Background: Previous studies have found no association between serum concentrations of total oestradiol and cognitive function, but these measurements may not reflect concentrations of hormone available to the brain. We tested the hypothesis that concentrations of non-protein-bound (free) and loosely bound (bioavailable) sex hormones are associated with cognitive function in older women.
Methods: We measured cognitive performance with a modified mini mental status examination (mMMSE) at baseline (1986-88) and 6 years later in 425 women (65 years or older). Concentrations of non-protein-bound and bioavailable oestradiol and total and non-protein-bound testosterone were measured by RIA in serum samples taken at baseline.
Findings: Initial cognitive scores did not differ by tertile of non-protein-bound oestradiol, bioavailable oestradiol, or testosterone. Cognitive impairment (a decrease of 3 points or more in mMMSE score) occurred in five (5%) of 94 women in the high tertile for non-protein-bound oestradiol and in 17 (16%) of 106 in the low tertile (odds ratio 0.3 [95% CI 0.1-0.8]). After adjustment for age, years of education, body-mass index, current oestrogen use, history of surgical menopause, and baseline mMMSE score, the odds ratio was 0.3 (0.1-0.9). The results were similar for bioavailable oestradiol (five [5%] vs 15 [15%]; adjusted odds ratio 0.3 [0.1-1.0]). There was no association between risk of cognitive impairment and serum testosterone.
Interpretation: Women with high serum concentrations of non-protein-bound and bioavailable oestradiol, but not testosterone, were less likely to develop cognitive impairment than women with low concentrations. This finding supports the hypothesis that higher concentrations of endogenous oestrogens prevent cognitive decline.