Lower Free Thyroxine Levels Are Associated with Diabetic Kidney Disease in Males with Type 2 Diabetes Mellitus: An Observational Cross-Sectional Study

<b>Objectives</b>: We aimed to explore the correlation between thyroid function and diabetic kidney disease (DKD) in patients with type 2 diabetes mellitus (T2DM). <b>Methods</b>: A total of 7516 T2DM patients were enrolled and grouped according to DKD status. Clinical parameters, including blood glucose parameters, thyroid function, and indicators of renal impairment, were collected and compared between the DKD and Non-DKD groups. Correlation analysis and univariate/multivariate logistic regression analyses were performed. <b>Results</b>: Age, T2DM duration, the use of insulin and lipid-lowering drugs, systolic and diastolic blood pressure, body mass index, and fasting blood glucose levels were greater in the DKD group than in the Non-DKD group (<i>p</i> < 0.001). Notably, compared with those in the Non-DKD group, patients in the DKD group had lower triiodothyronine (T3), thyroxine (T4), free triiodothyronine (FT3), and free thyroxine (FT4) levels and higher thyrotropin levels (<i>p</i> < 0.001). Univariate logistic regression analysis revealed that T3, T4, FT3, and FT4 levels were negatively correlated with the risk of DKD. Spearman correlation analysis confirmed that T3, T4, FT3, and FT4 levels were negatively correlated with blood urea nitrogen levels, blood creatinine levels, and the urinary albumin-to-creatinine ratio (<i>p</i> < 0.05). Multivariate logistic regression analysis revealed that a greater FT4 level was a protective factor against DKD in T2DM patients, especially in males, with a cut-off value of 13.35 pmol/L (area under the curve = 0.604). <b>Conclusions</b>: Thyroid hormone levels, especially FT4 levels, were significantly negatively correlated with DKD in T2DM patients.