Women who take the hormone therapy estrogen plus progestin have double the risk for venous thrombosis

Mary Cushman, M.D., M.Sc., of the University of Vermont, Colchester, Vt., and colleagues examined the effects of postmenopausal hormone therapy on VT in the presence of other thrombosis risk factors, such as age and obesity.

The researchers analyzed final data from the Women's Health Initiative Estrogen Plus Progestin clinical trial, a double-blind randomized controlled trial of 16,608 postmenopausal women between the ages of 50 and 79 years, who were enrolled in 1993 through 1998 at 40 U.S. clinical centers, with 5.6 years of follow up. Baseline gene variants related to thrombosis risk were measured in the first 147 women who developed thrombosis and in 513 controls. Participants were randomly assigned to 0.625 mg/d of conjugated equine estrogen plus 2.5 mg/d of medroxyprogesterone acetate, or placebo.

Venous thrombosis occurred in 167 women taking estrogen plus progestin and in 76 taking placebo (twice the risk for venous thrombosis for women taking hormone therapy). Compared with women between the ages of 50 and 59 years who were taking placebo, the risk associated with hormone therapy was higher with age: 4.3 times the risk for women aged 60 to 69 years and 7.5 times the risk for women aged 70 to 79 years. Compared with women who were of normal weight and taking placebo, the risk associated with taking estrogen plus progestin was increased among overweight (3.8 times the risk) and obese women (5.6 times the risk). Participants with the hereditary blood coagulation disorder Factor V Leiden had a 6.7 times increased risk of thrombosis compared with women in the placebo group without the genetic mutation.

"Based on projections for 10 years for 1,000 women taking estrogen plus progestin, the estimated excess number of events is 18 for VT, 6 for coronary heart disease, 8 for invasive breast cancer, and 8 for stroke," the authors write.

"The implications of these findings may be important for the use of postmenopausal hormone therapy in the treatment of menopausal symptoms among younger postmenopausal women," the researchers conclude.