More Findings on Thyrotropin and Alzheimer's Disease

Several epidemiological studies have linked abnormal thyrotropin levels with an increased risk of Alzheimer's disease (AD) in women, researchers found. Some results have indicated the risk applied only to women, and other studies had reported that there in fact was no link. Investigators had completed a systematic review and meta-analysis of cohort studies, published in the journal Molecular Neurobiology, assessing serum thyrotropin levels’ impact of on Alzheimer’s disease risk.

The investigators included cohort studies examining the relationship between serum thyrotropin levels and AD risk in the elderly. They identified 8 prospective cohort studies that had enrolled 9456 participants. Among participants, 640 developed AD.

Patients with low thyrotropin level were significantly more likely to develop AD, with a relative risk (RR) of 1.69 compared to participants with normal thyrotropin levels.

Participants with high thyrotropin levels were similarly at increased risk, with a RR of 1.7.

The researchers also applied a random effect model, a process that removed omitted variable bias by measuring change within a group. Low thyrotropin level was still significantly associated with risk of AD (random RR=1.65). High thyrotropin level, however, was not.

Several possible theories explain this relationship:

· AD-associated neurodegeneration in the brain may also reduce thyrotropin-releasing hormone secretion in the brain.

· Thyrotropin-releasing hormone, a general central nervous system neurotransmitter, could disturb thyrotropin-releasing hormone in the entire CNS, reducing serum thyrotropin, which may cause AD.

· Low thyrotropin levels may reflects true thyroid overactivity and thyroid hormone excess, which may be toxic to the brain.

The researchers indicated that this meta-analysis provides strong evidence for the relationship between low serum thyrotropin levels and increased AD risk. Clinically, they suggested that patients with low thyrotropin levels may need intense and early interventions to reduce AD risk.