PURPOSE: Although thyroid nodules are less common in the pediatric population, the risk of malignancy is higher than in adult patients. The aim of this study was to evaluate the ultrasonographic predictive factors of malignancy in thyroid nodules and to validate American College of Radiologists (ACR) TI-RADS performance in transition age patients. METHODS: One hundred forty-two patients aged between 14 and 21 years referred to the participating centers for FNA biopsy of a thyroid nodule between 2007 and 2022 were included and ultrasound reports and sonographic images were retrospectively analyzed. Nodule features were defined according to the ACR-TIRADS lexicon. Two reference standards were applied: FNA cytology and surgical histology. The diagnostic performance of single sonographic features was estimated. Significant predictors were then included in a multivariate regression model. RESULTS: Nodules included in ACR-TIRADS categories TR4 or TR5 had 10-fold increased risk of indeterminate or suspicious/malignant cytology [p <
0.001]. In univariate analysis, solid composition [p = 0.016] and presence of hyperechoic foci [p = 0.040] significantly increased the likelihood of malignant histology. In multivariate regression analysis, irregular margins [p = 0.011] and hyperechoic foci [p = 0.019] were independent predictors of indeterminate or suspicious/malignant cytology. CONCLUSION: Nodules included in ACR-TIRADS categories TR4 or TR5 had 10-fold increased risk of indeterminate or suspicious/malignant cytology in transition age. ACR-TIRADS was not able to rule-out malignancy compared to FNAB alone, suggesting the need to reconsider recommendations in the transition age group.