Association of triiodothyronine levels with future development of metabolic syndrome in euthyroid middle-aged subjects: a 6-year retrospective longitudinal study.

Background: Several cross-sectional studies have reported that thyroid hormone levels are associated with cardiovascular risk markers and metabolic syndrome (MetS) even in euthyroid subjects. However, the prognostic role of serum thyroid hormone levels in the risk of incident MetS has not been elucidated. Aim: We aimed to investigate the associations of baseline serum thyroid hormone levels with the development of MetS in healthy subjects. Methods: This 6-year, cross-sectional, longitudinal and follow-up study was conducted in 12 037 euthyroid middle-aged subjects without MetS subjected to comprehensive health examinations. Subjects were grouped according to total triiodothyronine (T3) quartiles. The hazard ratio (HR) for the development of MetS according to T3 quartiles was estimated using Cox proportional hazards model. Results: During the 6-year period, 3544 incident cases of MetS (29%) were identified. The proportion of subjects with incident MetS increased across the T3 quartiles (P for trend <0.001). The HR and 95% confidence interval (CI) for the development of MetS were significantly higher in the highest T3 quartile compared with the lowest T3 quartile even after adjusting for confounding variables including gender, age and smoking (HR: 1.238, 95% CI: 1.128-1.358, P <0.001). Conclusion: In euthyroid middle-aged subjects, serum T3 levels are associated with increased risk for future development of MetS.