Objective:To investigate the alteration in iodine nutritional status and influence on thyroid function in the elderly aged≥65 years following water source modification in high iodine areas.Methods:Data from Yaoji Town, Xuzhou, Jiangsu(an area with high iodine due to water sources) of the national epidemiological survey on thyroid diseases, iodine nutrition, and diabetes(TIDE study) in 31 provinces and cities in China from 2015 to 2017 were utilized. Additionally, data from the screening, monitoring, and intervention on thyroid diseases(TOPS study) in the elderly(≥65 years) in Shunhe Town, Suqian, Jiangsu(an area with iodine levels exceeding the recommended amount), and Yaoji Town, Xuzhou from May to August 2021, are included. Each subject completed a questionnaire, physical examination, laboratory tests and thyroid ultrasound examinations. A total of 2 717 subjects aged≥65 years were included, including group 1, 258 subjects in TIDE study; Group 2, 1 313 subjects in TOPS Xuzhou area; Group 3, 1 146 subjects in TOPS Suqian area.Results:The urinary iodine concentration(UIC) in group 2 was significantly lower than that in group 1 [(235.16±67.09)μg/L vs (491.58±384.93)μg/L, P<0.001], but no significant difference compared with group 3 [(235.16±67.09) μg/L vs(231.62±66.11) μg/L, P>0.05]. The serum TSH level in group 2 was significantly lower than that in group 1 [(2.92±5.14)μIU/mL vs (4.15±9.19)μIU/mL, P<0.001]. Compared with group 2 and 3, the prevalence of subclinical hypothyroidism in the elderly in group 1 was the highest(22.48% vs 10.13% and 8.12%, P<0.001). TSH levels were linearly correlated with age in both excessive iodine and more than adequate iodine nutrition areas. TSH level was gradually increased with age. Conclusion:The alteration in TSH levels among the elderly is notably linked to both aging and iodine status. The prevalence of hypothyroidism in the elderly can be significantly reduced when the iodine nutrition status of the elderly returns to normal.