APOE and Schnyder corneal dystrophy: Specifically, considering that the apolipoprotein E (APOE) ε4 allele is not only a genetic risk factor for sporadic AD but also for earlier stages such as MCI or even SCD (14, 15), we hypothesized that thyroid hormones as well as TSH may have distinct effects on cognitive performance in participants depending on their APOE ε4 status.