Objective: Thermal ablation (TA) is a safe and effective treatment for benign thyroid nodules (BTNs). However, there has been no consensus on the optimal maximum diameter (MD) of BTNs for TA. This study aimed to identify the optimal MD of BTNs for TA based on complete disappearance rate after TA. Materials and methods: This retrospective study included 639 BTNs treated with TA from June 2014 to January 2022. The complete disappearance rate of BTNs after TA was summarized, related influencing factors were explored, and the optimal MD of BTNs for TA was identified. Results: At the final follow-up (median: 40 months, range: 24-95 months), the overall volume reduction rate was 95.4 +/- 9.0%, and 50.5% of the BTNs (323/639) completely disappeared. The MD was significantly negatively correlated with complete disappearance (odds ratio 0.89, 95% confidence interval 0.87-0.92; p < 0.001). Calcification, comet-tail artifacts, multilocular cysts, and composition of BTNs, as well as diabetes were negatively correlated with complete disappearance. Restricted cubic spline indicated that an MD of 25.0 mm was the optimal threshold of BTNs for TA, which was confirmed by subgroup logistic regression analysis. Compared with BTNs with MD <= 25.0 mm, those with MD > 25.0 mm had a greater complication rate (6.5% vs. 2.4%, p = 0.012). Conclusions: The MD of BTNs was negatively correlated with complete disappearance after TA; an MD > 25.0 mm indicated a reduced likelihood of complete disappearance compared with an MD <= 25.0 mm. An MD of 25.0 mm is an appropriate threshold of BTNs for TA on the basis of complete disappearance rate.
