Thyroid-Gut-Axis: How Does the Microbiota Influence Thyroid Function?

Nutrients. 2020;12(6)

Plain language summary

Thyroid and gut disease often coexist together. This literature review highlights the strong interplay between gut, microbiota and thyroid disease. In autoimmune thyroid disease (AITD) gut bacteria imbalances, bacterial overgrowth, Coeliac's disease or non-coeliacs wheat sensitivity, increased gut permeability and resulting deficiency of thyroid nutrients are not uncommon. Inflammation and intestinal wall damage that lead to increased permeability are thought to be one of the driving factors for autoimmune activity. Allergens, certain drugs, impaired gut flora and nutrient deficiencies are some of the contributors to heightened intestinal permeability. Furthermore, the gut walls host deiodinase enzymes that convert thyroid hormone to its active form. The gut microbiota however influence thyroid function in their own rights. The bacteria are crucial for nutrient synthesis, absorption and availability, including those essential for thyroid health. Gut bacteria and their metabolites also play a significant role in the regulation, development and training of immune cells, relevant to AITD. After all, the gut also houses a large proportion of the immune system known as gut-associated lymphatic tissue (GALT). Besides, some bacteria species seem to be capable of balancing fluctuating thyroid hormone levels in the blood. The writings further elaborate on thyroid-essential nutrients and the gut such as iodine, iron, zinc, selenium and Vitamin D. And the impact of bariatric surgery on thyroid function and the presence of certain gut bacteria in thyroid cancers. In summary, the authors concluded that the thyroid-gut axis seems to exhibit a strong connection. Limited evidence from human studies showed promising results of probiotics and synbiotics on thyroid function and targeting the microbiota as a novel strategies for the management of thyroid disease is encouraged to be explored further. This article may be of interest to those looking for an informative summary on the many ways in which the gut influences thyroid function in health and disease.

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.

Lifestyle medicine

Fundamental Clinical Imbalances : Hormonal ; Immune and inflammation
Patient Centred Factors : Mediators/Thyroid-Gut-Axis
Environmental Inputs : Diet ; Nutrients ; Microorganisms
Personal Lifestyle Factors : Nutrition
Functional Laboratory Testing : Not applicable
Bioactive Substances : Thyroxine ; Levothyroxine ; Probiotics

Methodological quality

Jadad score : Not applicable
Allocation concealment : Not applicable
Publication Type : Journal Article ; Review

Metadata