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Is There a Thyroid-Gut Axis?

I was doing some recent research and came across a journal article entitled “Thyroid-Gut-Axis: How Does the Microbiota Influence Thyroid Function? [1]“.  Even before finding this article I realized that there was a relationship between the gut and thyroid health, but it was nice to see a published journal article discussing this.  So I’d like to go ahead and summarize the relationship between the thyroid and the gut based on the information in this recent journal article.

In the introduction the authors mention how an impaired microbiota can lead to the development of autoimmune conditions, including Graves’ disease and Hashimoto’s.  They remind us that thyroid autoimmunity likely develops due to the combination of genetics, immune impairment, and environmental factors such as nutrient deficiencies and the gut microbiota.  They end the introduction by mentioning that patients often report changes in their quality of life and thyroid function in relation to dietary changes.  This of course is true, although as I’ve mentioned in previous blog posts, there can be other factors as well.

Intestinal Influences on the Thyroid

The next section starts off by discussing how the gut microbiota modulates both the innate and adaptive immune system, and how it is fundamental in the development of gut-associated lymphatic tissue (GALT). You might have heard that most of the immune system cells are located in the gut, and this article reinforces this, mentioning that more than 70% of the entire immune system is situated in the GALT. It adds that the GALT plays an important role in the development of tolerance to self-antigens.  For more information on immune tolerance you can check out my article entitled ” Thyroid Autoimmunity and Loss of Self Tolerance [2] “.

What You Need To Know About Butyrate and Thyroid Health

In past articles I’ve discussed the importance of regulatory T cells [3] (Tregs), and how these help to keep autoimmunity in check. As a result, you want to have an abundance of Tregs, and the journal article mentions that there is a positive correlation between butyrate and the number of Tregs.  Butyrate is a short chain fatty acid, and the authors also mention how it can help to strengthen intracellular tight junctions together with thyroid hormones.  In other words, butyrate can increase Tregs AND help to maintain the integrity of the gut barrier. I briefly spoke about butyrate in an article entitled “What Thyroid Sufferers Need To Know About Fiber, Resistant Starch, and SCFA [4]”.

Next they discussed how an alteration in the composition of intestinal bacteria, an increase in intestinal permeability (leaky gut), and a shift to proinflammatory cells are some of the ways the gut microbiota impacts the thyroid.  They also mentioned another study where the authors found that intestinal dysbiosis (imbalance of the gut flora) and bacterial overgrowth was more common in those with hypothyroidism/Hashimoto’s when compared to a euthyroid group (1) [5]. I will say that it’s possible that bacterial overgrowth can be a consequence of hypothyroidism. In other words, while intestinal dysbiosis might be a factor in the development of Hashimoto’s (and Graves’ disease), hypothyroidism can lead to altered GI motility, which in turn can lead to small intestinal bacterial overgrowth [6] (2) [7].

Celiac Disease, NCWS, and Thyroid Autoimmunity

The authors then went on to discuss how there is a higher prevalence of thyroid autoimmunity in both Celiac Disease and Non-Celiac Wheat Sensitivity (NCWS).  As for why this is the case, they brought up different possible mechanisms, including 1) shared cytokines, 2) cross-reaction of antibodies (molecular mimicry), malabsorption of micronutrients essential for thyroid health, and 4) increased intestinal permeability.

Nutrient Deficiencies and Thyroid Health

Next they discussed how the microbiota influences the uptake of minerals that play an important role in thyroid health.  This includes iodine, selenium [8], zinc, and iron [9].  They also mention how Lactobacillus and Bifidobacterium species are decreased in those with Hashimoto’s thyroiditis and Graves’ disease, and this can also have an effect on the minerals.  This is yet another reason to eat probiotic-rich foods and perhaps consider taking a probiotic supplement.

Speaking of probiotics, the authors discuss how probiotics can help support thyroid health.  They specifically mentioned a study in mice that showed that supplementing with Lactobacillus reuteri resulted in an increase in free T4, and another animal study which showed that supplementing with probiotics increased T3 and T4 (3) [10].  I’m sure there might be a few people with hyperthyroidism/Graves’ disease who are wondering if they should avoid probiotics, and all I can say is that I’ve worked with many hyperthyroid patients over the years, I commonly recommend probiotic supplements, and I don’t recall anyone’s hyperthyroidism worsening by supplementing with probiotics.

The authors brought up another study looking at the influence of Lactobacilli and Bifidobacterium species on thyroid hormone replacement…specifically levothyroxine.  They found a significantly lower adjustment requirement of T4 in the study compared to the control group, reasoning that microbiota modification increases the availability of levothyroxine and stabilizes thyroid function. They also concluded that probiotics can play a role in lowering serum hormone fluctuations, and mentioned how probiotics seem to be able to accumulate trace elements such as selenium, zinc, and copper, and incorporate them into organic compounds.

Bariatic Surgery and Thyroid Health

The article also discussed how bariatric surgery can potentially affect the thyroid.  Bariatric surgery is considered for some people who are obese, as it can result in substantial weight loss. In this journal article they discussed a meta-analysis that showed a significant decrease in the TSH, free T3 and total T3 in obese patients after receiving bariatric surgery, but it didn’t affect the free T4 or total T4 (4) [11].  They also said that subclinical hypothyroidism was completely resolved in 87% of Roux-en-Y gastric bypass (RYGB) patients one year after surgery.  Other studies observed an improvement in hypothyroidism in around 35% of patients and resolution in about 15% (5) [12].

Thyroid Cancer, Nodules, and the Gut Microbiota

The final section discusses the relationship between the gut microbiota and thyroid cancer.  They start off by mentioning how those people with thyroid cancer and thyroid nodules [13] present with higher microbial richness and distinct composition compared to the healthy control group, which suggests that imbalances in the gut microbiota is correlated with thyroid cancer and nodules. They then mention that in thyroid cancer there are higher levels of Clostridiaceae, Neisseria, and Streptococcus, while in thyroid nodules there are increases in Streptococcus and Neisseria when compared to healthy control groups (6) [14].  So while in past articles and blog posts I’ve discussed the role of insulin resistance and estrogen metabolism in thyroid nodules, a healthy gut can also play a role.

Then they mentioned how Lactobacillus is significantly decreased in those with thyroid cancer and nodules.  They remind the reader how Lactobacillus is important for the utilization of trace elements such as selenium, which plays an important role in thyroid health. While most probiotic supplements include Lactobacillus, unfortunately no specific species or strains of Lactobacillus was mentioned in the study.

So hopefully you understand how a compromised gut can have a negative effect on thyroid health.  In the past I’ve mentioned numerous times how a leaky gut is part of the triad of autoimmunity [15], but this journal article gave a much more comprehensive overview of how problems with the gut can affect thyroid health.  Here is a summary of some of the key points included in this journal article: