Microbiota and Thyroid Disease: An Updated Systematic Review

Studies analyzing the relationship between microbiota composition and the thyroid have been increasing rapidly in recent years, and evidence has recently come to light about the involvement of the gut microbiota in various aspects of thyroid pathology. Recently, besides studies analyzing the microbiota composition of different biological niches (salivary microbiota or thyroid tumor microenvironment) in patients with thyroid disorders, some studies have been carried out in peculiar subcategories of patients (pregnant women or obese). Other studies added a metabolomic insight into the characterization of fecal microflora in an attempt to enlighten specific metabolic pathways that could be involved in thyroid disorder pathogenesis. Lastly, some studies described the use of probiotics or symbiotic supplementation aimed at modulating gut microbiota composition for therapeutic purposes. The aim of this systematic review is to analyze the last advancements in the relationship between gut microbiota composition and thyroid autoimmunity, extending the analysis also to nonautoimmune thyroid disorders as well as to the characterization of the microbiota belonging to different biological niches in these patients. The overall results of the present review article strengthen the existence of a bidirectional relationship between the intestine, with its microbial set, and thyroid homeostasis, thus supporting the newly recognized entity known as the gut-thyroid axis.