"Older women who took these [hormones] ... they had greater brain atrophy in these brain regions that are critical for the maintenance of memories," said Susan Resnick, a senior investigator at the National Institute on Aging, Laboratory of Personality and Cognition, and the study's primary author.
Hormone replacement therapy, a regimen of progestin and estrogen given to relieve symptoms of menopause, initially fell from favor in 2002 when the Women's Health Initiative Memory Study was stopped because of concerns that the treatment was putting the women involved at greater risk for stroke and heart problems.
The International Menopause Society in May released new guidelines saying the original guidelines were flawed and advised that the therapy may be safe for women younger than 65.
In this new study, women older than 65 were shown to have an decrease in brain volume if they took the hormone therapy. In the past, such reduction in brain size had been associated with a decline in memory, and Resnick expects those results to replicate when they analyze cognition test scores.
"We actually found negative effects, both on memory and the risk for dementia," she said of studies done in the past in this area.
She noted, however, that the hormone regimen seemed only to accelerate a mental decline in women who had shown signs of decline before starting treatment.
"If we take women with healthy looking brains, we didn't see a negative effect," Resnick said.
She noted that when women first began hormone treatments, it was expected to boost rather than impair brain power. But studies have since shown otherwise.
Women now on hormone therapy probably have little to fear at this time from the study, as recommendations would exclude any women of the ages looked at in the study from taking hormone therapy.
Could HRT Affect Brains of Younger Women Too?
Still, Resnick said that her study might ultimately affect younger women following the therapy.
"This suggests that a lot more research is needed in that younger age range," she said.
While praising the study as a whole, others were a bit more skeptical about the affects on women following the treatment.
"It's hard to extend these to the younger subjects, because they might have a whole different subset of risk factors for stroke," said Dr. James Brewer, a neurologist at the University of California at San Diego.
He said that the study does reinforce suspicions that hormone therapy could lead to mental decline.
"I think that this gives biological support to the findings about the effects of hormone therapy on cognition," Brewer said.
He noted that most women probably don't need to be concerned, even if the findings hold up, because the negative effects were limited and affected a smaller subset of women. But he said the number of people involved in the study leads him to believe those effects are very real.
"They're small effects, but I think the repeatability across studies was convincing to me in such a large group of subjects," he said.
While the research may open new avenues of study for the safety of hormone replacement therapy, Resnick is not prepared to make any new recommendations now based on her findings.
"It does give people a broader perspective in terms of trying to make decisions about hormone therapy," she said, adding, "This suggests that a lot more research is needed in that younger age range."