July 29 (HealthDay News) -- High or low levels of the hormone thyrotropin may be associated with an increased risk of Alzheimer's disease in women. Thyrotropin affects thyroid gland function and thyroid hormone levels.

Between 1977 and 1979, researchers at Beth Israel Deaconess Medical Center and Harvard Medical School measured thyrotropin levels in 1,864 people, average age 71, without cognitive problems. The participants were then assessed for dementia every two years.

After an average of 12.7 years of follow-up, 209 participants developed Alzheimer's disease. After they adjusted for the other factors, the researchers found that women with the lowest (less than one milli-international unit per liter) and highest (more than 2.1 milli-international units per liter) levels of thyrotropin had more than a twofold increased risk of Alzheimer's.

No association between thyrotropin levels and Alzheimer's risk was noted in men.

"Whether altered thyrotropin levels occur before or after onset of Alzheimer's disease, the neuropathologic mechanism is unclear," the study authors wrote.

Brain changes caused by Alzheimer's disease may cause a decrease in the amount of thyrotropin released or changes in the body's responsiveness to the hormone, the researchers said. Or, it may be that high or low thyrotropin levels damage neurons or blood vessels, resulting in cognitive problems.

"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," the researchers wrote.

The study was published in the July 28 issue of the journal Archives of Internal Medicine.

SOURCE: JAMA/Archives journals, news release, July 28, 2008