The study researchers believe that treatments affecting estrogen may pose different risks depending on women's normal circulating estrogen levels.
Jennifer Lee and colleagues from the University of California Davis examined estrogen levels in women aged over 65 years.
The researchers calculated the free estradiol index (FEI) by comparing estradiol levels with sex hormone-binding globulin concentrations. They then compared FEI scores in 196 women who had suffered a stroke with 219 controls.
When adjusted for age, the odds of women experiencing a stroke increased at higher FEI scores, the researchers found.
Women in the highest quartile of FEI scores had a 2.3-fold higher risk of stroke than women in the lowest quartile. Dr Lee said the association might be the result of lipid metabolism dysfunction, insulin resistance or inflammation in these women.
The link between estrogen levels and stroke risk was stronger in women who had greater central adiposity, and this aspect needs to be investigated further, the researchers believe.
'Estradiol is known to affect inflammation, lipid metabolism, insulin resistance and other processes in atherothrombosis,' they said. The researchers also point to studies suggesting that treatment with estrogen may increase stroke risk. But they believe this is the first study to examine stroke risk associated with high endogenous estrogen.
'The study implies that estrogen-altering agents might be harmful or beneficial depending on endogenous circulating free estradiol levels in older postmenopausal women, especially those with greater central adiposity,' they said.
Research will need to examine whether raising levels, or enhancing the effects, of estradiol has adverse outcomes in women with low endogenous levels. Studies should also look at whether decreasing estradiol levels benefits women with high endogenous levels, the researchers said.