ABSTRACT Background Klinefelter syndrome is associated with a markedly increased risk of venous thrombosis. While hypogonadism and metabolic dysfunction are known contributors, the role of the contact activation system in Klinefelter syndrome remains unexplored. contact activation system mediates crosstalk between coagulation, fibrinolysis, and inflammation and may contribute to thromboinflammatory risk. Objective To investigate whether contact activation system is dysregulated in Klinefelter syndrome and assess the influence of hypogonadism, body composition, and testosterone replacement therapy on contact activation system activity. Methods We conducted a longitudinal study including 22 testosterone‐treated Klinefelter syndrome, 15 untreated Klinefelter syndrome, and 14 age‐matched controls. Untreated Klinefelter syndrome initiated testosterone replacement therapy at baseline. Plasma levels of contact activation system proteins (coagulation factor XII, prekallikrein, high‐molecular‐weight kininogen, cleaved high‐molecular‐weight kininogen, and C1 esterase inhibitor were measured by enzyme‐linked immunosorbent assays. Contact activation system activity was assessed functional assays describing the endogenous kallikrein potential and contact‐induced thrombin generation. Associations with sex hormones and body composition were analyzed using mixed‐effects models. Results At baseline, untreated Klinefelter syndrome exhibited coagulation factor XII, prekallikrein, and C1 esterase inhibitor levels compared with controls. Following testosterone replacement therapy, untreated‐Klinefelter syndrome showed significant reductions in coagulation factor XII and C1 esterase inhibitor. Testosterone was inversely associated with coagulation factor XII, while body fat correlated positively with prekallikrein, endogenous kallikrein potential, and contact‐induced thrombin generation. Endogenous kallikrein potential and contact‐induced thrombin generation were not correlated, suggesting a bias toward inflammatory rather than procoagulant contact activation system activation. Discussion Contact activation system is dysregulated in hypogonadal Klinefelter syndrome and partially normalized by testosterone replacement therapy. contact activation system dysregulation in Klinefelter syndrome appears multifactorial, involving both hormonal and metabolic components. The strongest associations were with body fat, indicating that metabolic dysfunction may be a key regulator of contact activation system activity. Conclusion Inflammatory skew in contact activation system activation may contribute to thromboinflammatory risk in Klinefelter syndrome. Trial Registration Clinicaltrials.gov identifier: NCT02526628
Chang et al. (Fri,) studied this question.