Let us begin by discussing the ultrasonographic features of normal thymic tissue, which is frequently visible in the superior mediastinum of children. The normal thymus is hypoechoic with multiple punctate internal echoes giving the characteristic ‘starry sky appearance’. See figure 1 and 2.
Intrathyroidal thymic tissue is often incidentally identified in the paediatric population and shares imaging characteristics similar to the normal thymus. In many cases, the normal thymus can also be visualised in the superior mediastinum, allowing for direct comparison with ectopic intrathyroidal thymic tissue. The normal thyroid gland is hyperechoic. Therefore, the hypoechoic ectopic thymic tissue within it will stand out. Intrathyroidal thymic tissue typically appears well-defined, with no evidence of mass effect or local invasion of surrounding structures.[3,4] When the location within the thyroid gland is evaluated, the ectopic thymic tissue is mostly found in the middle and lower parts of the gland.[12] See figures 3 and 4.
Intrathyroidal thymic tissue typically exhibits well-defined margins, although its shape can vary significantly. According to current literature, ectopic thymic tissue may appear round or ovoid, as well as triangular or polygonal/angular in form. [3,4] In our case, as shown in Figure 4, the intrathyroidal thymic tissue demonstrates the possible angular shape.
The diagnostic challenge occurs when the ultrasound features of intrathyroidal thymic tissue are misinterpreted as those of a malignant thyroid nodule. Specifically, the internal echogenic foci within intrathyroidal thymic tissue can be mistaken for microcalcifications. According to the British Thyroid Association (BTA) "U" classification system for thyroid nodules, a solid hypoechoic nodule with an irregular outline and microcalcifications is classified as U5, indicating a high likelihood of malignancy.[5] This appearance is typically associated with papillary thyroid carcinoma.[5] It is therefore understandable how intrathyroidal thymic tissue could be misinterpreted as a malignant thyroid nodule if clinicians are unfamiliar with the existence of this entity.
The distinguishing sonographic features of papillary thyroid carcinoma include the following: [5] See figures 5 and 6 which demonstrate the sonographic appearances of papillary thyroid carcinoma.
• Irregular, indistinct, and infiltrative margins.
• Evidence of local invasion extending beyond the thyroid gland into surrounding structures.
• Increased intra-nodular vascularity compared to the rest of the thyroid gland.
• Presence of associated malignant cervical lymphadenopathy.
If any of the above features are present, further investigation with fine needle aspiration biopsy (FNAB) should be conducted to assess for papillary thyroid carcinoma. Discussion at the relevant paediatric multidisciplinary team (MDT) meeting is recommended.
Thymic involution refers to the process by which the thymus decreases in size with advancing age. The gland has its greatest relative size at birth and continues to grow until it reaches its largest absolute size during puberty.[6,8] Since the incidence of intrathyroidal thymic tissue is inversely correlated with age, intrathyroidal thymic tissue is expected to follow the same growth patterns as normal thymic tissue due to their shared embryologic origin.[6,7] The prevalence of thyroid malignancies increases with age, whereas intrathyroidal thymic tissue typically involutes by adulthood.[6,7] Therefore, if a lesion with characteristics of intrathyroidal thymic tissue is observed in older children, ultrasound-guided FNAB should be performed to exclude thyroid malignancy. If FNAB is performed on intrathyroidal thymic tissue, the cytological findings typically show benign-appearing lymphocytes, often with minimal or no epithelial components.[4,9,10]
A few studies have reported on the ultrasound follow-up of intrathyroidal thymic tissue. Chang et al.[6] reviewed the literature on ultrasound follow-up of suspected intrathyroidal thymic tissue and found that most lesions either remained stable or decreased in size.[6] If the clinician is experienced and the sonographic appearances are characteristic with no concerning features, an ultrasound follow-up protocol may be appropriate. This approach can demonstrate the expected involution of intrathyroidal thymic tissue, helping to avoid unnecessary invasive procedures such as FNAB or surgery.
However, it is important to note that during the thymic involution phase, these lesions may develop less well-defined borders and increased echogenicity.[6,11] This change is believed to result from the fatty replacement of the thymic cellular components, which leads to hyperechoic transformation that blends with the surrounding hyperechoic thyroid gland.[6,8] The lack of well-defined borders can pose a diagnostic challenge when differentiating these lesions from malignant thyroid nodules.
In cases of diagnostic uncertainty, proceeding with ultrasound-guided FNAB to exclude malignant pathology is the safest course of action.