Last Updated on June 20, 2026
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If you have been managing a thyroid condition for any length of time, you have probably noticed that the symptoms sometimes go well beyond the thyroid itself. Fatigue that lingers despite optimized medication. Bloating that shows up without obvious cause. Constipation, brain fog, and nutrient deficiencies that never seem to fully resolve. For many people, these nagging symptoms have a gut component. And often, that gut component has a name: small intestinal bacterial overgrowth, or SIBO.
SIBO is not a rare or fringe condition. It is a recognized gastrointestinal disorder in which bacteria that normally live in the large intestine migrate upward and colonize the small intestine in excessive numbers. When that happens, the consequences ripple outward well beyond the digestive tract, and emerging evidence makes it increasingly clear that the thyroid and the gut are far more connected than most people (and many clinicians) realize.
This article explains what SIBO is, why some thyroid patients are at disproportionate risk of developing it, what SIBO does to thyroid hormone metabolism, and what the latest science says about managing both conditions together.
What is SIBO exactly?
The small intestine is designed to have a relatively low bacterial population. Its job is to absorb nutrients (proteins, carbohydrates, fats, vitamins, and minerals), and it does that job best when the bacterial load is kept in check. SIBO occurs when bacteria overpopulate the small intestine, fermenting carbohydrates there instead of allowing them to pass through, producing hydrogen and methane gas, and triggering inflammation in the intestinal lining.
The symptoms of SIBO can look a lot like irritable bowel syndrome: bloating (often starting soon after eating), excessive gas, abdominal discomfort, alternating constipation and diarrhea, and nausea. But SIBO goes deeper than IBS. Because it damages the lining of the small intestine and disrupts normal digestive function, it also impairs nutrient absorption, which is where its impact on the rest of the body, including the thyroid, begins. The Cleveland Clinic notes that SIBO can cause fat malabsorption, weight loss, and deficiencies in key vitamins and minerals when left untreated.
How hypothyroidism sets the stage for SIBO
The connection between an underactive thyroid and SIBO comes down to a fundamental fact of physiology: thyroid hormones regulate gut motility. When thyroid hormone levels are low, digestion slows.
In a healthy gut, there is a protective mechanism called the migrating motor complex (MMC). This is a pattern of muscular contractions that sweeps through the small intestine during fasting periods, clearing bacteria, food residue, and cellular debris down into the colon, where the bacterial population belongs. Think of it as the gut’s self-cleaning cycle. It operates between meals and overnight, and it is crucial for preventing bacterial accumulation in the small intestine.
Hypothyroidism weakens this cycle. Reduced thyroid hormone production slows the muscular contractions of the gut, impairs the MMC, and reduces the secretion of gastric acid and bile, both of which have antimicrobial properties that also help keep bacterial populations in the small intestine low. When these defenses are compromised, bacteria get a foothold where they should not be. The result is SIBO.
The clinical evidence for this link is particularly striking. A landmark study published in the Journal of Clinical Endocrinology & Metabolism tested 50 patients with a history of overt hypothyroidism due to autoimmune thyroiditis, all of whom had been treated with levothyroxine and had normalized thyroid hormone levels by the time of the study. Despite their treated thyroid status, 54% of those patients tested positive for SIBO on a hydrogen glucose breath test, compared with just 5% of healthy controls. Abdominal discomfort, bloating, and flatulence were significantly more common in those who had SIBO.
That study, conducted at the Catholic University of the Sacred Heart in Rome, was notable not only for the striking prevalence it found, but for what happened after treatment: patients given a one-week course of rifaximin (an antibiotic that acts locally in the gut) showed significant improvement in their digestive symptoms and, importantly, a measurable change in thyroid hormone levels. This suggested that the bacteria themselves were having a direct effect on thyroid hormone absorption and metabolism.
A bidirectional risk?
More recent and much larger evidence has confirmed and deepened these findings. At ENDO 2025, the annual meeting of the Endocrine Society, researchers from Cedars-Sinai Medical Center presented data drawing on two separate datasets. In the prospective REIMAGINE cohort, SIBO prevalence in hypothyroid patients was more than double that of healthy controls: 33% versus 15%. In a large-scale analysis of over 1.1 million patients from the TriNetX medical records database, people with hypothyroidism had a relative risk of developing SIBO over a 10-year period that was 2.2 times higher than matched controls. For those whose hypothyroidism was caused by autoimmune thyroiditis (Hashimoto’s disease) the relative risk was even higher, at 2.4.
A separate but related study, also published in the Journal of the Endocrine Society, examined the reverse direction of that relationship: whether having SIBO increases the risk of later developing Hashimoto’s thyroiditis. Analyzing six years of patient records, the researchers found that SIBO patients had a meaningfully higher risk of subsequently developing Hashimoto’s, pointing to a potentially bidirectional loop. One possible mechanism behind this is likely to be molecular mimicry, which is the idea that some gut bacteria carry proteins that structurally resemble thyroid antigens such as thyroid peroxidase (TPO) or thyroglobulin, potentially triggering autoimmune responses that attack the thyroid gland.
This bidirectional picture is increasingly recognized in the scientific literature. A 2025 review in a peer-reviewed journal described the emerging concept of the gut–thyroid axis as a framework for understanding the reciprocal influences between the gut microbiome and thyroid function. As the review explains, thyroid hormone deficiency impairs gut motility and alters the mucosal environment, which disrupts microbial balance; that microbial disruption in turn impairs nutrient absorption, dysregulates immune function, and can further worsen thyroid status. Once this cycle begins, treating only one side of it may yield incomplete results.
How does SIBO undermine thyroid function?
Understanding why SIBO is so disruptive to thyroid health requires looking at three distinct mechanisms.
Nutrient malabsorption
The thyroid requires a steady supply of specific micronutrients to produce and activate its hormones. Iodine and tyrosine are the two primary raw materials from which T4 is synthesized. Selenium is essential for the deiodinase enzymes that convert the inactive thyroid hormone T4 into its biologically active form, T3. Zinc plays a role in both thyroid hormone synthesis and immune regulation. Iron is required for the enzyme thyroid peroxidase (TPO), which drives the iodination step in hormone production.
SIBO disrupts the absorption of virtually all of these nutrients. Bacterial overgrowth inflames and damages the lining of the small intestine, the very surface through which nutrients are absorbed. It can also compete for nutrients directly. The result is that people with SIBO are frequently deficient in selenium, zinc, iron, vitamin B12, vitamin D, and other nutrients that underpin thyroid health. These deficiencies can persist even in people eating a nutritious diet, because the absorptive machinery is compromised.
Disrupted T4-to-T3 conversion
Most thyroid hormone produced by the thyroid gland is T4, the inactive storage form. Before your cells can use it, T4 must be converted into T3, the active hormone. This conversion happens primarily in the liver, but the gut also plays a role: the intestinal wall contains deiodinase enzymes, and gut bacteria participate in the recycling of thyroid hormones through a process called enterohepatic circulation. When this system is working normally, a meaningful share of circulating T3 is produced with input from the gastrointestinal tract.
When SIBO and gut dysbiosis are present, this process is disrupted. Bacterial overgrowth increases levels of lipopolysaccharide (LPS), a molecule found in the outer wall of certain bacteria that, when it enters the bloodstream, has potent inflammatory effects. LPS has been shown to inhibit the deiodinase enzymes responsible for converting T4 into T3, effectively reducing the amount of active thyroid hormone available to your tissues.
This means that even if your thyroid is producing enough T4 (and your TSH looks normal on a lab report) ongoing SIBO could be quietly reducing how much of that T4 gets converted into the T3 your cells actually need. It is one reason some patients continue to experience hypothyroid symptoms despite apparently adequate treatment.
Immune dysregulation and autoimmunity
The gut houses approximately 70% of the body’s immune system. Its lining is the primary interface between the outside world (everything you ingest) and your internal tissues, and maintaining that barrier is fundamental to immune regulation. SIBO damages this barrier, contributing to increased intestinal permeability, a state sometimes called “leaky gut”, in which microbial products, partially digested food particles, and bacterial toxins pass into the bloodstream at a higher rate than normal.
This triggers a chronic low-grade immune activation that, in susceptible individuals, can fuel or amplify autoimmune thyroid disease. A 2023 Mendelian randomization study published in Frontiers in Endocrinology identified specific gut bacterial genera, including Bifidobacterium and Lachnospiraceae, that appear protective against hypothyroidism, while others were associated with increased risk, providing some of the strongest evidence yet for a causal gut–thyroid relationship. The molecular mimicry hypothesis mentioned above adds another layer: certain bacteria produce surface proteins that closely resemble thyroid antigens, and the immune response raised against those bacteria may, in some people, cross-react with the thyroid itself.
The symptom overlap problem
One of the most clinically frustrating aspects of the SIBO–thyroid relationship is how extensively their symptoms overlap. Both conditions can cause:
- Fatigue and low energy
- Brain fog and difficulty concentrating
- Bloating and digestive discomfort
- Constipation (or alternating bowel habits)
- Weight changes
- Mood changes, including depression or anxiety
This overlap creates a diagnostic problem. When a hypothyroid patient complains of persistent bloating and fatigue, it is easy (and common) for both the patient and their clinician to attribute everything to the thyroid, adjusting medication doses in search of relief that never fully comes. Alternatively, gastrointestinal symptoms may be labelled as IBS without any consideration of thyroid function. In both cases, half the picture is being missed.
For this reason, it may be helpful if doctors considered evaluating hypothyroid patients who complain of unexplained gastrointestinal symptoms (bloating, altered bowel habits, persistent abdominal discomfort) for SIBO. Equally, patients presenting with SIBO-like symptoms but no clear gastrointestinal cause might benefit from thyroid function testing.
A note on levothyroxine and SIBO
The relationship between levothyroxine — the most widely prescribed thyroid hormone replacement — and SIBO is worth examining separately, because the data is somewhat complex. On one hand, the ENDO 2025 Cedars-Sinai findings showed that taking levothyroxine appears to significantly reduce the risk of developing SIBO in hypothyroid patients, likely by restoring gut motility toward normal and protecting the MMC. Among patients with unspecified hypothyroidism taking levothyroxine, the relative risk of SIBO was markedly lower than in those who were not treated.
On the other hand, a separate retrospective cohort study published in the World Journal of Gastroenterology found levothyroxine use itself to be one of the strongest independent predictors of a positive SIBO breath test—more so than hypothyroidism alone. It is not entirely clear whether this reflects the drug’s pharmacological effects, the underlying thyroid condition that necessitates the drug, or some combination. What it does suggest is that levothyroxine and gut health interact in ways that remain incompletely understood, and that for some patients, taking thyroid hormone replacement does not fully insulate them against SIBO risk.
Testing for SIBO
The most widely used diagnostic test for SIBO is the hydrogen and methane breath test. After fasting overnight, you ingest a sugar solution (glucose or lactulose); if bacteria are overgrowing in the small intestine, they ferment the sugar and produce hydrogen and/or methane gas, which is exhaled and detected in breath samples taken at timed intervals. The test is non-invasive, relatively affordable, and can be done at home or in a clinic.
A more definitive but more invasive diagnostic method involves directly sampling fluid from the small intestine via endoscopy and culturing the bacteria found there. This is considered the gold standard but is far less commonly used due to its cost and complexity. ACG clinical guidelines on SIBO note that breath testing remains the most practical clinical tool for most patients.
If you have a thyroid condition and are experiencing persistent digestive symptoms that are not fully explained by your thyroid diagnosis, a SIBO breath test is a reasonable investigation to discuss with your doctor or a gastroenterologist.
Treatment
Treating SIBO typically involves antimicrobial therapy, dietary modification, and in some cases, prokinetic agents to restore gut motility. For patients who also have hypothyroidism, ensuring thyroid function is well-managed is an essential part of any SIBO treatment plan, not just for thyroid health, but because uncontrolled hypothyroidism will continue to slow gut motility and predispose to SIBO recurrence.
Rifaximin, a minimally absorbed, gut-targeted antibiotic, is the preferred first-line treatment for hydrogen-predominant SIBO. It works locally within the gut with limited systemic absorption, reducing the risk of side effects and systemic antibiotic resistance. In the original Italian study referenced earlier, a one-week course of rifaximin cleared SIBO in about 70% of hypothyroid patients who tested positive, with significant improvement in bloating and abdominal discomfort.
Herbal antimicrobial protocols are an alternative that some clinicians use, and evidence suggests their efficacy is broadly comparable to rifaximin in certain SIBO subtypes. For methane-predominant SIBO (now more precisely called intestinal methanogen overgrowth, or IMO), a combination of rifaximin and neomycin, or specific herbal protocols, may be more appropriate.
Dietary strategies, including low-FODMAP or elemental diets, are sometimes used as adjuncts to reduce bacterial fermentation and ease symptoms during treatment. A 2025 Cedars-Sinai elemental diet study reported 73% breath test normalization and 83% global symptom improvement in IMO patients over two to three weeks.
Because SIBO impairs the absorption of nutrients that are critical to thyroid health (selenium, zinc, iron, and B vitamins among them) addressing deficiencies that emerge from SIBO is also an important part of comprehensive thyroid care. A 2024 review in Frontiers in Cellular and Infection Microbiology highlighted how microbiota-targeted interventions, including probiotics and dietary modulation, have shown potential to improve microbial composition, reduce systemic inflammation, and enhance thyroid hormone utilization in patients with hypothyroidism.
It is also worth noting that SIBO has a recurrence rate that can be substantial, particularly when the underlying conditions that predispose to it, such as inadequately treated hypothyroidism or impaired gut motility, remain unaddressed. For thyroid patients, this underlines the importance of optimizing thyroid treatment not just as an endocrine goal, but as a gut health strategy.
Key takeaways
- SIBO (small intestinal bacterial overgrowth) is significantly more common in people with hypothyroidism than in the general population, with multiple studies pointing to a prevalence of over 30–50% in thyroid patients.
- Hypothyroidism slows gut motility, which disrupts a protective intestinal mechanism called the migrating motor complex, creating the conditions in which SIBO can develop.
- The relationship works in both directions: SIBO can also worsen thyroid function by impairing the absorption of nutrients critical to thyroid hormone production and by disrupting the gut’s role in converting inactive T4 into active T3.
- Research presented at ENDO 2025 found that people with hypothyroidism are more than twice as likely to develop SIBO and that treating the thyroid with levothyroxine appears to meaningfully reduce that risk.
- If you have unexplained digestive symptoms alongside thyroid disease (or vice versa), the two conditions may be more closely linked than your doctors have discussed.
