Alteration of the Intestinal Microbial Flora and the Serum IL-17 Level in Patients with Graves’ Disease Complicated with Vitamin D Deficiency

Intestinal flora is associated with Graves' disease (GD). This study explored the association of serum 25(OH)D with the diversity of the intestinal flora and serum IL-17 in GD patients.Patients newly diagnosed with GD at 2 centers between 2018 and 2021 were consecutively included. According to their 25(OH)D levels, they were divided into the deficiency group, the insufficiency group, and the sufficiency group. Some patients with vitamin D deficiency or insufficiency were randomly selected and were matched with healthy volunteers (normal control [NC]) in terms of sex, age, and case number. The diversity and differential species of the intestinal flora and serum IL-17 levels were compared.Serum 25(OH)D negatively correlated with serum IL-17, the platelet/lymphocyte ratio, and TSH receptor antibody. The diversity of the intestinal flora decreased in the GD group, with noticeable differences in the composition of the intestinal flora when compared with the NC group. At the phylum level, the GD group exhibited a significantly lower abundance of Firmicutes but a higher abundance of Actinobacteria. At the genus level, the GD group exhibited higher relative abundances of Bifidobacterium, Collinsella, and Pediococcus but lower abundances of Roseburia and Dialister.The changes in the vitamin D level and the composition of the intestinal flora may partially contribute to the development of GD.