Body mass index and malignant cytological findings in thyroid nodules: a multicentre retrospective cross-sectional study based on fine-needle aspiration biopsy in Guangzhou, China.

OBJECTIVE: To investigate the relationship between body mass index (BMI) and the risk of Bethesda VI cytological classification in thyroid nodules, and to analyse its non-linear characteristics. DESIGN: Multicentre retrospective cross-sectional study. SETTING: Four tertiary hospitals in Guangzhou, China. PARTICIPANTS: A total of 2246 patients with thyroid nodules who underwent fine-needle aspiration biopsy from January 2018 to June 2024, including 1770 with Bethesda II (benign) and 476 with Bethesda VI (malignant) cytology. Sex distribution: 481 male and 1765 female. Mean age was 50.0 years (IQR 38.0-59.0) in the benign group and 42.0 years (IQR 32.0-52.0) in the malignant group. PRIMARY AND SECONDARY OUTCOME MEASURES: The primary outcome was Bethesda VI cytological classification (malignant cytological outcome). Secondary measures included BMI, fasting plasma glucose (FPG), triglycerides (TG), total cholesterol and aspartate aminotransferase to alanine aminotransferase ratio. RESULTS: A total of 2246 patients were ultimately included in this study, comprising 1770 in the benign group and 476 in the malignant group. Statistically significant differences were observed between the two groups in terms of gender distribution, age, BMI, maximum nodule diameter and multiple metabolic indicators. Compared with the benign group, the malignant group had a younger age, a higher proportion of men, and significantly higher BMI, FPG and TG levels (all p<0.05). Multivariate logistic regression analysis showed that when adjusted only for age and sex, a 1 kg/m² increase in BMI was associated with odds of Bethesda VI cytological classification increased by 6% (OR=1.06, 95% CI 1.02 to 1.09, p=0.002); after adjusting for FPG and TG, the association was no longer statistically significant (OR=1.03, 95% CI 0.98 to 1.08, p=0.214); and after adjusting for all metabolic markers, the association was further attenuated (OR=1.01, 95% CI 0.96 to 1.06, p=0.751). Smoothed curve fitting suggested a potential non-linear pattern, with an exploratory inflection point around 26.1 kg/m² in the age-adjusted and sex-adjusted model. However, this pattern was attenuated and no longer statistically significant after further adjustment for glucose and lipid metabolism markers. CONCLUSION: There is a positive association between BMI and the risk of Bethesda VI cytological classification in thyroid nodules, and this association may be partly explained by abnormalities in glucose and lipid metabolism. Clinical evaluations should focus more on patients' metabolic status rather than BMI alone.