SUMMARY
To verify whether there is a difference between serum vitamin D averages in patients with Graves' disease and Hashimoto's thyroiditis, when compared with a group of healthy individuals. This is a systematic review and meta-analysis according to the PRISMA protocol. The test of difference of means with random effect and chi-square of heterogeneity was performed, co nsidering an alpha <0.05 for acceptance of the hypotheses. The tests were performed using the STATA® 16.0 software. There were differences between serum vitamin D averages: group with Graves' disease compared to healthy individuals (Mean = 10.06; 95% CI =0.99-19.14); Group with Hashimoto's disease compared to healthy individuals (Mean = 12.49; 95% CI = 5,9 - 19,79); group of children with Hashimoto compared to healthy children, (Mean = 19.46; 95% CI = 10.25-28.68). The mean serum vitamin D levels were significantly higher in healthy individuals, demonstrating that the depletion of this nutrient may be related to the pathophysiological process of these diseases, taking into account the immunomodulatory function that this vitamin has.