Hashimoto’s Thyroiditis And Gut Health

Hashimoto’s thyroiditis is the most common autoimmune disease worldwide. It affects millions of people — the vast majority of them women — causing the immune system to slowly attack and destroy thyroid tissue, often leading to hypothyroidism and a constellation of debilitating symptoms: fatigue, brain fog, weight gain, depression, hair loss, and digestive problems. Yet despite being so common, Hashimoto’s is frequently managed with a single treatment: the thyroid hormone replacement drug levothyroxine (L-T4). For many patients, this normalises the TSH blood test — but symptoms persist. In clinic, this is one of the most frustrating experiences patients describe: being told their levels are “fine” while they continue to feel anything but. What conventional medicine has been slow to recognise is that the gut plays a profound role in the development, perpetuation, and — potentially — the management of Hashimoto’s thyroiditis. A rapidly growing body of research is revealing a bidirectional “gut-thyroid axis” in which gut dysbiosis, increased intestinal permeability, and impaired immune regulation in the gut all contribute to thyroid autoimmunity. This article explores Hashimoto’s Thyroiditis and gut health in depth — what the research actually says, why it matters, and what you can practically do about it.

What Is Hashimoto’s Thyroiditis?

Hashimoto’s thyroiditis (HT) is a chronic autoimmune condition characterised by lymphocytic infiltration of the thyroid gland, progressive destruction of thyroid tissue, and the production of autoantibodies — most notably anti-thyroid peroxidase (anti-TPO) and anti-thyroglobulin (anti-Tg) antibodies.

The disease follows a spectrum. In its earlier stages, thyroid hormone levels may remain normal (a state called euthyroidism) despite elevated antibodies and subclinical inflammation. Over time, as the thyroid gland sustains more damage, hypothyroidism typically develops, requiring lifelong hormone replacement therapy.

Key features of Hashimoto’s include:

• Elevated anti-TPO and/or anti-Tg antibodies
• Progressive hypothyroidism over time
• Characteristic changes visible on thyroid ultrasound
• A strong female predominance (approximately 7–10:1 female to male ratio)
• Significant overlap with other autoimmune conditions (coeliac disease, type 1 diabetes, rheumatoid arthritis)

While the exact cause of Hashimoto’s remains incompletely understood, it is considered a multifactorial disease involving a combination of genetic susceptibility, environmental triggers, and — increasingly — gut-related factors.

The Gut-Thyroid Axis: An Overview

The term “gut-thyroid axis” refers to the bidirectional communication between the intestinal microbiome and thyroid function. This is not merely a theoretical concept — peer-reviewed research has now firmly established that the gut microbiome influences the thyroid, and the thyroid (and its dysfunction) influences the gut.

A 2025 review published in the Archives of Endocrinology and Metabolism (PMID: 41337660) described the gut-thyroid axis as a “dynamic interaction between the intestinal microbiota and thyroid function,” with growing evidence linking gut dysbiosis to multiple thyroid diseases including Hashimoto’s. The review highlighted four key mechanisms through which the gut affects thyroid health:

1. Immune dysregulation originating in the gut
2. Increased intestinal permeability (leaky gut)
3. Chronic low-grade systemic inflammation
4. Impaired absorption of nutrients essential for thyroid function

A separate 2025 review from Frontiers in Microbiology (PMC: 12521424) reinforced these findings, noting that gut microbial metabolites — including short-chain fatty acids and secondary bile acids — influence thyroid hormone metabolism by regulating hepatic deiodinase activity, enterohepatic circulation, and tissue sensitivity to the active thyroid hormone, triiodothyronine (T3).

Understanding this axis reframes Hashimoto’s not simply as a thyroid disease, but as a systemic autoimmune condition with significant roots in gut health.

What Does the Research Say About the Microbiome in Hashimoto’s?

Altered Microbial Composition

Multiple studies have now directly compared the gut microbiome of Hashimoto’s patients with healthy controls. The findings reveal consistent and meaningful differences.

A 2021 study published in Frontiers in Immunology by Cayres et al. analysed stool samples from 40 Hashimoto’s patients and 53 healthy controls. The researchers found:

• A significant increase (p < 0.05) in Bacteroides species in HT patients
• A significant decrease in Bifidobacterium species
• Higher Lactobacillus in patients not yet taking levothyroxine, suggesting the drug itself may alter microbial composition

A 2022 study published in BMC Microbiology confirmed distinct microbiota differences between HT patients and healthy individuals across different stages of disease progression. The study identified Bifidobacterium and Klebsiella as two genera most dramatically altered in HT patients.

A 2021 systematic review and meta-analysis published in Frontiers in Endocrinology synthesised the available evidence, finding consistent patterns of dysbiosis across studies — notably reduced populations of butyrate-producing bacteria alongside shifts in the Firmicutes/Bacteroidetes ratio.

A 2025 paper in the International Journal of Molecular Sciences found Bacteroides fragilis to be significantly enriched in Hashimoto’s patients, proposing a role in immune modulation. Meanwhile, the butyrate-producing species Faecalibacterium prausnitzii was found to be significantly reduced in hypothyroid and Hashimoto’s populations across multiple studies.

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Why Does Faecalibacterium prausnitzii Matter?

Faecalibacterium prausnitzii deserves special attention. It is one of the most abundant bacteria in a healthy gut and one of the primary producers of butyrate — a short-chain fatty acid (SCFA) with wide-ranging anti-inflammatory and immune-regulatory effects.

Butyrate:

• Feeds and maintains the integrity of intestinal epithelial cells (aka prevents leaky gut)
• Promotes the differentiation of regulatory T cells (Tregs) — the immune cells that suppress inappropriate autoimmune responses
• Inhibits pro-inflammatory NF-κB signalling
• Reduces intestinal permeability by strengthening tight junction proteins

Research from Frontiers in Endocrinology confirmed that a reduction in SCFA-producing bacteria “weakens immune tolerance, promoting inflammatory cytokine release and autoantibody production” — a mechanism directly relevant to Hashimoto’s pathogenesis.

When Faecalibacterium prausnitzii is depleted, the gut loses a key immune thermostat. The result is a microenvironment more prone to inflammation and aberrant immune activation — precisely the conditions that allow autoantibodies to thyroid tissue to persist and escalate.

Leaky Gut: The Link Between Gut Dysbiosis and Thyroid Autoimmunity

Perhaps the most compelling mechanism connecting gut health to Hashimoto’s is increased intestinal permeability — often referred to as “leaky gut.” When the tight junctions between intestinal epithelial cells become disrupted, the gut barrier loses its selective function, allowing bacteria, bacterial fragments (such as lipopolysaccharide, or LPS), and undigested food antigens to pass into systemic circulation.

This triggers immune activation and — in genetically susceptible individuals — may initiate or perpetuate autoimmunity.

Zonulin: A Biomarker of Gut Permeability in Hashimoto’s

Zonulin is a protein that regulates intestinal tight junction permeability. Elevated serum zonulin is considered a reliable biomarker of increased intestinal permeability.

Multiple studies have now confirmed that zonulin levels are significantly elevated in Hashimoto’s patients:

• The 2021 Cayres et al. Frontiers in Immunology study found serum zonulin was significantly higher in HT patients compared to controls. Elevated zonulin also positively correlated with IFN-γ levels — a pro-inflammatory cytokine.
• A 2022 case-control study published in the Endocrine Journal compared 77 HT patients with 66 matched healthy controls and confirmed that plasma zonulin levels were significantly elevated in HT patients, proposing zonulin as a potential biomarker linking intestinal permeability with autoimmune disease aetiology.
• A pilot study published in Pediatric Diabetes examined 30 children and adolescents with HT. Zonulin levels were significantly higher in HT patients than in matched controls. Critically, zonulin levels positively correlated with levothyroxine dose — suggesting a relationship between gut permeability and disease severity.

The consistent finding of elevated zonulin across independent studies in different populations — adult, paediatric, Brazilian, Turkish — strengthens the case that increased intestinal permeability is not incidental but a core feature of Hashimoto’s pathophysiology.

LPS: The Inflammatory Trigger From the Gut

Lipopolysaccharide (LPS) is a component of the outer membrane of gram-negative bacteria. When gut barrier function is compromised and dysbiosis is present — particularly an overgrowth of gram-negative species — LPS can translocate into the bloodstream.

This triggers toll-like receptor 4 (TLR-4) signalling, activating macrophages and dendritic cells, and driving the release of pro-inflammatory cytokines including TNF-α and IL-6. In the context of Hashimoto’s, this chronic low-grade endotoxaemia creates a persistent inflammatory state that amplifies thyroid autoimmunity.

The 2025 Frontiers in Microbiology review confirmed that elevated systemic endotoxin levels are associated with immune dysregulation in Hashimoto’s, with LPS described as a sensitive indicator of bacterial translocation.

SIBO and Hashimoto’s: A Bidirectional Relationship

Small intestinal bacterial overgrowth (SIBO) — a condition characterised by an abnormal increase in bacteria in the small intestine — has a well-documented association with hypothyroidism and Hashimoto’s thyroiditis. The relationship appears to be bidirectional: hypothyroidism predisposes to SIBO through slowed gut motility, while SIBO perpetuates gut dysbiosis and may worsen thyroid autoimmunity.

The Evidence for SIBO in Hashimoto’s

One of the earliest studies examining this relationship was published in the Journal of Clinical Endocrinology & Metabolism in 2007 by Lauritano et al. This landmark study enrolled 50 consecutive patients with a history of overt hypothyroidism due to autoimmune thyroiditis and found that 54% tested positive for SIBO on glucose hydrogen breath test — compared to just 5% of controls. Importantly, those with SIBO experienced significantly more abdominal discomfort, flatulence, and bloating.

A more recent 2025 study published in the Journal of Clinical Endocrinology & Metabolism, from the well-regarded REIMAGINE cohort, found that SIBO prevalence was significantly higher in hypothyroid patients (32.65%) than in controls (15.17%). Using the TriNetX platform’s data from over 130 million patient records, the authors calculated a relative risk of 2.40 for developing SIBO in patients with autoimmune thyroiditis — a clinically meaningful association that was consistent across sex, age, and BMI subgroups.

Also presented at the Endocrine Society’s annual meeting in 2025 (ENDO 2025) was data suggesting that SIBO was associated with a significantly higher risk of subsequently developing Hashimoto’s thyroiditis — indicating the relationship may not only be hypothyroidism causing SIBO, but SIBO potentially contributing to the development of autoimmune thyroid disease itself.

Why Does Hypothyroidism Predispose to SIBO?

Thyroid hormone is essential for maintaining normal gastrointestinal motility. When thyroid hormone levels fall, the entire GI tract slows down — a phenomenon clinically observed as constipation, bloating, and gastroparesis. Slowed small intestinal transit provides the optimal environment for bacterial overgrowth in the small bowel, where bacterial counts should ordinarily be very low.

This creates a potential vicious cycle: Hashimoto’s leads to hypothyroidism, which slows the gut and promotes SIBO, which increases intestinal permeability and dysbiosis, which drives further immune activation and autoantibody production.

Clinical Implications: SIBO Can Impair Levothyroxine Absorption

SIBO has a particularly important practical implication for Hashimoto’s patients taking levothyroxine. Levothyroxine is absorbed throughout the small intestine, and consistent absorption is essential for optimal treatment. A 2021 case report published in Thyroid Research documented a Hashimoto’s patient with gastroparesis and SIBO in whom malabsorption of L-T4 was causing suboptimal TSH control — despite adequate dosing. Addressing the SIBO and GI conditions resolved the medication management problem.

This means that patients with Hashimoto’s who struggle to maintain stable thyroid hormone levels despite consistent levothyroxine use should consider SIBO as a contributing factor worth investigating.

Test for SIBO at Home

If you have Hashimoto’s and experience persistent bloating, abdominal discomfort, constipation, or erratic responses to levothyroxine, SIBO may be a missing piece of your clinical picture. Our at-home SIBO breath test (hydrogen and methane) is simple, accurate, and can be completed from the comfort of your own home. Results are reviewed by a functional medicine practitioner.

The Immune System: Where Gut and Thyroid Converge

Approximately 70–80% of the body’s immune cells reside in or around the gut. The gut microbiome plays a foundational role in training, calibrating, and maintaining the immune system — and when gut health is compromised, systemic immune dysregulation follows.

In the context of Hashimoto’s, three immune mechanisms are particularly relevant:

1. Regulatory T Cells (Tregs) and Loss of Self-Tolerance

Regulatory T cells (Tregs) are a specialised subset of immune cells that suppress immune responses to self-tissues — they are, essentially, the immune system’s brakes. In autoimmune diseases including Hashimoto’s, Treg function is impaired, allowing autoreactive immune cells to attack thyroid tissue unchecked.

The gut microbiome — specifically butyrate-producing bacteria — is essential for the differentiation and function of Tregs. When Faecalibacterium prausnitzii and other SCFA producers are depleted, Treg generation declines and the balance shifts towards pro-inflammatory Th1 and Th17 cells — both of which are implicated in thyroid destruction in Hashimoto’s.

2. Molecular Mimicry

Molecular mimicry occurs when a bacterial or viral antigen shares structural similarities with a host tissue antigen, causing the immune system to inadvertently attack self-tissue when it mounts a response to the pathogen.

Several gut bacteria have been implicated in molecular mimicry with thyroid antigens. Bacteroides fragilis — found to be enriched in Hashimoto’s patients in the 2025 IJMS study — has been proposed as a candidate through its polysaccharide A production, though the direct mechanistic link in HT requires further investigation.

3. Th1/Th17 Imbalance and Pro-Inflammatory Cytokines

The gut microbiome profoundly influences the Th1/Th2/Th17 immune balance. Dysbiosis, reduced SCFAs, and increased LPS translocation collectively drive a Th1 and Th17 dominant state — characterised by elevated interferon-gamma (IFN-γ), interleukin-17, and tumour necrosis factor-alpha (TNF-α). These cytokines directly promote thyroid inflammation, thyrocyte destruction, and autoantibody production.

Research published in Frontiers in Endocrinology (2023) confirmed that a reduction in SCFA-producing bacteria promotes inflammatory cytokine release and autoantibody production in the context of autoimmune thyroid disease — directly connecting gut microbial changes to the immunological hallmarks of Hashimoto’s.

Nutrition, the Gut, and Hashimoto’s: What the Evidence Shows

The Gluten Question

The relationship between gluten and Hashimoto’s is one of the most debated topics in functional medicine — and the evidence, while not yet definitive, is increasingly interesting.

Coeliac disease and Hashimoto’s co-occur at rates significantly higher than chance. Both share HLA haplotypes, suggesting common genetic susceptibility. Moreover, the intestinal damage caused by coeliac disease increases gut permeability — a recognised driver of Hashimoto’s autoimmunity.

Beyond coeliac disease, research has explored whether a gluten-free diet benefits Hashimoto’s patients without overt coeliac disease (so-called non-coeliac gluten sensitivity, or NCGS).

A 2025 systematic review and meta-analysis published in Nutrients specifically examined the effects of a gluten-free diet in non-coeliac Hashimoto’s patients. The review found evidence of reductions in thyroid autoantibodies (anti-TPO and anti-Tg) in patients following a gluten-free diet, though the authors noted that some study participants may have had subclinical coeliac disease not identified due to the absence of intestinal biopsies.

Proposed mechanisms for gluten affecting Hashimoto’s patients include: increased intestinal permeability from gluten exposure, activation of zonulin release (disrupting tight junctions), and molecular mimicry between gliadin — a gluten protein — and thyroid antigens.

Clinical takeaway: Until further robust RCT data emerges, the decision to trial a gluten-free diet in Hashimoto’s should be made on an individual basis — ideally after testing for coeliac disease and non-coeliac gluten sensitivity. For patients with documented gut permeability issues, a structured elimination trial may be clinically worthwhile.

Selenium and the Gut Microbiome

Selenium is an essential micronutrient required for the synthesis of selenoproteins — including the antioxidant enzyme glutathione peroxidase and the thyroid enzyme thyroid peroxidase itself. The thyroid gland contains the highest selenium concentration of any organ in the body, reflecting its profound dependence on this mineral.

A 2024 systematic review and meta-analysis published in Thyroid by Huwiler et al. evaluated multiple randomised controlled trials on selenium supplementation in Hashimoto’s. The analysis found that selenium supplementation reduced anti-TPO antibody levels, with some studies also demonstrating improvements in thyroid ultrasound echogenicity.

A separate 2024 network meta-analysis comparing selenium, vitamin D, and myo-inositol found that selenium supplementation significantly reduced both TPOAb and TgAb compared to placebo.

Mechanistically, the gut microbiome affects selenium absorption and metabolism, and selenium deficiency in turn alters gut microbial composition — creating another potential feedback loop. A 2024 study published in Communications Biology specifically examined how iodine and selenium intake modify gut microbiota composition in Hashimoto’s patients, finding significant microbiota shifts with changes in mineral intake.

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Probiotics, Prebiotics, and the Hashimoto’s Microbiome

Given the dysbiosis documented in Hashimoto’s, the use of probiotics and prebiotics to modulate the gut microbiome represents a logical therapeutic avenue. The evidence here is still maturing, but several studies offer promising signals.

A 2024 meta-analysis published in PLOS ONE examined eight randomised controlled trials on the effects of probiotics or prebiotics on thyroid function. The meta-analysis found that probiotic and prebiotic interventions produced modest but measurable effects on thyroid hormone levels, with statistically significant changes in free T4.

More directly relevant to Hashimoto’s, a 2025 double-blind randomised study tested the probiotic Lactiplantibacillus plantarum 299v alongside nutritional education in women with Hashimoto’s. The study found that probiotic supplementation enhanced the effects of the nutritional intervention on quality of life measures — a clinically meaningful outcome given how profoundly HT affects daily wellbeing.

The 2025 Frontiers in Microbiology review also noted that “probiotic intervention demonstrates significant therapeutic efficacy in treating autoimmune-mediated hypothyroidism,” and that “probiotic supplementation significantly decreased serum oxidative stress markers while increasing antioxidant capacity in Hashimoto’s thyroiditis patients.”

Of particular therapeutic interest is restoring Faecalibacterium prausnitzii. While direct supplementation with this species is not yet commercially available — it is extremely oxygen-sensitive — feeding it through prebiotic substrates such as resistant starch is a realistic strategy. Supplementation with butyrate itself (sodium butyrate or tributyrin) offers another route to replicate its key functions at the intestinal level.

An Important Note on Study Limitations

It is important to be honest about the current state of evidence. Most probiotic studies in Hashimoto’s and hypothyroidism are small, vary in the strains used, and differ in treatment duration. A 2023 meta-analysis noted only modest and not always statistically significant effects on TSH. The science is promising but not yet definitive — which is precisely why personalised assessment of the microbiome is far more valuable than simply recommending a generic probiotic off the shelf.

What This Means in Clinical Practice

If you have Hashimoto’s and are reading this, here is the practical takeaway from the research above:

1. Your Gut May Be Driving Your Symptoms

Persistent fatigue, brain fog, bloating, constipation, and mood changes in Hashimoto’s may not be solely explained by thyroid hormone levels. Gut dysbiosis, increased intestinal permeability, and SIBO can each contribute independently to symptoms — and to ongoing immune activation — even in patients who are biochemically euthyroid on levothyroxine.

2. A Thorough Gut Assessment Should Be Part of Your Work-Up

Testing the gut microbiome via comprehensive stool analysis, and assessing for SIBO via breath testing, gives you and your practitioner an evidence-based, actionable picture of what is happening in your gut. This is not guesswork — these are validated clinical tools used in functional medicine practice worldwide.

3. Gut-Directed Interventions Can Complement Thyroid Treatment

Addressing gut dysbiosis, repairing intestinal barrier function, treating SIBO where present, and optimising nutrition (selenium, dietary fibre, fermented foods) represents a systems biology approach to Hashimoto’s that goes well beyond the pill-per-symptom model. The goal is to address root-cause drivers of immune activation — not merely to replace the thyroid hormone that the autoimmune process has destroyed.

4. Individual Assessment Is Essential

There is no single “Hashimoto’s gut protocol.” The microbiome findings in research studies show common patterns — but your specific microbial composition, SIBO status, dietary sensitivities, and nutrient status are unique to you. Personalised testing and interpretation by a qualified functional medicine practitioner is the foundation of effective gut-thyroid support.

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Conclusion

Hashimoto’s thyroiditis is not simply a thyroid disease. The emerging research on the gut-thyroid axis reveals a condition deeply interconnected with gut health — one in which dysbiosis, leaky gut, SIBO, and impaired microbial immune regulation each play meaningful roles in both the onset and perpetuation of thyroid autoimmunity.

The evidence base is now substantial enough that gut assessment should be considered a routine part of the functional medicine approach to Hashimoto’s — not an optional extra. For patients who continue to struggle with symptoms despite “normal” thyroid levels, looking to the gut is not alternative medicine. It is evidence-based, systems-level thinking.

Understanding your own gut microbiome, identifying whether SIBO is present, and working with a knowledgeable practitioner to interpret and act on this data may be one of the most impactful steps you can take in managing Hashimoto’s thyroiditis.

References

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Disclaimer: This article is for educational purposes only and does not constitute medical advice. Always consult a qualified healthcare professional before making changes to your treatment or supplementation.