Intrathyroidal Ectopic Thymus: The Trick of the Trade

Introduction: Intrathyroidal ectopic thymus (IET) is a rare benign entity in children, most often detected incidentally during thyroid or neck ultrasonography. Its sonographic appearance may mimic malignant thyroid nodules, potentially prompting unnecessary diagnostic or surgical interventions. The aim of our study was to describe the occurrence of IET in a tertiary pediatric endocrinology center, defining its clinical, biochemical, and sonographic characteristics and suggesting the best clinical approach. Methods: We retrospectively reviewed all pediatric patients (<18 years) evaluated at our tertiary pediatric endocrinology service between 2015 and 2025 who presented with thyroid nodules classified as EU-TIRADS >= 3 and were subsequently diagnosed with IET based on characteristic imaging findings and stability over time. Results: Twenty-one children with nodules classified as EU-TIRADS >= 3, with a median age of 10.6 (0.55-15) years, were identified. Ten patients (50% males) with a mean age of 6.19 +/- 3.93 years were presumed to have IET. All lesions were hypoechoic, well-circumscribed, and demonstrated punctate or linear hyperechogenicities. No lesion showed significant change in size or echotexture during follow-up. Thyroid function was normal in 9 patients, while 1 had subclinical hypothyroidism that resolved during follow-up. None of these 10 patients underwent surgical intervention or fine-needle aspiration biopsy (FNAB). Conclusion: IET should be considered in the differential diagnosis of pediatric thyroid nodules especially in the first years of life. Although ultrasonography alone cannot reliably distinguish benign from malignant lesions, recognition of the characteristic imaging pattern of IET, together with clinical and laboratory correlation, can help prevent misdiagnosis of thyroid cancer and avoid unnecessary FNAB.