Gut Microbiota and Thyroid Diseases.

The gut microbiota plays a pivotal role in maintaining the homeostasis of the digestive and immune systems. Dysbiosis compromises the intestinal barrier and triggers systemic inflammation, thereby contributing to metabolic and immune dysregulation. Emerging evidence suggests a bidirectional relationship between the gut microbiota and thyroid function. The microbiota influences the absorption and metabolism of micronutrients essential for thyroid hormone synthesis, including iodine, selenium, iron, and zinc. Dysbiosis enhances intestinal permeability, facilitating the translocation of bacterial components into systemic circulation and potentially triggering autoimmune thyroid diseases (AITD) such as Hashimoto's thyroiditis and Graves' disease. Animal studies further demonstrate that hypothyroidism, hyperthyroidism, and thyroidectomy can alter the diversity and composition of gut microbiota. Environmental exposures such as particulate matter 2.5 reshape microbial and metabolic profiles, influencing the hypothalamus-pituitary-thyroid axis and thyroid hormone homeostasis. Clinical studies show reduced abundance of beneficial bacteria (Bifidobacterium and Lactobacillus) and enrichment of proinflammatory taxa in AITD patients. These findings highlight the complex bidirectional interactions between gut microbiota, thyroid function, and immune tolerance. Microbiotatargeted therapies may represent promising strategies for the prevention and management of thyroid disorders. This review aims to summarize current evidence regarding the bidirectional relationship between gut microbiota and thyroid disorders, with particular emphasis on dysbiosis, immune regulation, micronutrient metabolism, and autoimmune thyroid disease. Cite this article as: Beyhan YE, Işık Ş. Gut microbiota and thyroid diseases. Eurasian J Med. 2026, 58(3), 1232, doi: 10.5152/eurasianjmed.2026.251232.