Association Between TSH Levels and Mortality, Microvascular and Macrovascular Complications in Euthyroid Patients with Type 2 Diabetic Retinopathy: A Three-Year Retrospective Cohort Study

Background: /Objectives: This study evaluated the relationship between baseline TSH levels and mortality, micro- and macrovascular complications during a three-year follow-up in T2DR patients with T2DM for at least 5 years. Methods: This single-center, retrospective cohort study included 363 euthyroid adult patients with T2DM and diabetic retinopathy in 2019. Patients were divided into three tertiles according to TSH levels (Group 1: 0.35–1.24 mIU/L, Group 2: 1.24–1.94 mIU/L, Group 3: 1.94–4.50 mIU/L) and followed for three years. Clinical outcomes were compared; multivariable logistic regression and nonlinear models were used for one-year mortality and complication risks. Results: At the end of the first year, mortality and mortality + microvascular complication rates were significantly higher in Group 1 compared to Group 2 (p = 0.025 and p = 0.041). Group 2 showed a lower risk for both outcomes (OR for mortality = 0.349, p = 0.004; OR for combined outcome = 0.358, p = 0.007). There was an inverted U-shaped non-linear relationship between TSH and mortality risk, with the lowest risk at ~2.0 mIU/L. Conclusions: Our findings indicate that, even within the normal reference range, TSH levels may influence outcomes in euthyroid T2DR patients with a ≥5-year T2DM history. Patients with TSH < 1.24 mIU/L had higher one-year mortality and microvascular complication rates, while the lowest risk was observed in those with TSH between 1.24–1.94 mIU/L. This range may represent a potential target for safer TSH levels, and closer monitoring may be warranted in patients with lower values.