Oestrogen contained in HRT is likely to be the key hormone
Some forms of hormone replacement therapy (HRT) can slightly shrink the brains of post-menopausal women, a US study has suggested.
The findings may help explain previous work linking HRT to an increased risk of memory loss and dementia.
A team led by researchers at Wake Forest University carried out brain scans on 1,400 women aged 71 to 89 who took part in an earlier HRT trial.
But UK experts said the study, published in Neurology, had flaws.
Significant numbers of women take hormones to reduce the unpleasant symptoms of the menopause, such as hot flushes, mood changes, and thinning of the bones.
However, research has linked HRT to a raised risk of some forms of cancer.
The latest study found two key areas of the brain involved in thinking and memory were smaller in women who had taken HRT than in those who had been given a "dummy" placebo pill.
Brain volume was 2.37 cubic centimetres lower in the frontal lobe and 0.10 cubic centimetres lower in the hippocampus.
However, the researchers admit they were unable to carry out brain scans before the women began taking HRT.
And the results suggest shrinkage was most pronounced in women who may already have started to develop memory problems before they started taking hormones.
Lead researcher Dr Susan Resnick, from the US National Institute on Ageing, said: "Our findings suggest that hormone therapy in older post-menopausal women has a negative effect on brain structures important in maintaining normal memory functioning.
"However, this negative effect was most pronounced in women who already may have had some memory problems before using hormone therapy, suggesting that the therapy may have accelerated a neurodegenerative disease process that had already begun."
Dr David Sturdee, president of the International Menopause Society, said 49% of the women in the study were over 70.
In contrast, women in the UK are typically offered HRT between the ages of 45 and 60.
He said brain shrinkage among women of a relatively advanced age was not a surprising finding.
"The benefits (of HRT) are still way in excess of the risks."
HRT is known to increase the risk of a stroke, and it had been assumed that women's memory might be affected by the build up of damage caused by "silent" strokes.
However, researchers found no evidence of an increase in the volume of such damage among women taking HRT.
The forms of HRT examined in the latest study contain a variety of oestrogens referred to as conjugated equine oestrogens.