Hashimoto’s and Gut Issues: Is SIBO Being Overlooked?
If you have Hashimoto’s disease, you already know that managing it can be a complex and difficult task. You track your labs, adjust your medication, pay attention to what you eat, and find yourself constantly wondering if there’s more you should be doing. And you may have discovered that your thyroid isn’t the only thing in your body that needs attention.
For many people with Hashimoto’s, the gut becomes its own separate battle. Bloating that doesn’t quit. Constipation that won’t budge no matter how much water you drink or fiber you eat. Diarrhea that shows up without warning. Food sensitivities that seem to expand over time, seriously limiting your options. Abdominal pain that flares and fades on an exhausting and unpredictable schedule.
And the most frustrating part is that these symptoms often stick around, even when your thyroid labs have improved. In fact, that experience is quite common, so you are not alone. Many Hashimoto's patients continue to deal with gut symptoms even when their thyroid hormone levels are well-managed, and research is starting to explain why.
One piece of the puzzle that often gets missed entirely is a group of conditions collectively called microbial overgrowths: Small Intestinal Bacterial Overgrowth (SIBO), Intestinal Methanogen Overgrowth (IMO), and Intestinal Sulfide Overproduction (ISO), all of which can produce various gut symptoms even when your thyroid levels are normal.
In this article we’ll look at the gut-thyroid connection, why Hashimoto’s can create conditions favorable to microbial overgrowth, and what that could mean for anyone trying to figure out what’s actually driving their ongoing gut symptoms.
What Is Hashimoto’s Disease?
Hashimoto’s thyroiditis (also called autoimmune thyroiditis or chronic lymphocytic thyroiditis) is an autoimmune condition in which the immune system produces antibodies that attack the thyroid gland.[1] Over time, that damage chips away at the thyroid’s ability to produce the hormones that regulate metabolism, energy, body temperature, digestion, and dozens of other functions throughout the body.
Hashimoto’s is the most common cause of hypothyroidism in the United States and other countries where iodine deficiency has been largely eradicated.[2] It affects women significantly more often than men, with women estimated to be up to ten times more likely to develop it, and it’s most commonly diagnosed between the ages of 30 and 50, though it can occur at any age.[3]
One important distinction worth noting: Hashimoto’s disease and hypothyroidism aren’t quite the same thing. Hashimoto’s is the autoimmune condition itself. Hypothyroidism is the state of low thyroid hormone production that often (but not always) results from it. Some people with Hashimoto’s antibodies still have normal thyroid function, especially earlier in the disease. The distinction matters here because much of the research on gut symptoms and microbial overgrowth has been done in hypothyroid populations, and the gut-related mechanisms we’ll get into are most relevant once Hashimoto’s has begun to impair thyroid hormone production.
Despite how common Hashimoto’s is, it often goes undiagnosed or misdiagnosed for years. Its symptoms, such as fatigue, weight gain, constipation, cold intolerance, brain fog, depression, are easy to write off as stress, aging, or other conditions before thyroid testing is even on the table.
There is no cure for Hashimoto’s, but patients have options for managing its effect on their health. When Hashimoto’s is diagnosed, treatment usually involves thyroid hormone replacement, most often with levothyroxine, to bring TSH levels back into the normal range. For many patients, this works well. For others, symptoms persist or worsen over time.
How Hashimoto’s Affects the Gut
The thyroid and the gut are more closely connected than most people realize, and that connection runs in both directions.
Thyroid hormones play a direct role in regulating the pace and rhythm of digestion. They influence the contraction of the smooth muscle that moves food through the digestive tract, and they affect other digestive functions, including secretion and absorption.[4] When thyroid hormone production drops, the effects on the gut can be significant and varied.
Gastrointestinal complaints are among the most commonly reported symptoms in hypothyroidism, and research confirms they aren’t imagined. A peer-reviewed study of the thyroid-gut relationship published in the Journal of Clinical Gastroenterology confirms that abdominal discomfort, flatulence, and bloating are recognized features of hypothyroidism-associated bacterial overgrowth, and constipation can result from diminished intestinal motility leading to slowed or stalled transit.[5]
These symptoms can persist even when thyroid labs are normalized through medication. For those patients, thyroid hormone levels alone may not fully explain ongoing symptoms, and microbial overgrowth becomes an important possibility to consider.
The Motility Connection
To understand why Hashimoto’s patients with hypothyroidism face a higher risk of microbial overgrowth, it helps to look at how the gut normally keeps bacterial populations in check.
Your small intestine is designed to stay relatively low in bacteria. As Dr. Mark Pimentel of the MAST Program at Cedars-Sinai has described, a healthy small intestine works like a fast-moving river, keeping food and bacteria moving through at a regular pace before they can build up and cause problems. The main mechanism for this is the Migrating Motor Complex (MMC), a series of coordinated muscular contractions that sweep through the small intestine roughly every 90 to 120 minutes between meals, clearing out residual food particles and bacteria before they can accumulate.
When that cleaning mechanism gets disrupted, conditions can become favorable for microbes like bacteria and archaea to overpopulate the small intestine, resulting in SIBO, IMO, or ISO.
Hypothyroidism is a well-documented cause of exactly that kind of disruption. Thyroid hormones influence gut motility by regulating the enteric nervous system and modulating the smooth muscle function that drives the MMC.[6] Studies in both animals and humans have shown that hypothyroidism is associated with delayed gastric emptying, decreased frequency of intestinal peristalsis, and slower orocecal transit time, all of which mean food and bacteria spend more time in the small intestine than they should.
One published study put specific numbers to this. Researchers measuring esophageal and gastric motor function in hypothyroid patients found that mean gastric emptying time was nearly 50 minutes, compared to just over 30 minutes in healthy controls. That's a significant difference, and one with direct implications for what happens downstream.[7]
For patients whose Hashimoto’s has progressed to hypothyroidism, this motility disruption may improve but not fully resolve once thyroid medication brings TSH back into range. So, conditions favoring chronic or recurring microbial overgrowth may persist.
What the Research Says About SIBO and Hashimoto’s
Peer-reviewed research is now documenting this connection directly, including work from the Medically Associated Science and Technology (MAST) Program at Cedars-Sinai Medical Center, one of the world's leading research institutions in GI microbiome science.
A 2026 study published in the Journal of Clinical Endocrinology & Metabolism by researchers from the MAST Program analyzed small bowel fluid samples from 49 patients with hypothyroidism and 323 controls enrolled in the REIMAGINE cohort. Using duodenal culture and DNA sequencing, the study found that SIBO was present in almost 33 percent of patients with hypothyroidism, compared to 15 percent of controls. That’s a prevalence more than twice that of the control group.[8]
Notably, the elevated SIBO rate was observed in patients who were already being treated with levothyroxine, with their TSH levels largely normalized. The study found that thyroid medication appeared to mitigate, but not completely eliminate, the excess SIBO risk, suggesting that effective thyroid management can improve, but doesn't necessarily resolve, the gut conditions that make overgrowth more likely for some patients.
The same paper also included a much larger analysis drawn from the TriNetX medical records database, examining ten-year risks of developing SIBO in people with hypothyroidism or autoimmune thyroiditis compared to matched controls. These findings, presented alongside the REIMAGINE results at ENDO 2025, showed that the risk of developing SIBO was 2.2 times greater in patients with hypothyroidism than in matched controls, and 2.4 times higher specifically in those with autoimmune thyroiditis (Hashimoto's disease).
"We now know that people with hypothyroidism, especially those with autoimmune thyroiditis, are more likely to develop SIBO, but this risk appears to be mitigated in those taking thyroid medications," said Dr. Ruchi Mathur, Director of Clinical Operations of the MAST Program at Cedars-Sinai. In other words, the connection between thyroid health and microbial overgrowth is real, and addressing the thyroid side of the equation appears to help, even if it doesn't fully resolve the underlying gut conditions for every patient.[9]
Interestingly, a separate analysis using a large matched cohort, presented at the same meeting, found that the risk of developing Hashimoto's thyroiditis was more than 2.5 times higher in people with SIBO compared to those without it. This was a conference abstract rather than a fully published study, so it's best read as a strong association rather than an established cause-and-effect relationship.[10]
What the research does suggest is that hypothyroidism, autoimmune thyroiditis, and SIBO co-occur at rates substantially higher than those seen in the general population, and patients with one condition have a meaningfully elevated risk of the other.
What Should You Do If You Have Hashimoto's and Ongoing Gut Symptoms?
If your thyroid is being managed but you’re still experiencing gut symptoms like bloating, abdominal discomfort, gas, constipation, or diarrhea, it’s worth looking for more answers.
Start by tracking whether your symptoms respond to thyroid treatment. If they don't move when your labs improve, or if they've been a constant presence regardless of your TSH results, that's meaningful information. It suggests something beyond thyroid dysfunction may be at work, and microbial overgrowth is one of the most important things to rule in or out.
Fortunately, breath testing can identify hydrogen, methane, and hydrogen sulfide gas patterns associated with SIBO, IMO, and ISO through a non-invasive process. When bacteria and archaea ferment carbohydrates, they release gases that travel through the bloodstream and are exhaled through the lungs. A breath sample collected at specific intervals captures those gases and gives you a direct, measurable look at what's happening in your gut.
Not all breath tests measure the same things, and that distinction matters more than most people realize. A test that only measures hydrogen will miss methane and hydrogen sulfide entirely, and one that measures hydrogen and methane still won't catch hydrogen sulfide. For Hashimoto's patients, where constipation is commonly associated with methane elevation and diarrhea patterns may be associated with hydrogen sulfide, those gaps aren't trivial. The Trio-Smart 3-Gas Breath Test is the only at-home SIBO test that measures all three gases simultaneously, giving you and your provider a more complete picture.
If you've spent months or years managing gut issues that no one has been able to fully explain, you're not out of options. The science connecting Hashimoto's, hypothyroidism, and microbial overgrowth is real, it's recent, and it's helping providers better understand a significant possible contributor to persistent gut symptoms.
Breath testing can provide objective information that may help clarify whether microbial overgrowth is contributing to symptoms. For some patients, that additional clarity can meaningfully change the direction of their care.
If you think breath testing might be right for you, take our short quiz or visit triosmartbreath.com to learn more.
Frequently Asked Questions
Can Hashimoto’s disease cause digestive problems?
Yes. Thyroid hormones help regulate gut motility and other aspects of digestion. When Hashimoto’s progresses to hypothyroidism, the resulting drop in thyroid hormone activity can disrupt motility in ways that lead to bloating, constipation, diarrhea, gas, and abdominal discomfort.
Why do I still have gut symptoms when my thyroid levels are normal?
Thyroid hormone replacement can normalize TSH and help with many symptoms, but it doesn’t always fully restore gut motility to its most regulated state. Hashimoto’s and hypothyroid patients also face an elevated risk of developing SIBO, which can produce ongoing gut symptoms independently of thyroid function. If your thyroid labs are well-controlled and you’re still symptomatic, other contributing factors deserve investigation.
Can Hashimoto’s cause bloating?
Yes. Bloating is one of the most commonly reported GI symptoms in Hashimoto’s and hypothyroidism. It can result from slowed gut motility, microbial overgrowth, and other digestive changes. SIBO, IMO, and ISO can all contribute to bloating, with each pattern more closely tied to particular symptom profiles.
Can Hashimoto’s cause constipation?
Yes. Constipation is among the most frequently reported gastrointestinal complaints in people with hypothyroidism. Thyroid hormones regulate the smooth muscle contractions that move stool through the colon, and when hormone activity drops, intestinal transit slows. Intestinal Methanogen Overgrowth (IMO), which produces methane that further slows gut motility, may also contribute when constipation persists despite thyroid treatment.
Can Hashimoto’s cause diarrhea?
Yes, though diarrhea is less commonly associated with Hashimoto’s than constipation. Some patients with hypothyroidism experience accelerated colonic transit as a secondary response to slowed small intestinal motility. Others may have diarrhea-predominant symptoms tied to SIBO or ISO, two overgrowth patterns associated with diarrhea, urgency, and abdominal pain.
Does Hashimoto’s cause SIBO?
Research supports a meaningful association between Hashimoto’s, hypothyroidism, and SIBO. Studies from the MAST Program at Cedars-Sinai have found that the risk of developing SIBO is more than twice as high in people with hypothyroidism compared to healthy controls, and 2.4 times higher specifically in those with autoimmune thyroiditis. The leading explanation is hypothyroidism-induced slowing of gut motility, which impairs the Migrating Motor Complex’s ability to keep bacterial accumulation in check.
How do I know if my gut symptoms are from Hashimoto’s, IBS, or SIBO?
Distinguishing between them based on symptoms alone is genuinely difficult, because they overlap significantly. A practical place to start is asking whether your symptoms have responded to thyroid optimization. If they haven’t, or if they’re present despite well-controlled thyroid labs, that warrants further investigation. Breath testing for SIBO, IMO, and ISO is a non-invasive, evidence-based way to find out whether microbial overgrowth is part of the picture.
Can SIBO be mistaken for IBS in Hashimoto’s patients?
Yes. SIBO and IBS share a meaningful amount of symptom overlap. Bloating, abdominal pain, gas, diarrhea, and constipation are common to both. Research has shown that a significant proportion of patients with an IBS diagnosis test positive for SIBO on breath testing. In Hashimoto’s patients, where the risk of microbial overgrowth is independently elevated, symptoms attributed to IBS may in some cases be driven in whole or in part by an underlying overgrowth that hasn’t been tested for.
Should I get a breath test if I have Hashimoto’s and chronic gut symptoms?
If you have Hashimoto’s, your thyroid is being treated, and you’re still experiencing unexplained or persistent gut symptoms, a breath test for SIBO, IMO, and ISO is worth considering or discussing with your healthcare provider. It’s non-invasive, can be done at home, and provides objective data that can help identify whether microbial overgrowth is contributing to your condition. For patients who have spent months or years chasing gut symptoms without resolution, it can be a meaningful and scientifically grounded next step.