Background: Type 2 Diabetes Mellitus (T2DM) has been associated with an increased risk of neurodegenerative changes, while abnormal tau phosphorylation is a key feature of Alzheimer’s disease (AD).
Objective: This observational case-control study evaluated serum phosphorylated tau protein levels in T2DM patients compared with healthy controls and assessed their potential relevance as an early biomarker of AD-related changes.                  
Methods: Serum samples were collected from 35 individuals, including 25 patients with T2DM and 10 healthy controls. Fasting blood glucose, lipid profile, creatinine, urea, insulin, and phosphorylated tau levels were measured using standard biochemical assays and ELISA.
Results: Phosphorylated tau levels were significantly higher in T2DM patients than controls (142.56 ± 106.82 vs. 15.30 ± 34.66 ng/L; p = 0.001). LDL cholesterol was also higher in T2DM patients (137.68 ± 72.33 vs. 55.30 ± 22.01 mg/dL; p = 0.001), as were total cholesterol (249.44 ± 76.40 vs. 110.60 ± 23.35 mg/dL; p < 0.001) and HDL cholesterol (86.28 ± 56.62 vs. 35.90 ± 10.08 mg/dL; p = 0.009). No significant differences were observed in creatinine, urea, triglycerides, or insulin levels.
Conclusion: The findings suggest a possible association between T2DM and increased serum phosphorylated tau levels. However, larger studies including cognitive assessment, body mass index, dietary evaluation, and AD-specific phosphorylated tau isoforms are required before clinical use can be established.