Investigating the Link between the Intestinal Microbiome and Th17/Treg Dysregulation in Hashimoto’s Thyroiditis, and the Therapeutic Potential of Vitamin D

Methods:A systematic literature search was conducted in PubMed using predefined inclusion/exclusion criteria across three tranches. A search of the review literature pertaining to thyroid autoimmunity, HT and the intestinal microbiome identified 21 papers for inclusion. A mechanistic literature search covering animal, in vitro and human studies on HT, autoimmunity, ID, intestinal microbiota, intestinal permeability (IP) and Th17 yielded 43 papers. A final search of human studies examining vitamin D status or supplementation in relation to ID, IP, Th17 and interleukin (IL)-17 identified 45 relevant papers. Accepted studies were critically appraised and synthesized in a narrative analysis.Results:Evidence across the included studies suggests an association between ID and pathogenesis of HT. Increased IP and alterations in Th17/Treg balance emerged as key contributing mechanisms. Vitamin D status was also associated with immune modulation, particularly involving Th17 activity, and was strongly linked to both ID and HT pathogenesis.Conclusions:ID may play a contributory role in HT pathogenesis through immune modulation involving CD4+ T cell subsets. Vitamin D supplementation demonstrates potential as an adjunctive strategy; however, further targeted human studies are required to clarify causality and therapeutic efficacy.