ABSTRACTObjectives:This study aimed to investigate the triglyceride-glucose (TyG) index, which is a simple surrogate marker of insulin resistance that is associated with various cardiometabolic diseases, in patients with Klinefelter syndrome (KS).Subjects and methods:A total of 30 patients with KS (mean age: 21.53 ± 1.66 years) and 32 healthy controls (mean age: 22.07 ± 1.01 years) were included in the study.The clinical and laboratory parameters,TyG index, asymmetric dimethylarginine (ADMA) level, homeostatic model assessment of insulin resistance (HOMA-IR) score, and high-sensitivity C-reactive protein level were measured in patients with KS and healthy subjects.Results:Patients with KS had higher HOMA-IR score (p = 0.043), ADMA levels (p < 0.001), and TyG index (p = 0.031) and lower high-density lipoprotein cholesterol levels (p < 0.001) than healthy subjects. TyG index was positively correlated with plasma ADMA (r = 0.48, p < 0.001) and HOMA-IR (r = 0.36, p = 0.011). Multivariate analyses showed that total testosterone level (β = −0.44, p = 0.001) and TyG index (β = 0.29, p = 0.045) were independent determinants of plasma ADMA levels.Conclusion:Patients with KS had higher TyG indices than healthy subjects. Moreover, TyG index was independently associated with endothelial dysfunction in patients. TyG index may be a practical and useful measure to show the increased endothelial dysfunction in patients with KS.