Does a three-degree hypoechogenicity grading improve ultrasound thyroid nodule risk stratification and affect the TI-RADS 4 category? A retrospective observational study.

Objective: The aim of this study was to determine whether classifying hypoechogenicity in three degrees (mild, moderate, and marked) could improve the distinction between benign and malignant nodules and whether such an approach could influence Category 4 of the Thyroid Imaging Reporting and Data System (TI-RADS). Materials and methods: In total, 2,574 nodules submitted to fine needle aspiration, classified by the Bethesda System, were retrospectively assessed. Further, a subanalysis considering solid nodules without any additional suspicious findings (n = 565) was performed with the purpose of evaluating mainly TI-RADS 4 nodules. Results: Mild hypoechogenicity was significantly less related to malignancy (odds ratio [OR]: 1.409; CI: 1.086-1.829; p = 0.01), compared to moderate (OR: 4.775; CI: 3.700-6.163; p < 0.001) and marked hypoechogenicity (OR: 8.540; CI: 6.355-11.445; p < 0.001). In addition, mild hypoechogenicity (20.7%) and iso-hyperechogenicity (20.5%) presented a similar rate in the malignant sample. Regarding the subanalysis, no significant association was found between mildly hypoechoic solid nodules and cancer. Conclusion: Stratifying hypoechogenicity into three degrees influences the confidence in the assessment of the rate of malignancy, indicating that mild hypoechogenicity has a unique low-risk biological behavior that resembles iso-hyperechogenicity, but with minor malignant potential when compared to moderate and marked hypoechogenicity, with special influence on the TI-RADS 4 category.