A healthy gut microbiota not only has beneficial effects on the activity of the immune system, but also on thyroid function. Thyroid and intestinal diseases prevalently coexist-Hashimoto's thyroiditis (HT) and Graves' disease (GD) are the most common autoimmune thyroid diseases (AITD) and often co-occur with Celiac Disease (CD) and Non-celiac wheat sensitivity (NCWS).
This can be explained by the damaged intestinal barrier and the following increase of intestinal permeability, allowing antigens to pass more easily and activate the immune system or cross-react with extraintestinal tissues, respectively.
Dysbiosis has not only been found in AITDs, but has also been reported in thyroid carcinoma, in which an increased number of carcinogenic and inflammatory bacterial strains were observed. Additionally, the composition of the gut microbiota has an influence on the availability of essential micronutrients for the thyroid gland. Iodine, iron, and copper are crucial for thyroid hormone synthesis, selenium and zinc are needed for converting T4 to T3, and vitamin D assists in regulating the immune response.
Those micronutrients are often found to be deficient in AITDs, resulting in malfunctioning of the thyroid. Bariatric surgery can lead to an inadequate absorption of these nutrients and further implicates changes in thyroid stimulating hormone (TSH) and T3 levels.
Supplementation of probiotics showed beneficial effects on thyroid hormones and thyroid function in general. A literature research was performed to examine the interplay between gut microbiota and thyroid disorders that should be considered when treating patients suffering from thyroid diseases.
Multifactorial therapeutic and preventive management strategies could be established and more specifically adjusted to patients, depending on their gut bacteria composition. Future well-powered human studies are warranted to evaluate the impact of alterations in gut microbiota on thyroid function and diseases.
There is accumulating data that a strong thyroid'gut axis exists. It appears to display a not wellknown but important correlation regarding the effect of the gut bacteria on the immune system and thyroid function. Furthermore, there is higher prevalence of the coexistence of thyroid and gut related diseases, just as Hashimoto's Thyroiditis/Graves' Disease and Celiac Disease/Non-celiac wheat sensitivity.
Dysbiosis is a common finding in thyroid disorders. On the one hand, it alters the immune response by promoting inflammation and reducing immune tolerance, damaging the intestinal membrane and causing an increase in intestinal permeability, which again leads not only to a high
exposure of antigens, but also local inflammation. On the other hand, it can directly impact thyroid hormone levels through its own eiodinase activity and the inhibition of TSH.
Gut microbiota also influences the absorption of minerals that are important to the thyroid, including iodine, selenium, zinc, and iron. All of them are essential for thyroid function and there is a clear link between thyroid dysfunction and altered levels of these minerals. For example, iodine deficiency may lead to goiter, presumably thyroid nodules, and even follicular thyroid cancer. High iodine intake can either induce hypothyroidism or hyperthyroidism in susceptible patients. Iron is essential for bacterial growth, iron availability influences the composition of the microbiota, and at the same time, the microbiota influences iron availability. Iron is vital for efficient iodine utilization and thyroid hormone synthesis and ID could cause thyroid disorders, including impaired thyroid hormone synthesis, storage, and secretion.
Probiotics have shown beneficial effects in thyroid diseases and are able to have a positive effect on trace elements such as selenium, zinc, and copper. Additionally, microbes function as a reservoir for T3 and are able to prevent thyroid hormone fluctuating and thus may be able to reduce the need for T4 supplementation.
Probiotics could constitute an adjuvant therapy for thyroid diseases.However, most studies on probiotics rely on animal models, therefore well-designed human studies are needed to further elucidate the importance of the thyroid'gut axis and the possibilities of intervention.
Taking into account the various potential effects of the microbiota and micronutrients on thyroid functions and medications, novel therapeutic strategies for the management of thyroid diseases could be established and more specifically adjusted to patients, depending on their gut bacteria composition.
Future adequately powered human studies would be necessary to evaluate the impact of gut microbiota on thyroid function and diseases
Full Text : https://www.mdpi.com/2072-6643/12/6/1769