The connection between your thyroid and IBS (Irritable Bowel Syndrome) is frequently overlooked, but it’s critical to look into for anyone struggling with chronic bloating, constipation, diarrhea, and abdominal pain. Whether you’ve been diagnosed with hypothyroidism, Hashimoto’s thyroiditis, or even hyperthyroidism, your thyroid may be playing a bigger role in your digestive symptoms than you realize.
My name is Katrina and I am a registered dietitian specializing in IBS, SIBO, and the Low FODMAP diet. Too often we isolate the GI system when talking about IBS and SIBO and fail to look at the bigger picture of what else could be impacting our symptoms. In this article, I will cover how thyroid disorders like hypothyroidism and Hashimoto’s can impact IBS symptoms. Learn what every IBS and SIBO patient should know about thyroid function and gut health.
Function of Thyroid
Your thyroid is a small, butterfly-shaped gland located at the base of your neck. Despite its small size, it plays a huge role in keeping your body in homeostasis and running smoothly.
The thyroid produces hormones, primarily T3 (triiodothyronine) and T4 (thyroxine) and your body furthers to convert T3 to T4. The production of these hormones influence how fast or slow your cells work. It essentially functions as you body’s thermostat or engine regulator, controlling:
- Digestion and gut motility
- Metabolism rate
- Heart rate
- Body temperature
- Brain function and mood
- Menstrual cycles and fertility
How the Thyroid Affects Gut Function
One of the main systems that the thyroid is in charge of regulating is the GI system or your gut, specifically how quickly food moves through your digestive tract.
When your thyroid is off, here is how it will present in your gut:
- Impact on Motility: Underactive or sluggish thyroid hormones (also known as hypothyroidism) can slow down your digestive tract, leading to bloating and constipation. The impact a dysregulated thyroid has on motility makes is one of the leading causes of SIBO. (1) In hyperthyroidism (overactive thyroid), motility can speed up too much, contributing to diarrhea, nutrient malabsorption, and gut inflammation. (2)
- Nutrient absorption: Thyroid dysfunction can impair digestion and lead to poor absorption of key nutrients like iron, B12, and zinc, nutrients your gut lining needs to stay strong.
- Immune and inflammation connection: Thyroid disorders, especially autoimmune disorders like Hashimoto’s Thyroiditis, can trigger gut inflammation or increase your risk of gut permeability and gut dysbiosis.
In short, if your thyroid is off, your digestion likely feels off, too. That’s why understanding and supporting your thyroid health is essential for anyone struggling with chronic IBS symptoms or the development of SIBO.
Thyroid Dysfunction Can Drive SIBO
If you do a quick google search on SIBO (Small Intestinal Bacterial Overgrowth), the buzzword most used is “root cause.” The reason behind this is that SIBO just doesn’t appear nor is it something that you can “catch.” It is a symptom of a bigger issue, one that is affecting your digestion and/or motility.
When the thyroid is underactive, the mechanism that is most affected is gut motility. This happens when your Migrating Motor Complex (MMC), the wave-like contractions that clear bacteria and undigested food from the small intestine, slows down. Think of the MMC as being the house keeper of your gut, sweeping undigested food and bacteria throughout your GI tract. When it is not functioning properly, it creates the perfect environment for bacteria to overgrow. That’s why many patients with hypothyroidism are more likely to get SIBO and experience bloating after meals, excessive gas, and poor digestion.
Key takeaway: If you’ve been treated for SIBO multiple times and it keeps coming back, your thyroid could be the missing piece of the puzzle!
Hypothyroidism and IBS
A growing body of research shows a strong link between IBS and hypothyroidism, especially in women. In fact, studies suggest that people with IBS are more likely to have thyroid dysfunction, and vice versa.
Thyroid imbalances often go hand-in-hand with IBS and SIBO. Low thyroid function can slow down gut motility, leading to constipation and bacterial overgrowth. Autoimmune thyroid disease known as Hasimoto’s Thyroiditis, the most common cause of hypothyroidism, can also drive low-grade inflammation that disrupts the gut lining and microbiome. This can also explain why IBS sufferers also report symptoms of fatigue, brain fog, joint pain, and food sensitivities.
As a gut health dietitian, I’ve seen clients finally turn a corner with their symptoms once their thyroid function was properly assessed and supported. Optimizing thyroid health can make a major difference in how your gut feels and functions, even if your doctor says your labs are “normal.”
Symptoms That May Suggest a Thyroid and IBS Link
- Bloating, especially worse in the evening
- Chronic constipation or alternating constipation/diarrhea
- Unexplained fatigue and cold intolerance
- Hair thinning, dry skin, or brittle nails
- Brain fog and mood changes
- A history of autoimmune conditions
Testing Your Thyroid If You Have IBS
If you have IBS and suspect your thyroid may be involved, it’s essential to go beyond a standard TSH test. I recommend a comprehensive thyroid panel that includes:
- TSH (Thyroid Stimulating Hormone): Reflects the signal from your brain to your thyroid, but doesn’t tell us how well your body is using thyroid hormones.
- Free T4 (Thyroxine): Measures the inactive form of thyroid hormone produced by the thyroid gland.
- Free T3 (Triiodothyronine): The active thyroid hormone that powers your metabolism and gut motility. Low levels can mean your body isn’t converting T4 into usable T3 efficiently.
- Reverse T3 (rT3): Acts like a “brake” on thyroid function. Elevated rT3 can block T3 at the cellular level, common in times of stress, chronic inflammation, or nutrient deficiency.
- Thyroid Antibodies (TPO & TgAb): These help detect autoimmune thyroid conditions like Hashimoto’s thyroiditis, which can be a hidden driver of both thyroid and gut issues.
Your average thyroid bloodwork will only test for thyroid stimulating hormone (TSH) which does not give us a complete picture of thyroid health, especially when subclinical hypothyroidism is at play.
Why You Need a Full Thyroid Panel with IBS
If you have IBS and suspect your thyroid may be involved, it’s essential to go beyond a standard TSH (Thyroid Stimulating Hormone) test. While TSH can be a helpful screening tool, it doesn’t give the full picture of how your thyroid is actually functioning, especially when it comes to subclinical hypothyroidism or functional imbalances that can contribute to digestive symptoms like bloating, constipation, or reflux.
Functional vs. Conventional Thyroid Ranges
In conventional medicine, thyroid levels are usually interpreted based on broad lab reference ranges, which are designed to catch overt disease. However, many people fall into a gray area where symptoms are present, but labs appear “normal.”
Functional ranges are often narrower than conventional ones, and as a gut health dietitian, I’ve seen many clients benefit from optimizing thyroid levels, even if their doctor says everything is “normal.”
Functional medicine uses tighter, symptom-based ranges to catch early imbalances and guide supportive care.
Even if your labs fall within the conventional range, symptoms like constipation, fatigue, brain fog, cold intolerance, or slowed digestion may still indicate that your thyroid function isn’t truly optimal
Nutrition Strategies for Supporting Thyroid, IBS, and SIBO
Here are a few dietitian-approved tips that support both your thyroid and digestive system:
- Eat enough protein and selenium-rich foods (e.g., Brazil nuts, eggs, wild-caught fish) to support thyroid hormone production.
- Support optimal digestion + motility. Because many people with dysfunctional thyroids struggle with motility, ensuring you are eating enough fiber – both soluble and insoluble – to support healthy bowel movements is key.
- Support your gut with low FODMAP or SIBO-specific protocols if needed, but always personalize your plan. This is where working with a practitioner is best – my Bloating Breakthrough Group Coaching Program is a great place to start!
- Support your nervous system through stress management. Chronic stress negatively affects both IBS and thyroid health and is a common denominator in both cases.
When to Work With an IBS + SIBO Dietitian
If you’re managing IBS, SIBO, and thyroid dysfunction at the same time, you don’t have to go it alone. As a specialized dietitian, I can help you:
- Navigate conflicting dietary recommendations
- Reduce inflammation, support your immune system, thyroid, and gut health
- Personalize your nutrition plan based on labs, symptoms, and lifestyle
Quit guessing and trying to make do with the pieced together advice you find on social media – it’s time to truly live symptom free!
Final Thoughts: Healing the Gut-Thyroid Connection
Small Intestinal Bacterial Overgrowth (SIBO) is a symptom of a bigger picture. Understanding the thyroid-IBS connection could be the breakthrough you need to rid your SIBO for good. If you’ve been spinning your wheels with bloating, constipation, or recurrent SIBO, it may be time to look beyond the gut and into your thyroid function.
Ready to uncover the root cause of your gut issues?
Schedule a free strategy call with me and take the first step toward lasting relief.
