A healthy gut microbiota not only has beneficial effects on the activity of the nervous and immune system, but also both directly and indirectly on thyroid function. A growing number of studies support the link between gut microbiota composition and thyroid health. Many thyroid and intestinal diseases coexist like Hashimoto’s thyroiditis (HT) and Graves’ disease (GD) are the most common autoimmune thyroid diseases and often co-occur with Celiac Disease (CD) and Non-celiac wheat sensitivity. The negative influence on the immune system and the inflammatory regulation of an impaired microbiota (dysbiosis) seems to be likely to promote autoimmune diseases such as autoimmune thyroid diseases. Hashimoto’s thyroiditis (HT) is the most common thyroid disorder worldwide with a general prevalence of around 10–12% leading to hypothyroidism, and often, destruction of the thyroid gland. Graves’ disease (GD) has a prevalence of 1–1.5% and is marked by autoantibodies against the thyroid stimulating receptors, causing hyperactivity of the thyroid. Dysbiosis has not only been found in autoimmune thyroid diseases, but has also been reported in thyroid carcinoma.
The gut microbiota contributes to thyroid hormone synthesis and hydrolysis of thyroid hormones and the composition of the gut microbiota and Dysbiosis can directly impact thyroid hormone levels. In thyroid disorders, the microbiota may affect L-thyroxine uptake. It has also been shown that microbial metabolites may play a role in autoimmune thyroid diseases via modulating the immune system.
Gut microbiota also influences the absorption of minerals that are important to the thyroid, including iodine, selenium, zinc, and iron which are needed for converting T4 to T3 and are often found to be deficient in autoimmune conditions, resulting in malfunctioning of the thyroid. 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. The gut microbes influence iodine uptake, degradation, and cycling. In addition, there is a pronounced influence of minerals on interactions between host and microbiota, particularly selenium, iron, and zinc.
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 dysbiosis could cause thyroid disorders, including impaired thyroid hormone synthesis, storage, and secretion.
On the positive side supplementation of probiotics has been shown to have beneficial effects on thyroid hormones and thyroid function in general. 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 or maybe even play a role in prevention?
In support of this a study of the gut microbiome in primary hypothyroidism patients found four intestinal bacteria (Veillonella, Paraprevotella, Neisseria, and Rheinheimera) were prevalent in untreated primary hypothyroidism patients compared to healthy individuals. In addition, mice receiving a fecal transplant from hypothyroidism patients displayed decreased total thyroxine levels. Suggesting that primary hypothyroidism causes changes in gut microbiome. In turn, an altered flora can affect thyroid function in mice.
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Knezevic, J.; Starchl, C.; Tmava Berisha, A.; Amrein, K. Thyroid-Gut-Axis: How Does the Microbiota Influence Thyroid Function? Nutrients 2020, 12, 1769.
Clin Sci (Lond) . 2020 Jun 26;134(12):1521-1535.
Endocr Metab Immune Disord Drug Targets. 2020;20(3):344-350. doi: 10.2174/1871530319666190930110605. Our Little Friends With Big Roles: Alterations of the Gut Microbiota in Thyroid Disorders Hanieh-Sadat Ejtahed 1 2 , Pooneh Angoorani 1 , Ahmad-Reza Soroush 1 , Seyed-Davar Siadat 3 , Nooshin Shirzad 2 , Shirin Hasani-Ranjbar 1 2 , Bagher Larijani 2