Hypothyroidism and SIBO: What the 2026 Research Reveals About Your Gut

If you live with an underactive thyroid and struggle with bloating, wind or an unsettled tummy, the two may well be connected. A 2026 study from Cedars-Sinai found SIBO (small intestinal bacterial overgrowth) in around a third of people with hypothyroidism — more than double the rate seen in people without thyroid problems. This article walks through what that research shows, why an underactive thyroid may make bacterial overgrowth more likely, and the practical steps that may help. As always, we’ll be honest about what the evidence can and cannot tell us.

Key takeaways

• People with a history of hypothyroidism appear more likely to have SIBO than people without thyroid problems.
• A 2026 study found SIBO in 32.65% of people with hypothyroidism, compared with 15.17% of controls — a more than twofold increase.
• The likely mechanism is a slower “migrating motor complex” (the gut’s between-meal cleansing wave), alongside changes in the gut lining and microbiome.
• The link is an association, not proof that one condition causes the other.
• Testing can show what’s happening in your own gut, so support can be tailored to you rather than generic.

What is SIBO, and why does it matter for your thyroid?

SIBO is a build-up of bacteria in the small intestine, which is normally home to relatively few. When bacteria overgrow there, they ferment food before your body has properly absorbed it, which can cause bloating, wind, abdominal discomfort and changes in bowel habits (Pimentel et al., 2020).

It matters for your thyroid specifically because SIBO may interfere with how well levothyroxine — the standard thyroid medication — is absorbed (Lauritano et al., 2007). An unsettled gut isn’t only uncomfortable; it may also affect how your thyroid treatment works, which is why understanding your gut is part of the bigger picture.

How are hypothyroidism and SIBO connected?

People with hypothyroidism are consistently more likely to have SIBO than people without thyroid problems — a pattern first reported in 2007 and repeated in several studies since, with the strength of the link varying by study (Lauritano et al., 2007).

What’s been missing is a close look at the small intestine itself. Most gut studies rely on stool samples, which mainly reflect the large intestine — a different environment with a different mix of microbes to the small bowel, where SIBO actually happens. That gap is what the 2026 Cedars-Sinai study, described below, was designed to close.

It’s also worth distinguishing subclinical from overt hypothyroidism. Much of the original 2007 evidence focused on overt (clearly under-treated or untreated) hypothyroidism specifically. If your thyroid levels are only mildly out of range, or well-controlled on medication, your personal risk may look different from the averages reported here.

What did the 2026 study find?

The headline numbers:

• 32.65% of people with a history of hypothyroidism had SIBO, versus 15.17% of people without thyroid problems — more than double the rate, independent of sex (odds ratio 2.71).
• In a smaller subgroup with confirmed autoimmune thyroiditis (Hashimoto’s), the rate was even higher — 40% had SIBO — though this subgroup was very small, so treat that figure as suggestive rather than definitive.
• Separately, in a health-records database analysis spanning 488,798 matched pairs of hypothyroid patients and controls (drawn from a network of over 140 million patient records), people with hypothyroidism were roughly twice as likely to be newly diagnosed with SIBO over a ten-year window. The figure was slightly higher again for autoimmune thyroiditis.

A team at Cedars-Sinai in Los Angeles (Wei et al., 2026) reached these numbers two ways. First, they sampled fluid directly from the small intestine in 372 people undergoing an endoscopy — 49 with a history of hypothyroidism and 323 without — which produced the 32.65%-vs-15.17% figures above. Second, they searched a large US health-records database (TriNetX) for new SIBO diagnoses over ten years, which produced the roughly-twofold risk figure.

The gut microbes looked different too

In the 2026 Cedars-Sinai study, the type of bacteria involved also differed by group. People with hypothyroidism tended to have more of a genus called Neisseria in the small intestine — reassuringly, harmless species, not the kind linked to infection (Wei et al., 2026).

The overgrowth pattern itself also differed. In people with SIBO but no thyroid problem, Escherichia/Shigella bacteria were the dominant group. In people with both hypothyroidism and SIBO, Klebsiella took over instead (roughly five times more likely to dominate than in SIBO-only cases). This suggests thyroid-related overgrowth may have its own microbial signature, rather than being one single condition (Wei et al., 2026).

Why might an underactive thyroid lead to bacterial overgrowth?

The most established mechanism is slower gut movement. Your small intestine has a natural “housekeeping wave” — technically the migrating motor complex (MMC) — that sweeps bacteria and debris along between meals. Thyroid hormone helps keep this movement brisk; when thyroid function is low, transit can slow, giving bacteria more time to settle and multiply (Lauritano et al., 2007).

Beyond motility, the 2026 review points to a few other plausible routes (Wei et al., 2026):

• The gut lining. Changes to gut barrier function have been described in Hashimoto’s. A more permeable barrier may shape which bacteria thrive.
• Selenium. This mineral supports healthy thyroid function, and low levels are common in Hashimoto’s. Selenium also appears to influence gut microbial diversity.
• Thyroid hormone recycling. Gut bacteria help recycle thyroid hormones, so a disrupted microbiome could feed back on thyroid balance.
• Molecular mimicry. Some microbes carry proteins that resemble thyroid tissue, which may in theory confuse the immune system — a promising but not yet proven idea.

The gut and thyroid appear to influence each other in both directions — a conversation, not a one-way street.

Does levothyroxine change your SIBO risk?

The evidence here is genuinely mixed. A 2026 study found levothyroxine treatment associated with a lower SIBO risk; an earlier 2017 study found the opposite. The two weren’t designed the same way, so they aren’t directly comparable — but neither should be dismissed.

The Wei et al. authors suggest that properly treating the thyroid may help keep the gut moving and lower overgrowth risk. The honest summary is that the role of thyroid medication in SIBO risk is not yet settled — which makes it an important area to review with your GP or endocrinologist, and never something to start, stop or adjust yourself based on gut symptoms.

How is SIBO tested and identified?

SIBO is usually assessed with a breath test: after drinking a sugar solution, you breathe into a series of tubes over a couple of hours, and bacteria fermenting the sugar release hydrogen (and sometimes methane) that shows up in your breath (Pimentel et al., 2020).

Because SIBO and a wider gut imbalance can look similar from the outside, Healthpath offers two options depending on what you’re trying to find out. The SIBO Test uses a hydrogen and methane breath test to look specifically for overgrowth, and suits bloating or wind that comes on soon after eating. The Ultimate Gut Health Test takes a wider view of the gut microbiome and key markers, and suits longer-standing or more varied digestive symptoms, or wanting the fuller context around your thyroid. Both include a practitioner-led review of your results, so you’re not left to interpret numbers alone.

Which supplements are worth considering for thyroid-related gut issues?

Selenium, vitamin D, herbal antimicrobials and probiotics all have some supporting evidence for thyroid-related gut support — but none is a quick fix, and the right combination depends on your own test results rather than a generic list.

Because slowed motility is a leading theory behind thyroid-related SIBO, some practitioners also suggest supporting the gut’s natural rhythm — for example, leaving a genuine gap between meals rather than grazing continuously, to give the migrating motor complex time to do its “housekeeping” sweep, and in some cases prescribed prokinetic support. The evidence specifically linking meal-timing habits to SIBO outcomes is limited, so treat this as a plausible, low-risk habit rather than a proven fix, and raise prokinetic options with your practitioner rather than self-prescribing.

The common thread: the most useful plan is the one built around your own results, not a generic list — which is where testing and practitioner guidance earn their place.

Frequently asked questions

Can SIBO affect how well my thyroid medication works? It may. Some research suggests bacterial overgrowth could interfere with levothyroxine absorption, which is one reason a settled gut matters for thyroid care (Lauritano et al., 2007). Discuss any concerns about your medication with your GP.

If I have hypothyroidism, should I be tested for SIBO? Not everyone needs testing. It’s most worth considering if you have ongoing digestive symptoms such as bloating, wind or discomfort. A practitioner can help you decide whether a breath test or a wider gut assessment is the better fit.

Does treating SIBO improve thyroid symptoms? Some small studies hint that addressing gut overgrowth may ease symptoms, but the evidence is early and not conclusive. It’s best seen as one supportive piece of a wider plan.

Can SIBO come back after it clears? Yes, recurrence is common, particularly if the underlying reason for the overgrowth isn’t addressed — which is why understanding root causes, including thyroid function and gut motility, matters.

What’s the difference between testing for SIBO and testing overall gut health? A SIBO breath test looks specifically for bacterial overgrowth in the small intestine. A broader gut microbiome test looks at the wider picture — including the large intestine — which is more useful if your symptoms are longer-standing, varied, or not clearly tied to eating.

The bottom line

The connection between hypothyroidism and SIBO is becoming harder to ignore. The 2026 research adds real weight to the idea that an underactive thyroid can shift the small intestine’s environment, making bacterial overgrowth more likely and giving it its own microbial signature (Wei et al., 2026).

At the same time, this is an association, not a proven cause. The good news is you don’t have to guess: testing your gut, reviewing your thyroid care with your doctor, and building support around your own results is a measured, evidence-informed way forward.

If you’d like to understand what’s happening in your own gut, the SIBO Test and Ultimate Gut Health Test both include a personal, practitioner-led review to help you make sense of your results and plan your next steps.

This article is for educational purposes only. It is not intended to diagnose, treat, cure or prevent any disease. Always speak to your GP or a qualified healthcare practitioner before making changes to your medication, diet or supplements, and never adjust prescribed thyroid medication without medical advice.

Scientific references

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2. Lauritano EC, Bilotta AL, Gabrielli M, et al. Association between hypothyroidism and small intestinal bacterial overgrowth. J Clin Endocrinol Metab. 2007;92(11):4180–4184.
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