Thyrotropin Levels in Women Increase Alzheimer's Risk
Women appear to have a higher risk of developing Alzheimer's disease as a result of high or low levels of thyrotropin. Thyrotropin is a hormone that affects both thyroid gland function and thyroid hormone levels. People with a clinically under active or overactive thyroid may mimic the symptoms of dementia. When treated these symptoms reverse. Because of this, screening for serum thyrotropin levels, has become a standard procedure when dementia is suspected.
Previous attempts at relating thyrotropin levels to cognitive performance have been inconsistent. However, recent studies are pointing to thyroid dysfunction as a possible risk factor for the onset of irreversible dementia.
Since 1977, as part of an extensive community-based research program known as the Framingham Study, volunteers have been monitored for the development of neurological disorders, including stroke and Alzheimer's disease. Zaldy S. Tan, M.D., of Hebrew SeniorLife, Beth Israel Deaconess Medical Center and Harvard Medical School, Boston, measured thyrotropin levels in volunteers with an average age of 71 years, who experienced no cognitive problems. Over an average of 12.7 years of follow-up, 209 of the volunteers developed Alzheimer's disease. Tan and his research team, discovered that women with the highest and lowest levels of thyrotropin had more than double the risk of developing Alzheimer's. Results of the study are published in the July 28 issue of Archives of Internal Medicine.
According to the report it remains unclear whether variation in thyrotropin levels occurs before or after the onset of Alzheimer's disease. The process of brain neurodegeneration could lead to a reduction in the secretion of thyrotropin-releasing hormone by the hypothalamus. Alternatively, depletion of thyrotropin could itself enhance changes in proteins associated with the onset of dementia. "Low thyrotropin levels may be the consequence, rather than the cause of Alzheimer's disease," (p.1518).
The American Association of Clinical Endocrinologists have proposed narrowing the target thyroid levels from 0.5 to 5.0 mIU/L (milli-international unit per liter) to 0.3 to 3.04 mIU/L. This, they believe, will help to identify mild thyroid disease more easily. However, the National Association of Clinical Biochemistry are arguing that the upper limit should be reduced even further to 2.5 mIU/L.
It is unclear why the association between thyrotropin levels affects women rather than men. The authors do point out that the sample size and the reduced number of men who developed Alzheimer's disease may account for the difference. The authors prefer their findings to be viewed with caution and to be considered "hypothesis generating" until more research is able to substantiate their assumptions.