Estrogen type may be associated with risk of venous thrombosis.(News)

Women who take conjugated equine estrogens have an increased risk of venous thrombosis, compared with those who take esterified estrogens or no hormones, according to a new study.

And the addition of progestin to conjugated equine estrogen (CEE) further increases the risk of venous thrombosis (VT), reported Nicholas L. Smith, Ph.D., of the University of Washington, Seattle, and his colleagues.

"If replicated, these findings for VT may have implications for the choice of hormones in treating menopause-related vasomotor symptoms in perimenopausal and postmenopausal women," they said (JAMA 2004;292:1581-7).

The population-based, case-control study was part of a larger study of VT, myocardial infarction, and stroke. It included 586 peri- and postmenopausal women and 2,286 matched controls, 30-89 years old, from Group Health Cooperative, which is a large HMO in western Washington state. The risk of first VT was assessed in relation to hormone use, type of estrogen used, and whether concomitant progestin was used.

The study found that, compared with women not using hormones, those using esterified estrogen (EE) had no increased risk of VT (odds ratio 0.92), while women taking CEE had an elevated risk (OR 1.65).

Even when the analysis excluded women who did not use hormones, thus minimizing a potential indication bias, CEE users were still at increased risk (odds ratio 1.78).

With the addition of progestin, which was almost exclusively medroxy-progesterone acetate, the risk of VT for CEE users was doubled (odds ratio 2.02), compared with the risk in EE plus progestin users.