Analysis of the correlation between high iodized salt intake and the risk of thyroid nodules: a large retrospective study
Authors: Wang Yaohui , Wang Jiangang , Chen Zhihen , Ma Min , Lin Changwei , He Qingnan , Ye Mingzhu 
Published: September 2021

Abstract
Background
Currently, whether daily excess iodized salt intake increases the risk of thyroid nodules and even thyroid cancer remains controversial. Our research group aimed to provide a theoretical basis for the clinical guidance of daily iodized salt intake and the prevention of thyroid nodules through a retrospective analysis of the correlation between daily iodized salt intake and the risk of thyroid nodules and thyroid cancer in Hunan, China.

Methods
This study retrospectively analyzed the data of subjects who underwent a physical examination at the Health Management Center, Third Xiangya Hospital of Central South University, between January 1, 2017, and December 31, 2019. Subjects enrolled in this study underwent thyroid ultrasonography and tests to urine routines and liver and kidney function, and all subjects completed a questionnaire survey. The daily iodized salt intake of the study subjects was estimated based on spot urine methods (Tanaka). A multivariate logistic regression model was used to analyze the relationship between daily iodized salt intake and thyroid nodules and thyroid cancer.

Results
Among the 51,637 subjects included in this study, the prevalence of thyroid nodules was 40.25%, and the prevalence of thyroid cancer was 0.76%; among all enrolled subjects, only 3.59% had a daily iodized salt intake less than 5 g. In addition, we found that a daily intake of more than 5 g of iodized salt was not only an independent risk factor for the occurrence of thyroid nodules (odds ratio (OR): 2.08, 95% confidence interval (CI): 1.86–2.31, p < 0.001) but also an independent risk factor for the occurrence of thyroid cancer (OR: 5.81, 95% CI: 1.44–23.42, p = 0.012). A pooled analysis showed a significantly higher risk of thyroid nodules in subjects aged > 60 years with a daily iodized salt intake of more than 5 g compared to subjects aged < 60 years with a daily iodized salt intake of no more than 5 g (OR: 4.88, 95% CI: 4.29–5.54, p < 0.001); the risk of thyroid cancer was not significantly different between subjects aged > 60 years with a daily iodized salt intake of more than 5 g and those aged < 60 years with a daily iodized salt intake of no more than 5 g (OR: 2.15, 95% CI: 0.52–8.95, p = 0.281). The risk of thyroid nodules was not increased in physically active subjects with a daily iodized salt intake of more than 5 g compared to physically inactive subjects with a daily iodized salt intake of no more than 5 g (OR: 1.12, 95% CI: 0.97–1.28, p = 0.111). The same protective effect of physical activity was observed for thyroid cancer in subjects whose daily iodized salt intake exceeded 5 g. The risk of thyroid nodules was reduced for subjects with an education level of postgraduate and above, even when the daily iodized salt intake exceeded 5 g, compared to those with high school education and below and a daily iodized salt intake of no more than 5 g (OR: 0.79, 95% CI: 0.66–0.93, p = 0.005); however, a protective effect of education level on the occurrence of thyroid cancer was not observed. Independent risk factors affecting daily iodized salt intake greater than 5 g included age, triglycerides, family history of tumors, physical activity, and marital status.

Conclusions
Daily intake of more than 5 g of iodized salt increased the risk of thyroid nodules and thyroid cancer, while increased physical activity and education level reduced the risk of thyroid nodules and thyroid cancer caused by iodized salt intake.