodules are less common in children (1.5%) as compared to adults (4-7%), however they are more likely to be malignant. (26% in children vs 5-10% in adults) Although rare, it is important to be cognizant of IET to avoid unnecessary biopsy and treatment. IET is typically discovered during ultrasound evaluation of the thyroid gland with findings of irregular, triangular, polygonal hypoechoic or hyperechoic area with punctate, granular, linear echogenic foci surrounded by normal thyroid tissues. As for papillary carcinoma, it can share some common sonographic features including a hypoechoic echotexture with microcalcifications. When trying to distinguish between the two entities, demographics can help guide the medical-decision making process. Pediatric thyroid carcinomas tend to be diagnosed in adolescents, as seen in a large case study including 566 patients with an average age at presentation of 16 years of age. Given the young age of our patient, papillary thyroid carcinoma was felt to be much less likely.

Conclusion:

Intrathyroidal ectopic thymus is typically discovered during ultrasound evaluation of the thyroid gland with findings that may mimic differentiated thyroid carcinoma. IET should be included in the differential for abnormal sonographic findings of the thyroid of young children, who do not fit the expected demographic age for thyroid carcinoma, and clinicians should be aware of its characteristics to avoid further unnecessary testing.