Abstract
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.
Highlights
Humans have always been living together in a symbiotic community with their microbiota [1].Trillions of bacteria are found in the human intestine, in jejunum and ileum, but most of them are in the colon, where the two dominant strains are Firmicutes and Bacteroides [2]
All of them play a role in supporting thyroid function and there is a clear link between thyroid dysfunction and altered levels of these minerals
There is accumulating data that a strong thyroid–gut axis exists. It appears to display a not well known but important correlation regarding the effect of the gut bacteria on the immune system and thyroid function
Summary
Humans have always been living together in a symbiotic community with their microbiota [1]. The bacterial composition shows geographical differences and numerous factors influence the composition of the microbiota, starting in utero [3] and continuing with the way a baby is born and if it is being breastfed [4]—children born by Caesarean section have a lower diversity of intestinal bacteria [3]. This extends to the influence of diet, use of antibiotics, other drugs, genetics, environment, and diseases [1,5]. There is increasing evidence for the presence of an important thyroid–gut axis that modulates these autoimmune diseases and patients often report changes in their quality of life and thyroid function in relation to dietary changes
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