Abstract
The thyroid-gut axis represents a dynamic interaction between the intestinal microbiota and thyroid function, with growing evidence linking gut dysbiosis to thyroid diseases. The gut microbiome, comprising over 100 trillion microorganisms, influences immune modulation, iodine metabolism, and thyroid hormone regulation. Short-chain fatty acids, produced by beneficial gut bacteria, support immune homeostasis and thyroid function, while pathogenic bacteria and lipopolysaccharides trigger inflammatory pathways that impair thyroid activity. Alterations in gut microbiota composition have been associated with autoimmune thyroid diseases, including Hashimoto's thyroiditis and Graves' disease. Dysbiosis increases intestinal permeability, antigen exposure, and immune activation, exacerbating thyroid autoimmunity. A reduction in short-chain fatty acids-producing bacteria weakens immune tolerance, promoting inflammatory cytokine release and autoantibody production. Recent studies highlight microbial metabolites such as tryptophan derivatives and their role in immune regulation. Gut dysbiosis is also implicated in thyroid nodules and cancer. Decreased butyrate-producing bacteria and increased inflammatory bacterial taxa have been observed in thyroid malignancies. Microbiota influence iodine and selenium bioavailability, essential for thyroid hormone synthesis, and modulate sodium-iodide symporter expression, affecting thyroid cancer response to radioactive iodine therapy. Microbiome-targeted interventions, including probiotics, prebiotics, dietary modifications, and fecal microbiota transplantation, may restore microbial balance, enhance immune regulation, and improve thyroid treatments. This review synthesizes our current understanding of the thyroid-gut axis, indicating that the intestinal microbiota and its metabolites may act directly or indirectly on the thyroid gland, highlighting potential clinical implications and paving the way for therapeutic strategies targeting the intestinal microbiota.