Residential Greenness Is Associated with Lower Thyroid Nodule Prevalence: A Nationwide Study in China

Background: The prevalence of thyroid nodules is increasing globally. This study explored the association between residential greenness and thyroid nodule prevalence. Methods: Data were collected from a national cross-sectional survey of 73,728 participants across 31 provinces in mainland China. Residential greenness was assessed with the normalized difference vegetation index (NDVI) and the enhanced vegetation index (EVI). Thyroid nodules >10 mm in diameter were diagnosed via ultrasound. We used quartile comparisons of the NDVI and EVI to compare nodule prevalence and employed logistic regression and restricted cubic spline analyses to examine nodules' associations with greenness. Interaction and sensitivity analyses were performed to test robustness. Results: A total of 73,728 participants were included in this study. The prevalence of 10 mm thyroid nodules decreased across NDVI500 quartiles: Q1: 7.99% (7.59-8.39%), Q2: 10.04% (9.60-10.48%), Q3: 6.59% (6.23-6.95%), and Q4: 5.20% (4.88-5.52%) (p for trend <0.001). The prevalence was 5.25% (95% confidence interval [CI]: 5.02-5.49%) in males and 9.09% (CI: 8.80-9.39%) in females. Logistic regression analysis showed that greater residential greenness was associated with a lower prevalence of thyroid nodules after adjusting for all covariates. This association was observed for both continuous greenness measures (NDVI500: odds ratio [OR] = 0.20, CI: 0.16-0.25; EVI500: OR = 0.08, CI: 0.06-0.12) and across quartiles (NDVI500 Q4: OR = 0.53, CI: 0.48-0.58; EVI500 Q4: OR = 0.55, CI: 0.51-0.60; both compared to Q1). Multiple sensitivity analyses confirmed this negative association, including the use of an alternative thyroid nodule definition (5 mm threshold), exclusion of individuals with cysts on ultrasound, and subgroup analyses excluding individuals with autoimmune thyroid antibody positivity, goiter, or both. In all these sensitivity analyses, NDVI and EVI data were assessed using both 500 m and 1000 m buffers. Conclusion: Our study is the first to identify an association between higher levels of residential greenness and a lower prevalence of thyroid nodules.