Intrathyroidal ectopic thymus in childhood: ultrasound features for accurate diagnosis

Aims: To characterize the ultrasonographic features of intrathyroidal ectopic thymus (IET) in pediatric patients and emphasize the role of ultrasound (US) in its accurate identification to prevent misdiagnosis. Methods: Eighty-four pediatric patients (aged 3–12 years) were referred for neck US due to cervical lymphadenopathy, during which incidental lesions in the thyroid gland were detected. These lesions, identified within normal thyroid parenchyma, were classified as IET. In such cases, color Doppler US (CDUS) was also performed without additional cost or workload. Results: Among 84 pediatric patients, all IET lesions appeared as solid nodules with smooth margins and no mass effect. Ten lesions were uniformly hypoechoic, while 74 showed scattered internal bright echoes. Lesions measured 4–12 mm and were predominantly fusiform (59.5%), followed by triangular and round shapes. IET was unilateral in most cases (88%), with a marked left-lobe predominance, and all lesions were localized to the mid-to-lower thyroid lobe. CDUS demonstrated reduced or normal vascularity without suspicious flow patterns. Follow-up imaging in 62 children showed stable findings, with gradual size reduction in 8 adolescents and complete regression in one child. Echogenicity (p=0.001), lesion shape (p=0.01), and location (p=0.03) were significantly associated with IET characteristics, supporting their diagnostic relevance. Conclusion:The presence of internal echogenicities and mid-to-lower lobe localization, especially in the left lobe, are key diagnostic indicators. IET is more prevalent than previously recognized. Recognition of its US features may prevent unnecessary interventions. Routine US monitoring is recommended, while further diagnostic procedures are generally unwarranted.