Long-Term Ultrasound Surveillance of Solid and Predominantly Solid Thyroid Nodules Reveals Two Distinct Absorption Patterns.

Background: To characterize volume reduction in benign solid and predominantly solid thyroid nodules during long-term ultrasound surveillance, and to describe distinct absorption patterns over time. Methods: This retrospective study included 34 solid or predominantly solid thyroid nodules from 32 patients (median age, 46 years; 78.1% female), who underwent longitudinal ultrasound surveillance for a median of 28 months (interquartile range, 47-120 months). Volumetric measurements were obtained at multiple time points. Generalized additive mixed models (GAMMs) were employed to model nonlinear trends in volume over time. Unsupervised clustering was applied to identify representative regression trajectories based on individual volume curves. Clinical and ultrasound features were compared between trajectory-defined groups. Results: All nodules exhibited measurable volume decrease over time (median reduction: 85.6%). Two representative volume change patterns were identified: a rapid-absorption group with early shrinkage and a slow-absorption group with gradual decline. Nodules in the latter were more frequently located in the lower pole and lacked Doppler flow at baseline (p < 0.05). Some nodules developed hypoechoic or stiff appearances over time, leading to higher TIRADS categories despite continuous shrinkage and no clinical signs of malignancy. Conclusion: Solid and predominantly solid thyroid nodules can undergo significant volume reduction during long-term surveillance, resembling the well-documented absorption phenomenon of cystic nodules. This phenomenon may be underrecognized in clinical practice. Describing the diversity of volume change patterns may improve the understanding and interpretation of nodule evolution during follow-up.