The SIBO and Hypothyroidism Connection: How Gut Health Plays a Role

If you have hypothyroidism or Hashimoto's and your digestion has never felt right, you are not imagining the connection. I see this often in my practice: "The gut is the gateway to health." Your gut is the foundation of your whole body's health because 80% of your immune system resides there. Without a healthy gut, you can't have a healthy immune system. Small intestinal bacterial overgrowth, or SIBO, was one of the most common gut issues I saw in my hypothyroid and Hashimoto's patients, and the research backs this up: there is a strong, well-documented correlation between the two. (1)

I want to show you how your thyroid and your gut influence each other, why addressing SIBO is often the missing piece when thyroid symptoms will not resolve, and how I approach both through functional medicine. I cover this relationship in even greater depth in my book, “The Thyroid Connection™”.

The Gut-Thyroid Connection

Your gut and thyroid are in constant conversation. Roughly 20% of your inactive T4 is converted into active T3 in your gut, which means the state of your gut directly shapes how much usable thyroid hormone your body actually produces. When your gut is compromised, your thyroid feels it.

SIBO occurs when bacteria that belong throughout your digestive system overgrow in your small intestine. As I often say, too much of a good thing can be a bad thing. If you want the fundamentals on each condition first, my articles on SIBO symptoms and causes and hypothyroidism versus hyperthyroidism are great starting points. Here, we are focused on how the two feed into each other.

How Hypothyroidism Contributes to SIBO

Your thyroid regulates every metabolic process in your body. When it is underactive, as in hypothyroidism, all of those processes slow down, including your digestion and gut motility. That sluggish movement has real consequences:

• Food lingers longer in your intestines, creating a breeding ground for bacteria to overgrow.
• Slowed motility disrupts the natural "cleaning wave" that normally sweeps bacteria out of your small intestine.
• Many hypothyroid patients also have low stomach acid (HCl), which impairs digestion and allows bacteria to flourish where they should not.

Together, these factors help explain why hypothyroidism so often sets the stage for SIBO. (3)

How SIBO Worsens Hypothyroidism

The relationship runs in both directions. When your gut flora is disrupted by overgrowth, that critical conversion of T4 into active T3 is reduced, which can deepen hypothyroid symptoms even if you are taking supplemental thyroid hormone. (2)

SIBO also contributes to leaky gut, one of the primary triggers of autoimmunity. When the tight junctions of your small intestine open up, toxins, microbes, and partially digested food particles escape into your bloodstream, prompting your immune system to respond with inflammation. This is a key driver of Hashimoto's thyroiditis, the autoimmune form of hypothyroidism. To understand that link more fully, read: Is hypothyroidism an autoimmune disease?

This creates a chicken-and-egg cycle. Sometimes hypothyroidism comes first and drives the overgrowth. Other times SIBO comes first and worsens thyroid function. Because they so frequently appear together, I recommend testing for SIBO any time both symptom sets are present.

Can Hypothyroidism Cause Constipation, Diarrhea, and Other Gut Problems?

Yes. Hypothyroidism can cause a wide range of gastrointestinal problems, largely because it slows digestion and so often overlaps with SIBO. 

The most common include:

• Constipation: The most frequent digestive symptom, caused by slowed gut motility.
• Diarrhea: Often tied to SIBO, where certain bacteria break down bile salts before your body can use them, leading to fat malabsorption.
• Bloating, gas, and abdominal discomfort: Byproducts of bacteria fermenting food in the small intestine.
• Nutrient malabsorption: Overgrowth can interfere with absorption of B12 and other essential vitamins and minerals.

If these symptoms sound familiar and you also have thyroid symptoms, that overlap is a strong clue that your gut and thyroid need to be addressed together, not separately.

The Functional Medicine Approach to SIBO and Hypothyroidism

Before starting any protocol, it helps to know where you stand. My SIBO quiz can help you gauge whether you are likely positive. From there, I follow a 3-step approach:

Step 1: Starve the overgrowth. Remove the sugar, alcohol, and refined carbohydrates that feed bacteria in the small intestine. My SIBO Foods to Enjoy and Foods to Toss list makes this simple.

Step 2: Disrupt and prune back. Biofilm Disruptor breaks down the protective layer that shields bacteria, then Microb-Clear®, a blend of magnesium caprylate, berberine, and herbal extracts, prunes back the overgrowth with a gentler approach than broad-spectrum antibiotics.

Step 3: Restore and balance. Primal Earth Probiotic, a soil-based probiotic, restores balance without feeding the overgrowth. This step matters enormously for SIBO, because most probiotics contain Lactobacillus, and SIBO is often an overgrowth of Lactobacillus. Finally, ImmuneIG balances the gut immune environment with beneficial immunoglobulins.

You can follow the disrupt-and-restore steps conveniently with my SIBO Solution™ Protocol, and you can watch me walk through the protocol here. All of my supplements are physician-formulated and free of gluten, wheat, dairy, corn, soy, GMOs, and yeast. Pair this gut work with a nutrient-dense hypothyroidism diet to give your thyroid the foundation it needs.

Heal Your Gut to Support Your Thyroid

Your gut is the foundation everything else builds on. This is also a good time to make sure you're covering your daily foundation: I recommend the Essential 4 Supplements, a Complete Multivitamin, Fish Oils / Omega-3, a Multi-Strain Probiotic, and Vitamin D with Vitamin K2, for everyone, from 1 to 101 years old.

FAQs

• Can hypothyroidism cause constipation? 
• Yes, constipation is one of the most common digestive symptoms of hypothyroidism. Your thyroid regulates your metabolism, so when it is underactive, every process slows down, including the muscle contractions that move food through your digestive tract. This slowed motility means waste lingers longer in your intestines, which causes constipation and also creates an environment where bacteria can overgrow and contribute to SIBO. Supporting both your thyroid and your gut, rather than treating the constipation in isolation, is the most effective approach.
• Can hypothyroidism cause diarrhea and other stomach problems? 
• Yes. While constipation is more common, hypothyroidism can also cause diarrhea, bloating, gas, and abdominal discomfort, often when SIBO is involved. Certain overgrown bacteria break down bile salts before your body can use them to digest fats, leading to fat malabsorption and diarrhea. Others interfere with nutrient absorption, which can leave you deficient in B12 and other key nutrients. These overlapping gut symptoms are a strong signal that your thyroid and gut health are connected.

• How are SIBO and hypothyroidism connected? 
• The relationship runs in both directions. Hypothyroidism slows digestion and gut motility, creating conditions where bacteria overgrow in the small intestine. In turn, SIBO reduces your gut's ability to convert inactive T4 into active T3 and contributes to leaky gut, a primary trigger of the autoimmune form of hypothyroidism, Hashimoto's thyroiditis. Because each condition can drive the other, they frequently appear together, which is why I recommend investigating SIBO whenever someone has both digestive and thyroid symptoms.
• Will treating SIBO improve my thyroid function? 
• Addressing SIBO can meaningfully support your thyroid, because a healthier gut improves the conversion of T4 into active T3 and reduces the gut-driven inflammation that fuels autoimmune thyroid conditions. Resolving SIBO is not a replacement for working with your practitioner on supplemental thyroid hormone if you need it. Instead, healing your gut removes one of the root-cause obstacles standing between you and optimal thyroid function, which is the heart of the functional medicine approach.
• What is the best way to address SIBO when you have hypothyroidism? 
•  The approach typically focuses on addressing the bacterial overgrowth while also supporting thyroid function. This may include dietary changes that limit foods that can contribute to overgrowth, followed by strategies to support a balanced gut microbiome. Because SIBO and hypothyroidism can affect each other, work with a qualified healthcare professional to determine an individualized approach and address underlying factors that may be contributing to your symptoms.

Resources

1. Cedars-Sinai Study Links Hypothyroidism to GI Disorder. Cedars-Sinai. Accessed on July 21, 2026.
2. Small Intestinal Bacterial Overgrowth Syndrome and Thyroid Function. PMC. Accessed on July 21, 2026.
3. Hypothyroidism and Its Association With Small Intestinal Bacterial Overgrowth. The Endocrine Society. 2025. Accessed on July 21, 2026.