Thyroid Hormone Levels Linked to Alzheimer’s disease in Older Women

According to new research, older women who have “moderate” thyroid hormone levels have less risk of developing Alzheimer’s disease than those with lower levels of the thyroid hormone thyrotropin. The study, , published in the July 28 issue of the Archives of Internal Medicine, found that low thyrotropin levels may increase that risk two-fold.

One of the study authors, Zaldy S. Tan, MD, MPH, of Hebrew SeniorLife, Beth Israel Deaconess Medical Center and Harvard Medical School in Boston, Massachusetts says, "The most important thing to take away from this study is the question of whether our currently accepted standard of what normal thyroid levels are is too broad. ” However, the findings are not meant to dictate new standards of care, until the results are further validated.

Dr. Tan and his colleagues measured thyrotropin levels of 1864 participants in the Framingham longitudinal, community-based, observational study, between March 1977 and November 1979. The participants had been free of signs of dementia for three years before entered into the study, to minimize the risk of including those with early Alzheimer’s disease. The mean age initially was 71 years of age. The participants were studied at baseline and then twice yearly for signs of dementia. Alzheimer’s disease developed in 209 women, and the researchers observed that women with the lowest serum thyrotropin concentrations (< 1.0 mIU/L) and those with the highest (> 2.1 mIU/L) were more than twice as likely to have Alzheimer's disease when compared to women whose thyroid hormone was in mid-range. Currently accepted standards for thyroid measurement are 0.5 and 5.0 mIU/L.

Dr. Tan explains, "The fact that the brain tries to maintain thyroid levels at a relatively narrow range may suggest that for it to function optimally, it has to be maintained within this range and going below or above that is not a good thing."

The researchers aren’t clear how thyroid levels outside of ranges that are optimal contribute to Alzheimer’s disease. Supposition exists that high levels might increase oxidative stress, while lower levels may affect circulating levels of beta amyloid peptide and brain tissue. Further questions include why women seem to be most affected. According to Dr. Tan, "There's something about either the brain of women or the hormonal milieu of women that makes them more prone to this effect." Questions also remain about the association between thyroid problems and dementia that may occur merely as the result of aging. "One has to question whether these things are there just because one is getting old, or if there is some connection between the two conditions," said Dr. Tan.

Though more studies are needed, we do know is that thyroid function is closely linked to the central nervous system. Low levels of thyroid hormone, or hypothyroidism, causes depression, while high levels, or hyperthyroidism causes confusion. For now, we have some light on the possibility that honing in on thyroid levels may reduce a woman’s risk of developing Alzheimer’s disease, though further studies will take some time to complete.

Source: Arch Intern Med. 2008; 168:1514-1520.

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