In the vascular system, estrogen and serotonin have been shown to individually alter clotting, cholesterol, vasocon- striction, and heart attacks. Both high and low levels of E2 have been associated with increased risk of thromboem- bolism; high levels result in increased clot formation, while low levels result in slower clot breakdown. Unusu- ally high concentrations of estrogen (beyond normal physiological levels) directly increase the likelihood of clotting by increasing production of clotting factors VII through X in the liver [41]. In addition, these levels of E2 might increase clotting by increasing serotonin, which is constitutively present in human plasma and platelets and works to promote clotting[6,79] and increase density of platelets [58]. Increased.
