Gut Microbiota and Hashimoto's Thyroiditis: Microecological Regulatory Mechanisms and Clinical Significance Based on the Gut–Thyroid Axis

ABSTRACT The human gut microbiota is highly diverse and widely distributed throughout the gastrointestinal tract, playing an important role in immune homeostasis. With advances in microbiome research, the concept of the “gut–thyroid axis” has emerged to describe a potential bidirectional relationship between gut microbiota and thyroid function. Hashimoto's thyroiditis (HT), the most prevalent autoimmune thyroid disease, remains incompletely understood in terms of its etiology and pathogenesis. A narrative review of recent experimental and clinical studies was conducted to summarize current evidence on the relationship between gut microbiota and HT. Relevant literature was analyzed to discuss potential mechanisms linking microbial dysbiosis with thyroid autoimmunity and to evaluate microbiota‐targeted therapeutic strategies. Available human studies indicate that HT is associated with differences in gut microbial composition, whereas experimental studies provide plausible mechanisms involving trace‐element metabolism, intestinal barrier integrity, microbial metabolites, and Th17/Treg regulation. Microbiota‐targeted interventions—including traditional Chinese medicine, dietary modification, probiotics, and fecal microbiota transplantation—have shown potential in preclinical studies or limited clinical investigations, but their efficacy in HT remains uncertain. Gut microbiota alterations may be involved in the gut–thyroid axis and may be associated with HT, but current evidence does not establish dysbiosis as a cause of HT. Further longitudinal and controlled clinical studies are needed to clarify temporal and causal relationships and to determine whether microbiota‐targeted interventions provide clinically meaningful benefits.