Welcome to my blog post ‘Does Poor Gut Health Cause Thyroid Issues? The Gut-Thyroid Axis’ where I discuss some of the research exploring the connection between our gut microbiome and thyroid function.
You may also be interested in the section of my blog dedicated to gut health, click here, in particular:
- The Gut Microbiome In Multiple Sclerosis: What Bacteria Are Involved
- Psychobiotics: Probiotics For Mood And Mental Health
The microbiota has been identified as an important factor in health and in a variety of diseases. An altered microbiota composition increases the prevalence of Hashimoto’s thyroiditis and Graves’ disease. Microbes influence thyroid hormone levels by regulating iodine uptake, degradation, and enterohepatic cycling. In addition, there is a pronounced influence of minerals on interactions be- tween host and microbiota, particularly selenium, iron, and zinc. In manifest thyroid disorders, the microbiota may affect L-thyroxine uptake and influence the action of propylthiouracil (PTU). Although it is relatively well documented that thyroid disor- ders are linked to the composition of the microbiota, the role of specific genera and the potential use of therapies targeting the microbiota are less clear (1).
Role Of Poor Gut Health in Autoimmune Thyroid Diseases
Autoimmune thyroid diseases are the most prevalent organ-specific autoimmune diseases and affect 2–5% of the population. Microbial products, particularly short-chain fatty acids (SCFAs), can serve as an energy source for enterocytes and, together with thyroid hormones, enhance enterocyte differentiation and strengthen intercellular tight junctions. An altered microbiota composition in the gut, on the other hand, promotes the development of AID by several hypothesised mechanisms.
Interactions between Factors That Are Important for the Development and Treatment of Thyroid Diseases
How Does Poor Gut Health Cause Thyroid Disease?
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 extra-intestinal tissues, respectively. An imbalance in the microbiome has not only been found in autoimmune thyroid diseases, 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 (1).
Iodothyronine-deiodinases play an important role in the conversion of thyroxine (T4) to its active form T3 or reverse T3, its inactive form. Deiodinase activity has also been found in the intestinal wall and could contribute to total T3 body levels.
Another influencing factor of the microbiome is its effect on neurotransmitters such as dopamine, which can inhibit thyroid stimulating hormone (TSH).
The microbiome influence the uptake of minerals relevant to the thyroid including iodine, selenium, zinc, and iron.
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.
Beyond that, there seems to be a negative correlation between Lactobacillaceae and Bifidobacterium spp. with dietary iron and a positive correlation with selenium and zinc. As these bacteria are diminished in Hashimoto thyroiditis and Grave´s disease, it has been suggested that gut composition and mineral regulation may have an impact on these diseases (1).
Grave’s Disease And Gut Health
Intestines of Graves Disease patients contain:
- Higher levels of antibodies against the bacteria Yersinia enterocolica and Helicobacter pylori
- Greater colonisation by yeast
- Less colonisation by Bacteroides compared to healthy guts.
- Microbiota species diversity in hypothyroid patients is higher than in healthy controls.
This may be explained by effects related to the longer gastrointestinal (GI) transit time com- monly seen in hypothyroid patients. Low cell turnover, low redox potential, and long transit times were proposed as main reasons for the higher diversity of microbiota in the colon.
Although high diversity has been interpreted as positive for human health, negative effects may also result, namely increased protein catabolism, decreased polyphenol conversion and mucus secretion, and decreased epithelial turnover.
Poor Gut Health, Minerals, And Thyroid Function
Selenium (Se), iron (Fe), and zinc (Zn) are minerals that support thyroid function. The thyroid gland contains the highest amount of Se per mg tissue in the body.
It has been shown that thyroid dysfunction is linked to abnormal levels of these minerals. Mothers with goiter had lower iodine, Se, and Fe serum levels than healthy controls. Zn deficiency reduces free T3 and T4 levels by 30% in animals. In humans TSH, T4, and T3 serum levels are also decreased by Zn deficiency, hypothyroid individuals frequently present with low serum Zn levels. It appears that the relationship between Zn and thyroid metabolism is reciprocal because hypothyroidism induces Zn deficiency and insufficient supplementation with Zn causes hypothyroidism.
These minerals also have prominent roles influencing the gut microbiota. Bacteria compete with the host for Se. Resident microbes of the colon metabolize the Se, which is not absorbed by the host in the upper GI tract. Se increased microbial diversity in mice, with a relative increase in Bacteroidetes and a decrease in Parabacteroidetes. Dietary Se is positively linked with the abundance of Bifidobacterium adolescentis in the gut, and Se promotes growth of this genus.
Do Probiotics Help Thyroid Disease?
Supplementation of probiotics showed beneficial effects on thyroid hormones and thyroid function in general.
Interestingly, microbes like E. coli function as a reservoir for T3 by binding it to bacterial thyroid-binding hormone and are able to prevent thyroid hormone fluctuating and thus, possibly reduce the need for T4 supplementation. Spaggiari et al. investigated the influence of Lactobacilli and Bifidobacteriaceae probiotics on levothyroxine. They found a significantly lower adjustment requirement of T4 in the study compared to the control group, reasoning that microbiota modification increases levothyroxine availability and stabilizes thyroid function. They concluded that probiotics have a beneficial role in lowering serum hormone fluctuations, also considering that iodothyronines deconjugation is regulated by bacterial enzymes sulfatases and ß-glucuronidases, which could be more available due to probiotics.
Probiotics seem to be able to accumulate trace elements such as selenium, zinc, and copper and incorporate them into organic compounds. Considering that selenium, zinc, and probiotics operate via different pathways and all of them are favorable for the thyroid, there could be a synergistic effect for health when incorporating all of them, especially in deficient conditions.
Probiotics could constitute an adjuvant therapy for thyroid diseases.
Microbes, for instance Escherichia coli, may serve as reservoir for T3 owing to strong binding to bacterial thyroid-binding protein. An intestinal reservoir may prevent fluctuations of thyroid hormone levels and reduce the need for T4 supplementation. A study on probiotics showed that application of the probiotic VSL#3 – a mixture of four Lactobacillus spp., three Bifidobacterium spp., and Streptococcus thermophilus – reduced the number of L-thyroxine dose adjustments compared to the control group without probiotics, whereas mean L-thyroxine doses were similar. It was concluded that there is a positive role for probiotics in averting serum hormone fluctuations.
Summary of ‘Does Poor Gut Health Cause Thyroid Issues? The Gut-Thyroid Axis’
There is a clear relationship between poor gut health and thyroid function.
- The gut microbiota has important effects on human health and disease, and an altered composition of the gut microbiota was identified as a factor contributing to Hashimoto’s and Grave’s Disease.
- The microbiome may influence iodine uptake and recycling of thyroid hormones.
- There is a pronounced influence of minerals on interactions between host and microbiome, particularly iron, selenium, and zinc.
- In manifest thyroid disorders, the microbiome may affect L-thyroxine uptake.