Thyrotropin and Alzheimer's Disease Risk in the Elderly: a Systematic Review and Meta-Analysis.

Although several epidemiological studies assessed the relationship between thyrotropin and risk of Alzheimer's disease in the elderly, the results were inconsistent. A systematic review and meta-analysis of cohort studies was conducted to assess the impact of serum thyrotropin levels on Alzheimer's disease risk. PubMed, Embase, and Web of Science were searched through September 20, 2014 to identify cohort studies on the relationship between serum thyrotropin levels and risk of Alzheimer's disease in the elderly. Pooled relative risks (RR) and 95% confidence intervals (95% CI) were calculated to assess the risk of Alzheimer's disease according to serum thyrotropin levels. Eight prospective cohort studies were included, with a total of 9456 participants and 640 cases of Alzheimer's disease. Low thyrotropin level was significantly associated with an increased risk of Alzheimer's disease (fixed RR = 1.69, 95% CI 1.31-2.19, P < 0.001; I(2) = 38.0%). High thyrotropin level was also significantly associated with an increased risk of Alzheimer's disease (fixed RR = 1.70, 95% CI 1.18-2.45, P = 0.005; I(2) = 42.2%) when compared with normal thyrotropin level. When using random effect model, low thyrotropin level was still significantly associated with risk of Alzheimer's disease (random RR = 1.65, 95% CI 1.14-2.37, P = 0.007), but high thyrotropin level was not (random RR = 1.54, 95% CI 0.88-2.68, P = 0.129). When investigating thyrotropin levels continuously, an inverse but not significant association between serum thyrotropin levels and Alzheimer's disease risk was observed (per standard deviation increment of thyrotropin: RR = 0.89, 95% CI 0.78-1.01, P = 0.06; I(2) = 31.3%). This meta-analysis supports that low thyrotropin level is significantly associated with an increased risk of Alzheimer's disease in the elderly.