Association Between Thyroid Hormone Levels with Diabetic Kidney Disease Among Euthyroid Participants with Type 2 Diabetes Mellitus: A Cross-Sectional Study.

PURPOSE: Thyroid hormones are linked to prognostic outcomes for a range of diseases. The link between thyroid hormone levels and diabetic kidney disease (DKD) risk in euthyroid individuals with type 2 diabetes mellitus (T2DM) was investigated in this study. PATIENTS AND METHODS: Overall, 2945 T2DM cases were enrolled and grouped as with or without DKD. Thyroid hormones were analyzed both as continuous variables and by quartiles. Logistic regression models and smooth curve fitting were applied to assess associations. Stratified analyses and interaction tests were also conducted to examine effect modification. RESULTS: Patients with DKD had significantly decreased free triiodothyronine (FT3) levels compared to those without DKD, while free thyroxine (FT4) and thyroid-stimulating hormone (TSH) were comparable across groups. Relative to the bottom FT3 quartile (Q1), the respective adjusted odds ratios (ORs) for DKD in Q2, Q3, and Q4 were 0.63 (95% CI: 0.50-0.80, p < 0.001), 0.60 (95% CI: 0.47-0.77, p < 0.001), and 0.63 (95% CI: 0.49-0.82, p = 0.001). The relationship between FT3 and DKD followed a nonlinear, L-shaped pattern (p for nonlinearity < 0.001), with a threshold at approximately 3.09 pg/mL. Below this inflection point, an adjusted OR for DKD of 0.622 (95% CI: 0.515-0.751, p < 0.001) was evident. Subgroup analysis suggested that this association was generally consistent across most subsets of study subjects, but was not statistically significant in several strata. CONCLUSION: Lower FT3 levels are independently linked to greater prevalence of DKD in euthyroid individuals with T2DM. The observed L-shaped association suggests a threshold effect, with 3.09 pg/mL as the approximate inflection point.