Aims Studies confirm that microalbuminuria (MAU) can not only predict diabetic kidney disease (DKD) but also indicate of systemic vascular endothelial injury. Thyroid hormones (TH) have been reported to be associated with various cardiovascular diseases and endothelial cell damage. Currently, few studies have reported the association between free thyroxine (FT4) to free triiodothyronine (FT3) ratio and MAU. The purpose of this study was to investigate the association between FT4/FT3 ratio and MAU in euthyroid patients with type 2 diabetes mellitus (T2DM). Methods This study enrolled a total of 1734 T2DM patients with euthyroid status. MAU was defined as 30 ≤ urinary albumin-to-creatinine ratio (UACR) <
300 mg/g. The participants were divided into MAU group and normal albuminuria group (NAU group). The relationship between FT4/FT3 ratio and MAU was analyzed. Results In this study, there were 367 patients with MAU. Compared with the NAU group, the MAU group had higher FT4/FT3 ratio (P <
0.002). In univariate analysis, the FT4/FT3 ratio was positively related to both UACR and MAU (both P <
0.002). In multivariate linear stepwise regression, the FT4/FT3 ratio is independently related to UACR (Standardized coefficients: 0.148, P <
0.002). In binary logistic regression analysis, the FT4/FT3 ratio emerges as an independent risk factor for MUA (odds ratio: 1.947, 95% confidence interval: 1.522-2.491). Conclusion The FT4/FT3 ratio was independently and positively correlated with MAU in euthyroid patients with T2DM.