Lactobacillus in autoimmune thyroid diseases: benefit or risk?

Autoimmune thyroid diseases (AITDs), mainly Hashimoto's thyroiditis and Graves' disease, are common organ-specific autoimmune disorders driven by complex interactions among genetic susceptibility, immune dysregulation, environmental exposure, and microbial factors. Increasing evidence supports the relevance of the gut-thyroid axis, in which gut microbiota may influence thyroid hormone metabolism, intestinal barrier integrity, systemic inflammation, and thyroid-directed autoimmunity. Among gut commensals, Lactobacillus-related taxa have attracted attention because selected strains can modulate cytokine production, Treg/Th17 balance, short-chain fatty acid metabolism, epithelial barrier function, and inflammatory signaling pathways such as TLR4/NF-κB, AhR, and PPARδ-related pathways. However, most mechanistic evidence is derived from non-AITD animal, cellular, or metabolic disease models, and direct validation in AITD patients remains limited. This review summarizes current evidence on Lactobacillus-related strains in AITDs, emphasizing strain-specific effects, updated taxonomy, mechanistic plausibility, clinical heterogeneity, and translational limitations. We also discuss inconsistent microbial signatures, neutral or inconclusive clinical findings, and safety concerns, including probiotic-associated infection, antibiotic resistance genes, horizontal gene transfer, and potential pro-autoimmune effects. Overall, Lactobacillus-related interventions remain investigational, and future studies should prioritize standardized strain identification, disease-specific mechanisms, long-term safety, and well-designed clinical trials.