SIBO and Hashimoto's Hypothyroidism: The Connection

💡 How are SIBO and Hashimoto's hypothyroidism related?

Hypothyroidism slows the motility of the digestive tract: it delays gastric emptying and prolongs small intestine transit. That slowdown lets bacteria accumulate instead of being swept toward the colon, which is exactly the terrain that favors SIBO. The evidence is associational, not proven cause: several studies show that people with hypothyroidism have SIBO more often than controls (a 2007 study reported 54% versus 5%; a 2025 study reported 32.65% versus 15.17%, with an odds ratio of 2.71). In the subgroup with positive anti-TPO antibodies (a marker of Hashimoto's), the association was even greater (odds ratio 3.73). The relationship appears bidirectional, and there are three plausible mechanisms: slow motility, less stomach acid, and, in the reverse direction, SIBO that hinders the absorption of levothyroxine and of nutrients the thyroid needs.

If you have Hashimoto’s—the most common cause of hypothyroidism—and you also live bloated, gassy, and with heavy digestion, it is reasonable to wonder whether the two are connected. The short answer is that they probably are, through specific mechanisms, but it is worth being honest about the level of evidence: what we have are statistical associations and plausible mechanisms, not a causal relationship demonstrated in clinical trials.

This guide explains why the thyroid and the small intestine influence each other, what the real studies say along with their numbers, and what makes sense to ask your doctor. It is not a prescription: it is a map of the connection so you can come to your doctor with better questions.

Author’s note: I’m Beiker, a developer, not a doctor. I built this site when my sister was diagnosed with SIBO and I ran into the same question again and again on forums: “I have Hashimoto’s and SIBO, is it a coincidence?” Most pages answered with certainties the literature doesn’t support, or with theories without a single number. I wanted to do the opposite: organize what the public studies actually show, clearly mark where the evidence is solid and where it is only a hypothesis, and leave the rest in the hands of those who can actually diagnose and treat.

The connection in one sentence: the thyroid sets the gut’s pace

Thyroid hormones are one of the “accelerators” of the body’s metabolism, including the smooth muscle of the digestive tract. When the thyroid works too little (hypothyroidism), many processes slow down, and gut motility is one of them.

Why does this matter for SIBO? Because the small intestine stays relatively clean thanks, in large part, to a mechanical sweep: the contents (and the bacteria) move toward the colon within hours. If that sweep becomes slow, bacteria have time to settle and multiply where they shouldn’t. That is the essence of SIBO, and that is why motility is the thread that connects the two diseases. I develop this in depth in the guide to root causes of SIBO and in the one on the migrating motor complex.

Mechanism 1: hypothyroidism slows motility

This is the best-established mechanism. Hypothyroidism is associated with:

Delayed gastric emptying: the stomach takes longer to deliver its contents to the intestine.
Prolonged orocecal transit: the journey from the mouth to the cecum (where the colon begins) gets longer.
Lower peristalsis frequency: the waves that push the contents along occur less often.

The result is a “slower” small intestine, where the contents stagnate for longer. And an intestine that doesn’t sweep regularly is, literally, the terrain described as a risk factor for bacterial overgrowth. It’s the same principle that explains why scleroderma, diabetic neuropathy, and other motility disorders are associated with SIBO: the underlying cause changes, but the defense that fails—the sweep—is the same.

Mechanism 2: less stomach acid (hypochlorhydria)

Stomach acid doesn’t only digest: it also kills or holds back many bacteria before they reach the small intestine. It is one of the chemical barriers that keep the bacterial load low downstream.

Here a detail enters that distinguishes Hashimoto’s from “simple” hypothyroidism. Hashimoto’s is an autoimmune disease, and people with thyroid autoimmunity are more likely to have other associated autoimmune conditions, among them autoimmune atrophic gastritis, which destroys the acid-producing cells of the stomach. When that happens, acid drops (hypochlorhydria or achlorhydria) and exactly that chemical barrier weakens. Less acid means more bacteria surviving the passage into the small intestine.

Not everyone with Hashimoto’s develops atrophic gastritis, but the coexistence of autoimmune stomach problems is what makes the connection with SIBO appear stronger in the Hashimoto’s subgroup than in hypothyroidism from other causes.

Mechanism 3 (in the reverse direction): SIBO complicates thyroid treatment

So far, the thyroid “creates” terrain for SIBO. But the arrow also points the other way, and this is what makes the relationship described as bidirectional.

Levothyroxine absorption. Levothyroxine (the standard hormone replacement) is a sodium salt that needs an acidic gastric environment to dissolve before it can be absorbed in the intestine. If acid is low—because of atrophic gastritis, Helicobacter pylori infection, or proton pump inhibitors—the drug dissolves worse and is absorbed less. In a classic study published in The New England Journal of Medicine (Centanni et al., 2006), patients with atrophic gastritis needed on average a 27% higher dose of levothyroxine; those who also had H. pylori required 34% more, and those with non-atrophic gastritis 22% more, compared with people who had no gastric involvement. In other words: the same underlying problem (low acid) that favors SIBO also makes it harder for the thyroid pill to work well.

Nutrient absorption. SIBO can impair the absorption of micronutrients the thyroid uses to make and activate its hormones—among them iron, vitamin B12, and selenium. This is biologically plausible and is mentioned frequently, but it is worth taking it as a reasonable hypothesis, not as an effect quantified in each person. I go deeper into deficiencies in supplements and deficiencies in SIBO.

What the studies show (with their numbers and their limits)

This is where you have to be precise, because a lot of exaggeration circulates. Here is what the literature actually reports:

Study	Design	Main finding	What it means
Lauritano et al. (2007), JCEM	50 patients with hypothyroidism vs 40 controls; SIBO by glucose breath test	SIBO in 54% vs 5% (P less than 0.001)	Strong association, but small sample
*Wei et al. (2025), JCEM* (REIMAGINE data)**	Cohort with duodenal aspirate/sequencing	SIBO in 32.65% vs 15.17%; odds ratio 2.71 (95% CI 1.39–5.30)	Risk roughly 2.7 times higher; more modern methodology
Same study, anti-TPO+ subgroup	Hypothyroid patients with positive antibodies (a marker of Hashimoto’s)	SIBO in 40%; odds ratio 3.73 (95% CI 1.15–14.56)	The association appears greater in thyroid autoimmunity
Same study, population cohort	Cumulative 10-year incidence of SIBO	Relative risk 2.20 (hypothyroidism) and 2.40 (autoimmune thyroiditis)	Confirms the pattern at the population scale

How to read these numbers without overreaching:

The first two rows give very different percentages (54% vs 32.65%) because they are different studies, with different methods and populations. What they agree on is the direction: more SIBO in hypothyroidism than in controls. That consistency is what matters, not the exact number.
An odds ratio of 2.71 means SIBO was roughly 2.7 times more likely in the hypothyroid group, not that hypothyroidism “causes” SIBO in 2.7 out of every person. It is association, not demonstrated causality.
The wide confidence intervals (especially the one for the anti-TPO subgroup, 1.15–14.56) reflect small samples: the signal points to a greater risk, but the exact magnitude is uncertain.

One more data point, which is at once a mechanistic clue and possibly good news: in the population cohort, the risk of SIBO dropped in those taking levothyroxine (relative risk of 0.33 in treated hypothyroidism). It is consistent with the idea that correcting thyroid function helps normalize motility, even though it is an observational analysis—it does not prove that treating the thyroid cures or prevents SIBO on its own.

What to ask your doctor

If you have Hashimoto’s or hypothyroidism and persistent digestive symptoms, these questions help make the conversation with your doctor concrete:

Is my thyroid well controlled right now? A TSH within target matters because motility improves when thyroid function normalizes. If your digestive symptoms worsened when control worsened, it’s worth mentioning.
How am I taking my levothyroxine? The American Thyroid Association guidance suggests taking it on an empty stomach, about 60 minutes before breakfast or at bedtime (3 or more hours after the last meal). Coffee can reduce its absorption (roughly 29–36% in one study), as can calcium and iron taken at the same time. If the dose “isn’t quite working,” reviewing how you take it is the first thing to check.
Does it make sense to test for SIBO if the digestive symptoms don’t ease? The standard is the breath test. Not all gas and bloating is SIBO; it’s worth ruling out other causes and distinguishing it from irritable bowel syndrome.
If I keep needing more and more levothyroxine, could there be a stomach acid problem? Malabsorption from hypochlorhydria (atrophic gastritis, H. pylori, acid inhibitors) is a known cause of “refractory hypothyroidism” and can be investigated.
Is it worth checking iron, B12, and vitamin D? They are common deficiencies in both thyroid problems and SIBO, and they are measurable.

What this guide does NOT say

To avoid jumping to conclusions:

It does not say that treating SIBO cures Hashimoto’s, or vice versa. There is no evidence of that.
It does not say that everyone with Hashimoto’s has or will have SIBO. Most studies find SIBO in a minority of hypothyroid patients, just in a higher proportion than in controls.
It does not propose stopping or changing levothyroxine. The dose and timing are decided by your doctor.
It does not turn the thyroid into “the cause” of your SIBO. It may be one terrain factor among several; SIBO is usually multicausal.

Disclaimer: This guide is educational material summarizing public sources; it does not replace diagnosis or treatment by a health professional. SIBO and thyroid diseases require individual medical evaluation.

References

Lauritano EC, et al. Association between hypothyroidism and small intestinal bacterial overgrowth. Journal of Clinical Endocrinology & Metabolism, 2007. PubMed / PMC
Wei M, et al. Relationship Between Hypothyroidism, Risk of Small Intestinal Bacterial Overgrowth, and Duodenal Microbiome Alterations. Journal of Clinical Endocrinology & Metabolism, 2025. PubMed · Oxford Academic
Centanni M, et al. Thyroxine in Goiter, Helicobacter pylori Infection, and Chronic Gastritis. New England Journal of Medicine, 2006. NEJM
Virili C, et al. Levothyroxine Therapy in Gastric Malabsorptive Disorders. Frontiers in Endocrinology, 2021. Frontiers
American Thyroid Association. Patient information: hypothyroidism and levothyroxine administration. thyroid.org

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