The Gut–Thyroid Axis

The gut and thyroid are closely connected and may influence one another through the gut–thyroid axis. Gut microbes can affect nutrient absorption, immune activity, inflammation, and the integrity of the intestinal barrier, all of which may have downstream effects on thyroid health.

Read this article to learn more about the relationship between gut health and thyroid function, the role of the gut microbiome on nutrient absorption, immune regulation, and autoimmune thyroid disease, and discover emerging evidence for diet and probiotic interventions.

Understanding the gut–thyroid axis

The gut-thyroid axis is a two-way relationship between the gut and the thyroid. This means gut microbes, intestinal barrier function, and thyroid hormones all influence one another. Gut microbes can play an important role in endocrine homeostasis, influencing hormone production, metabolism, and signalling.

A healthy and diverse gut microbiota may help to support immune balance and normal thyroid function. Whereas dysbiosis, an imbalance of the gut microbiota, may increase intestinal permeability and promote immune activation, contributing to thyroid autoimmunity ¹. These mechanisms are especially relevant to autoimmune thyroid conditions such as Hashimoto’s thyroiditis and Graves’ disease ¹. Gut microbes may also influence the availability and absorption of micronutrients needed for thyroid function, including iodine, selenium, iron, zinc, and copper ¹.

Autoimmune thyroid disease and gut microbiome

Thyroid dysfunction commonly presents as hypothyroidism or hyperthyroidism and is assessed using circulating thyroid-stimulating hormone (TSH) and thyroid hormone concentrations. Autoimmune thyroid diseases include Hashimoto’s thyroiditis, a common cause of hypothyroidism, and Graves’ disease, a common cause of hyperthyroidism ². Current evidence has linked gut health to these autoimmune thyroid diseases.

One study comparing gut microbiota in healthy individuals and people with Hashimoto’s thyroiditis found distinct differences between the two groups. Certain bacteria, including Lachnoclostridium, Bilophila, and Klebsiella, were more abundant in healthy controls, while Akkermansia, Bifidobacterium, and members of the Lachnospiraceae family were more common in those with Hashimoto’s thyroiditis ³.

Similarly, a clinical study comparing people with Graves’ disease to healthy individuals found significant differences in gut microbiota composition. Participants with Graves’ disease also had lower levels of several short-chain fatty acid-producing bacteria, including Bacteroides, Dialister, and Coprococcus ⁴. It is unclear, however, whether these changes in gut health contribute to the development of Hashimoto’s thyroiditis or Graves’ disease or happen as a result of the conditions.

Gut bacteria and thyroid function

When it comes to gut bacteria and thyroid function, they may influence one another through four key mechanisms: micronutrient absorption, inflammation, gut barrier function, and microbial metabolites.

Micronutrient absorption

Gut bacteria may affect how well the body absorbs and processes micronutrients that are needed for thyroid hormone production and metabolism. These include iodine, selenium, iron, zinc, and copper ¹.

Inflammation

An imbalanced gut microbiome may promote ongoing, low-grade inflammation. This can disrupt immune regulation and may contribute to thyroid dysfunction, particularly in autoimmune conditions such as Hashimoto’s thyroiditis and Graves’ disease ⁴.

Gut barrier function

The intestinal barrier helps prevent bacteria and their by-products from entering the bloodstream. When this barrier becomes more permeable (i.e., leaky gut), microbial substances may trigger systemic inflammation and immune activity that could affect thyroid tissue ⁵.

Microbial metabolites

Beneficial gut bacteria produce microbial metabolites such as short-chain fatty acids (e.g., butyrate) when they ferment dietary fibre ². These metabolites can help to maintain the gut barrier and regulate immune responses. Reduced production of these metabolites may weaken immune tolerance and contribute to inflammation associated with thyroid disease ².

Diet and lifestyle considerations

Diet and lifestyle may play a role in supporting the gut–thyroid axis. Below is a breakdown of some of the key areas being explored, but it is important to note that research in these areas is still ongoing:

• Fibre-rich diets: A clinical study showed that dietary fibre supplementation in Hashimoto's thyroiditis patients positively reshaped gut microbiota composition ⁶. However, the relationship between hypothyroidism and gut health is complicated. An earlier study showed that fibre interacts with T4 bioavailability in some hypothyroid patients, potentially increasing the dose required to maintain treatment responses ⁷. So dietary fibre can have both positive and negative effects.
• Selenium, iodine and iron: These are the three essential trace elements that underpin the synthesis and action of thyroid hormones ⁸. A review found that in clinical studies, selenium supplementation was clinically beneficial for patients with mild to moderate Graves’ orbitopathy ⁹. However, selenium has a narrow optimal intake range, and unnecessary supplementation may be harmful.
• Stress and sleep management: Good sleep quality and stress management may also have benefits for thyroid function. In a clinical study, eight weeks of a stress management intervention improved anti-thyroglobulin antibodies and levels of stress, depression, and anxiety in women with Hashimoto's thyroiditis ¹⁰.
• Dietary diversity: A narrative review found that thyroid health may be influenced by the overall quality and variety of a diet. Micronutrients, protein, carbohydrates, fats, and beneficial plant compounds may work together to support gut and thyroid function ¹¹.

Probiotics and thyroid health

Research into probiotics and thyroid health is still emerging, but early findings suggest that certain strains may offer benefits.

• Lactiplantibacillus plantarum 299v (Lp299v): In a 12-week randomised controlled trial involving women with Hashimoto’s thyroiditis, Lp299v was given alongside nutritional education. Compared with placebo, participants receiving the probiotic reported significant improvements in quality of life ¹².
• Bifidobacterium longum: In a six-month clinical study involving people with Graves’ disease, Bifidobacterium longum was used alongside methimazole therapy. The probiotic plus methimazole group improved thyroid function through support of the gut–thyroid axis ¹³.

Two systematic reviews have also been conducted assessing probiotic supplementation for thyroid function ^14,15. One concluded that probiotic or prebiotic supplementation does not influence thyroid hormone levels, but may modestly reduce TSH receptor antibody levels in patients with Graves’ disease ¹⁴. However, the other review concluded that routine administration of probiotics, prebiotics, or synbiotics may result in little to no benefit in patients with primary hypothyroidism ¹⁵.

In theory, probiotics may support thyroid health by helping to restore a healthier balance of gut bacteria, strengthen the intestinal barrier, and increase the production of short-chain fatty acids. These effects may help reduce inflammation and improve immune regulation along the gut–thyroid axis.

However, evidence for this is currently limited by small sample sizes, short study durations, and research involving only a couple of probiotic strains. Probiotics should therefore be viewed as a potential supportive therapy rather than a treatment that modifies thyroid disease. Larger, longer-term randomised controlled trials are needed to determine which strains, doses, and treatment durations may be most beneficial.

Clinical takeaways

The thyroid and gut microbiome may influence one another through many interconnected pathways, such as micronutrient absorption, inflammation, intestinal barrier integrity, and the production of microbial metabolites. Differences in the gut microbiome have been observed in people with autoimmune thyroid conditions, including Hashimoto’s thyroiditis and Graves’ disease. However, it is still unclear whether these changes contribute to disease development or occur as a consequence of the condition.

Dietary fibre, adequate micronutrient intake, dietary diversity, sleep and stress management may help support overall gut and thyroid health. Probiotics and thyroid health are currently under investigation, but at the moment, evidence is limited and does not show that they can treat or modify thyroid disease.

Currently, the strongest evidence relates to associations between autoimmune thyroid disease and the gut microbiome ^3,4. But larger, longer-term studies are needed to determine whether targeting the gut microbiome can improve clinically meaningful thyroid outcomes.

References

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