Abnormal Thyroid Levels Can Increase Risk For Alzheimer’s Disease in Women

Thyroid disease, resulting from either low or high thyrotropin levels, has been found to be associated with an increased risk of Alzheimer's disease in women only. Thyrotropin is a hormone that affects thyroid gland function and thyroid hormone levels. When a patient has low levels of the hormone, the condition is known as hypothyroidism, while a patient having high levels is referred to as having hyperthyroidism.

Both conditions are recognized causes of reversible dementia based on previous studies. In fact, routine screening of serum thyrotropin levels is included in evaluation of patients with suspected dementia. However, the effects of a normally functioning thyroid gland on the brain's cognitive abilities have not been made clear. It is not known whether thyroid function affects Alzheimer's or if it is Alzheimer's that affects thyroid function.

Zaldy S. Tan, M.D., M.P.H., of Harvard and Beth Israel Deaconess Medical Center, and colleagues performed research on blood samples taken from what is known as the Framingham Health Study, in which the health of members of a Massachusetts town were analyzed from March 1977-November 1979. The participants were 1,864 healthy men and women, average age 71, with no known cognitive problems, with 59 percent of the participants being women. The researchers analyzed measured levels of thyrotropin in the drawn blood. After almost 13 years of follow-up, 209 of the volunteers had developed Alzheimer's disease. It was determined that women with the lowest and highest levels of thyrotropin had more than double the risk of developing Alzheimer's disease.

According to Tan's team, changes in the brain caused by Alzheimer's disease may reduce the amount of thyrotropin released, which may result in low or high thyrotropin levels that could damage brain cells or blood vessels. The researchers said that sex differences related to thyroid hormone are well known for bone density as well as higher incidences of thyroid disease in women compared with men, which suggests that the effects are changed according to sex. The group was careful to note that the study may have been limited by the single thyrotropin measure from the available blood samples and the lack of data on thyroxine levels, depression status, other illnesses that could affect thyroid levels, or the use of antithyroid medications.

Due to the almost exclusively Caucasian population in the study, the findings need validation in larger and more diverse populations. In addition, when analysis was limited to individuals with thyrotropin levels of 0.5 to 5.0 mIU/L, the relationship between thyrotropin levels and Alzheimer's disease decreased. This suggests individuals with more extreme thyrotropin values may have accounted for the relationship.