Estrogen therapy increases the risk of blood clots in some women

According to new research postmenopausal women on estrogen therapy who have undergone a hysterectomy have an elevated risk of developing a blood clots in veins.

The researchers say such women should only use estrogen therapy for the short-term treatment of severe menopausal symptoms, and only after careful consideration of the relative risks and benefits.

This applies especially to women who have other risk factors including a history of thrombosis, older age, obesity, and factor V Leiden, the most common hereditary blood coagulation disorder in the United States.

These latest findings, which are drawn from the Women's Health Initiative (WHI) trial, are not the first to link hormone therapy with venous thrombosis, but data from other studies, as a rule, only looked at the effect of combined estrogen/progestin therapy, not estrogen alone.

For the study Dr. J. David Curb, from the University of Hawaii in Honolulu, and colleagues analyzed data from nearly 10,739 women who participated in the WHI estrogen trial. The women were between 50 and 79 years of age, and had had a hysterectomy.

The women were randomized to receive estrogen or a placebo and according to the researchers, during an average follow up of 7 years, more women in the estrogen group than in the placebo group developed a blood clot in a vein, 111 women compared to 86.

The same was true for deep vein thrombosis which occur most often in large veins in the legs.

Eighty-five estrogen-treated women developed this type of blood clot compared with only 59 placebo-treated women.

The investigators say the risk of developing venous thrombosis appeared highest in the first two years of estrogen therapy.

However, the clotting risk with estrogen-only therapy did not appear to be as high as with estrogen/progestin combination therapy, say the team.

The report is published in the Archives of Internal Medicine, April 10, 2006.