In the study paper, researchers have said that a clinically detectable over- or under-active thyroid has long been recognized as a potentially reversible cause of cognitive (thinking, learning and memory) impairment.
In earlier studies, they examined whether levels of thyrotropin, a hormone that is secreted by the pituitary gland and helps regulate thyroid gland function, is associated with cognitive performance in individuals with normal thyroid function. But they always came out with inconsistent results.
For their research, the scientists measured thyrotropin levels in 1,864 individuals (average age 71) without cognitive problems between 1977 and 1979. All the participants, who were part of the community-based Framingham Study, were assessed for dementia at that time and again every two years.
After a follow-up of over an average of 12.7 years, it was found that 209 participants developed Alzheimer's disease. After adjusting for other related factors, women with the lowest (less than 1 milli-international unit per liter) and highest (more than 2.1 milli-international units per liter) levels of thyrotropin were found to have more than double the risk of developing Alzheimer's disease. But, the researchers couldn't find any relationship between thyrotropin levels and Alzheimer's disease risk in men.
"Whether altered thyrotropin levels occur before or after the onset of Alzheimer's disease, the neuropathologic mechanism is unclear," wrote the authors.
The changes in the brain caused by Alzheimer's disease may reduce the amount of thyrotropin released or changes in the body's responsiveness to the hormone. On the other hand, low or high thyrotropin levels could damage neurons or blood vessels, leading to cognitive difficulties.
"In conclusion, low and high thyrotropin levels were associated with an increased risk of incident Alzheimer's disease in women but not in men. These findings should be considered hypothesis-generating and should be validated in other populations before clinical conclusions are drawn," concluded the authors.
The study was published in the latest issue of Archives of Internal Medicine, one of the JAMA/Archives journals.