In this study involving patients with Hashimoto thyroiditis (HT), researchers examined the association of vitamin D levels with cognitive impairment. Study participants included 194 patients with HT and 200 healthy volunteers. Using a competitive protein-binding assay, levels of serum 25-hydroxyvitamin D (25(OH)D) were measured. Patients with mild cognitive impairment (MCI) had significantly lower levels of 25(OH)D vs those without MCI. There were significant differences in 25(OH)D quartiles in patients with HT between those with MCI and those without MCI. Overall, findings revealed an important association between serum vitamin D levels and cognitive impairment in patients with HT.