Purpose or Case Report: Ectopic thymic tissue may be found in the neck in up to 20% of the general population. Intrathyroidal thymic rest has been described as a rare entity, present in as many as 1% of children. The course of thymic migration parallels the thyroid and parathyroid glands, which explains their similar ectopic locations. While the natural history of these lesions has not been well studied, it is likely that there is involution of thymic rests with age. A few individual case reports have described the imaging features of intrathryoidal thymic rests confirmed by histopathology and flow cytometry. The aim of this case series is to raise awareness of the characteristic sonographic appearance of this entity to the radiology community.
Methods & Materials: This case series will review clinical and imaging features of 4 biopsy-proven intrathyroidal thymic rests. Characteristic imaging features will be elucidated. Differences between this entity and more concerning thyroid nodules that require biopsy and perhaps excision will be highlighted.
Results: The presence of dot-dash echogenic foci within a focal hypoechoic nodule are characteristic for thymic rests but can be mistaken for more worrisome calcifications in a thyroid nodule. Additionally, the border can be lobulated and somewhat irregular, which is another significant feature of suspicious thyroid nodules by TiRADS criteria.

Conclusions: The sonographic appearance of thymic rests within the thyroid gland is very characteristic and should be suggested in the differential diagnosis of an incidentally found characteristic nodule in an otherwise healthy child. These nodules do not follow TiRADS criteria for benignity, however, radiologists should be aware of this entity. When patients have this classic appearance , conservative management should be considered, with less invasive sampling with fine needle aspiration favored over surgical excision if there remains clinical concern.