The relationship between the gut microbiota and thyroid disorders

Unidades de investigação

• Patologia

Abstract

Disorders of the thyroid gland are common, more prevalent in women than in men, and range from inflammatory to neoplastic lesions. Autoimmune thyroid diseases (AITD) affect 2-5% of the population, while thyroid cancer is the most frequent endocrine malignancy. Treatment for AITD is still restricted to management rather than prevention or cure. Progress has been made in identifying genetic variants that predispose to AITD and thyroid cancer, but the increasing prevalence of all thyroid disorders indicates that factors other than genes are involved. The gut microbiota, which begins to develop before birth, is highly sensitive to diet and the environment, providing a potential mechanism for non-communicable diseases to become communicable. Its functions extend beyond maintenance of gut integrity: the gut microbiota regulates the immune system, contributes to thyroid hormone metabolism and can generate or catabolize carcinogens, all of which are relevant to AITD and thyroid cancer. Observational and interventional studies in animal models support a role for the gut microbiota in AITD, which has been confirmed in some reports from human cohorts, although considerable geographic variation is apparent. Reports of a role for the microbiota in thyroid cancer are more limited, but evidence supports a relationship between gut dysbiosis and thyroid cancer. The gut microbiota has been implicated in the increasing prevalence of thyroid disorders, including autoimmune thyroid diseases and thyroid cancers, through its effects on gut integrity, immune regulation and thyroid hormone metabolism, as outlined in this article. The increased incidence of thyroid disorders, such as autoimmune thyroid disease (AITD) and thyroid cancer, has driven the search for contributory environmental factors, including the microbiota.The gut microbiota has diverse functions, mainly exerted via metabolites such as short-chain fatty acids, including the modulation of immune balance, maintenance of thyroid hormone levels and generation or catabolism of carcinogens.Animal models of AITD clearly support a role for the gut microbiota; disease-associated taxa have been identified and antibiotic treatment reduces the incidence and severity of disease.Studies in humans with AITD have shown substantial geographic variation but all have shown reduced gut microbiota diversity; eight genera were associated with all types of thyroid autoimmunity, together with increased abundance.Reduced diversity is also a feature of thyroid cancer microbiomes, with the abundance of some species associated with the response to radioactive iodine treatment.Microbiome studies in thyroid disorders are uniquely complicated by the interplay between the microbiota and thyroid hormones and/or therapeutic drugs, making it difficult to identify taxa relevant to autoimmunity or cancers.

Keywords

• CANCER; AUTOIMMUNITY; ANTIBODIES; DYSBIOSIS; DATABASE; IMPACT; DIFFERENTIATION; TRANSMISSION; ANTIBIOTICS; LANDSCAPE