Venous Thrombosis and Conjugated Equine Estrogen in Women without a Uterus

April 2006

Findings Summary

Postmenopausal hormone therapy has been associated with an increased risk of venous thromboembolism (VT; blood clots), including deep vein thrombosis (blood clots in the legs) and pulmonary embolism (blood clots in the lungs). While these findings have been reported in observational studies, clinical trial data are very limited. Recently, the Women’s Health Initiative reported results from the clinical trial of estrogen plus progestin in women with a uterus showing increased risk of VT among women taking estrogen plus progestin compared to placebo. In another clinical trial, postmenopausal women with heart disease who were taking estrogen-alone had an increased risk of VTE.
The current report of Dr. Curb and colleagues from the Women’s Health Initiative describes the risk of VT among generally healthy postmenopausal women (without a uterus) enrolled in a clinical trial of conjugated equine estrogen (estrogen-alone) compared to placebo. The Women’s Health Initiative (WHI) estrogen-alone trial enrolled 10,739 women aged 50-79 without a uterus. Participants were randomly assigned to take estrogen-alone as conjugated equine estrogen (0.625 mg per day) or placebo.
Over an average 7.1 years, there was an increased risk of VT in women taking estrogen-alone: 111 women taking estrogen-alone (0.30% women per year) and in 86 women taking placebo (0.22% women per year) developed VTE. The increased risk was highest during the first two years. However, the risk was not as high as for women (with a uterus) in the WHI who were taking estrogen plus progestin compared to placebo. Deep venous thrombosis among women in the WHI estrogen-alone trial was reported in 85 women taking estrogen-alone and in 59 women taking placebo. The risk of deep venous thrombosis was similar to that for pulmonary embolism with 52 women taking estrogen-alone and 39 taking placebo developing pulmonary embolism. There were no significant correlations between estrogen-alone use and age, body mass index or most other VT risk factors (such as prior VT) for VT, deep vein thrombosis or pulmonary embolism.
Overall, the WHI estrogen-alone trial found an increased risk of venous thromboembolism with use of estrogen-alone, especially within the first two years, but this risk elevation was less than that for women in the WHI taking estrogen plus progestin. WHI results indicate that women should use estrogen-alone only after careful consideration of risks and benefits, especially if they have other risk factors for VT such as previous VT, being older, or being heavier.