Unveiling the Role of Gut Microbiota and Metabolites in Autoimmune Thyroid Diseases: Emerging Perspectives.

Autoimmune thyroid diseases (AITDs) are among the most prevalent organ-specific autoimmune disorders, with thyroid hormones playing a pivotal role in the gastrointestinal system's structure and function. Emerging evidence suggests a link between AITDs and the gut microbiome, which is a diverse community of organisms that are essential for digestion, absorption, intestinal homeostasis, and immune defense. Recent studies using 16S rRNA and metagenomic sequencing of fecal samples from AITD patients have revealed a significant correlation between a gut microbiota imbalance and the severity of AITDs. Progress in animal models of autoimmune diseases has shown that intervention in the gut microbiota can significantly alter the disease severity. The gut microbiota influences T cell subgroup differentiation and modulates the pathological immune response to AITDs through mechanisms involving short-chain fatty acids (SCFAs), lipopolysaccharides (LPSs), and mucosal immunity. Conversely, thyroid hormones also influence gut function and microbiota composition. Thus, there is a bidirectional relationship between the thyroid and the gut ecosystem. This review explores the pathogenic mechanisms of the gut microbiota and its metabolites in AITDs, characterizes the gut microbiota in Graves' disease (GD) and Hashimoto's thyroiditis (HT), and examines the interactions between the gut microbiota, thyroid hormones, T cell differentiation, and trace elements. The review aims to enhance understanding of the gut microbiota-thyroid axis and proposes novel approaches to mitigate AITD severity through gut microbiota modulation.