Clinical studies show a strong connection between vitamin D levels in serum with cognitive impairment in patients with Hashimoto’s thyroiditis. It is important to study the effect of cholecalciferol on the cognitive function in patients with autoimmune thyroiditis and hypothyroidism in the Western Ukrainian population. The study included 56 patients with hypothyroidism (H) caused by autoimmune thyroiditis (AIT). These patients were distributed into two groups. Patients in the Group 1 (n = 28) received cholecalciferol at a dose of 4000 IU/day (28,000 IU/week) and L-thyroxine (88.39 ± 12.70 μg / day). Patients in the Group 2 (n = 28) were prescribed only L-thyroxine (87.50 ± 12.73 μg / day). Examinations were performed at the beginning and end of the 12-week treatment. We detect a decline in cognitive function using the Mini-Mental State Examination. After the course of treatment, patients showed improvement in cognitive function. In Group 1 patients who received cholecalciferol and L-thyroxine after treatment, the percentage of patients with moderate (pre-dementia) cognitive impairment decreased from 50% to 21.4%. At the same time, no statistically significant changes were found in the group of patients taking L-thyroxine alone. Normalization of cognitive functions according to the MMCE test increased from 28.6% to 78.6% in the first group, and from 21.4% to 50% in patients of Group 2. Based on our results, patients with autoimmune thyroiditis and hypothyroidism should be given vitamin D supplements to improve cognitive function.