Women's Thyroid Levels Linked to Risk of Alzheimer's Disease

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If you are a woman with low or high levels of the hormone thyrotropin (also known as thyroid stimulating hormone, or TSH) you have an increased risk of Alzheimer's disease. The findings were reported in the medical journal, Archives of Internal Medicine.

The Research

In a large study, conducted as part of the community-based Framingham Study, almost 2,000 patients were periodically evaluated for dementia over a period lasting more than ten years. The group also underwent follow-up of almost 13 years. During that follow-up period, 209 of the patients studied developed Alzheimer's disease. In women, the TSH levels were significantly linked to Alzheimer's disease. In fact, for women with a TSH level below 1.0 or above 2.1, there was a greater than two-fold increased risk of Alzheimer's disease. Interestingly, no similar relationship was seen in men.

The researchers do not know whether the changes to thyroid function and TSH are occurring before or after the onset and diagnosis of Alzheimer's disease. They also do not know the scientific or biological mechanism behind the relationship. They do, however, recommend that further research be conducted to determine if there are any implications for treatment or prevention.

Perhaps the most important finding, however, was that the results were the same, whether patients were diagnosed with a thyroid condition and taking thyroid hormone replacement medication or not. The operative factor appeared to be the TSH level.

While measurable hyperthyroidism or hypothyroidism can cause cognitive problems, and difficulties with memory, thinking, and learning, these symptoms are considered to be reversible with proper thyroid treatment. This study, however, challenges the traditional dogma that thyroid problems are a reversible cause of cognitive impairment. Instead, the research findings suggest that an imbalance in thyroid function may even be a contributory factor to the risk of developing Alzheimer's disease.

One of the study's authors, Dr. Zaldy Tan, told Medscape:

"It was interesting to find that both low and high levels were associated with Alzheimer's disease.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."

It's not clear whether the TSH levels are causing Alzheimer's, protecting against Alzheimer's, or whether the Alzheimer's disease is ultimately affecting TSH. The study was observational and did not evaluate causality. The experts theorize that pituitary responsiveness may be damaged by Alzheimer's disease, or that thyroid hormone irregularities may be a contributing factor in developing Alzheimer's disease.

One interesting hypothesis by the study authors looks at the role that thyroid hormone plays in regulating the expression of a gene called the amyloid precursor protein (APP), which has a role in Alzheimer’s. It's possible that imbalances of thyroid hormone may lead to issues with the regulation of APP, thereby increasing the risk of Alzheimer's.

Further study into the relationship between the thyroid, TSH levels, and Alzheimer's disease is needed. Such research could, however, end up providing sound reasons for the medical community to eventually adopt a narrower TSH reference range.

This issue has been controversial. In 2002, the American Association of Clinical Endocrinologists (AACE) recommended narrowing the TSH reference range from the generally used 0.5 to 5.0 mIU/L, to a narrower range of 0.3 to 3.0, with the National Association of Clinical Biochemistry recommended an even lower top limit of 2.5. While some doctors and endocrinologists advocated for the recommendations, the AACE abandoned this recommendation, and laboratories and physicians still evaluate thyroid blood tests according to the old 0.5 to 5.0 range.

A Word from Verywell

In the meantime, however, this study provides further evidence to argue that, for patients receiving thyroid treatment, the target range for the optimal TSH level while on medication should be between 1.0 to 2.0, unless other more immediate factors are involved. (For example, some thyroid cancer patients are maintained on suppressive doses of thyroid medication, keeping the TSH level very low, or sometimes suppressed to levels near 0, as a way to prevent a recurrence of thyroid cancer.

View Article Sources
  • Tan, Zaldy et. al. "Thyroid Function and the Risk of Alzheimer Disease: The Framingham Study." Archives of Internal Medicine, 2008;168(14):1514-1520.