Supplementary Material for: High Serum Ferritin Levels Increase the Risk of Hyperuricemia: A Cross-Sectional and Longitudinal Study

<b><i>Background/Aims:</i></b> To examine the cross-sectional and longitudinal associations of serum ferritin levels with hyperuricemia. <b><i>Methods:</i></b> A cross-sectional and subsequently prospective study was performed among the employees of Zhenhai Refining and Chemical Company, Ningbo, China. In a cross-sectional study, the association between serum ferritin levels and the prevalence of hyperuricemia was analyzed. Subjects who were free of hyperuricemia at baseline were followed up annually to explore the prospective association between serum ferritin levels and hyperuricemia incidence. <b><i>Results:</i></b> Of the 10,074 subjects enrolled at baseline, 1,731 (17.18%) fulfilled the diagnostic criteria of hyperuricemia. Subjects with hyperuricemia presented significantly higher serum ferritin levels, and the levels were positively correlated with the prevalence of hyperuricemia. During a total of 22,367 person-years of follow-up, 502 subjects developed hyperuricemia. The overall incidence of hyperuricemia for 1,000 person-years of follow-up was 22.4, ranging from 17.6 in subjects with baseline serum ferritin levels in the first quintile to 19.2, 21.7, 23.9, and 30.7 in subjects in quintiles 2, 3, 4, and 5, respectively (p for trend < 0.001). Cox regression analyses showed that serum ferritin levels were positively associated with the risk of incident hyperuricemia. <b><i>Conclusions:</i></b> Our cross-sectional and longitudinal results indicate that high serum ferritin levels increase the risk of hyperuricemia.