Observational studies have shown apparently lower ischemic coronary disease risk in postmenopausal women receiving hormone replacement therapy (HRT). However, several recent studies have shown an increase in ischemic cardiac events when HRT is initiated in postmenopausal women with known coronary artery disease. It is postulated that estrogen may result in increased platelet aggregation.

We evaluated platelet activation, as measured by flow cytometric analysis using P selectin and PAC1 as activation markers, and aggregation, as measured by standard platelet aggregation using platelet-rich plasma, in 27 postmenopausal women (17 HRT, 10 placebo) who were participants in 2 placebo-controlled randomized angiographic trials evaluating the effect of HRT on coronary atherosclerosis or saphenous vein graft disease. All women had received HRT or placebo for >2 years and were on aspirin therapy. The estrogen component was either conjugated equine estrogen or 17β-estradiol.

Patients on HRT and those on placebo had comparable degrees of platelet aggregation when measured using various doses of agonists (adenosine diphosphate and epinephrine). There were no significant differences in levels of platelet activation measured by flow cytometry.

We conclude that long-term HRT does not appear to cause increased platelet activation and aggregation in women with coronary artery disease. There may be increased platelet activation in the early period after HRT initiation; however, this was not assessed in this study.