Objective:

To investigate the individual and combined associations of FT3 and the TyG index with mild cognitive impairment (MCI) in euthyroid patients with type 2 diabetes mellitus (T2DM) and evaluate their potential value for cognitive risk stratification.

Approach:

Key Findings:

• Lower FT3 levels were independently associated with higher odds of MCI, showing a significant linear negative dose–response relationship.
• The TyG index exhibited a nonlinear, asymmetric inverted J-shaped association with MCI odds, with an inflection point at TyG = 9.24.
• ROC analysis indicated good discriminative performance for FT3 alone (AUC = 0.761, 95% CI: 0.721–0.802), while the TyG index alone showed limited value (AUC = 0.571, 95% CI: 0.520–0.622).
• The combined FT3–TyG model yielded an AUC of 0.766 (95% CI: 0.726–0.807), not significantly higher than FT3 alone but provided additional granularity for higher TyG levels.

Interpretation:

Physiological variation in FT3 is independently associated with cognitive status in euthyroid T2DM patients, while the TyG index has a non-linear association with cognitive impairment.

Limitations:

• Retrospective design may limit causal inferences.
• Study conducted in a single center, which may affect generalizability.
• Cognitive assessments were based on a single MoCA score without longitudinal follow-up.

Conclusion:

FT3 and the TyG index may serve as accessible, noninvasive metabolic indicators for identifying individuals at higher risk of MCI in T2DM.

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