Clinical Value of Core Needle Biopsy as a Second-Line Approach After Non-Conclusive Fine-Needle Aspiration in Thyroid Nodules: A Paired Analysis

Background: Fine-needle aspiration biopsy (FNAB) is the standard initial diagnostic procedure for thyroid nodules; however, a considerable proportion of results are non-diagnostic or indeterminate, often requiring repeat procedures and delaying management. Core needle biopsy (CNB) has been proposed as a second-line option. This study evaluated the frequency of non-conclusive FNAB and CNB results and assessed the diagnostic contribution of CNB in nodules with initially non-conclusive FNAB findings. Methods: A retrospective–prospective study was conducted between 2019 and 2025 at a tertiary referral center, including 434 thyroid nodules. Ultrasound risk stratification followed ACR TI-RADS criteria. FNAB was performed in 430 nodules, and CNB in 85 nodules, including 82 evaluated by both methods. Biopsy results were classified according to the Bethesda system as conclusive or non-conclusive. Paired comparisons were analyzed using the McNemar test, and associations with ultrasound risk were assessed. Results: FNAB produced non-conclusive results in 56.5% of cases, compared with 23.5% for CNB. In paired analysis, 53.7% of nodules with non-conclusive FNAB were reclassified as conclusive after CNB (p < 0.001). CNB significantly distinguished benign from malignant lesions, unlike FNAB. Hypoechogenicity, irregular margins, and punctate echogenic foci were independent predictors of malignancy. Minor complications were more frequent after CNB, while major complications were rare in both groups. Conclusions: CNB improves diagnostic yield when used as a second-line procedure in nodules with non-conclusive FNAB findings. Selective use in higher-risk nodules may reduce repeat procedures and facilitate more structured clinical management.