Do you have an under-active thyroid and struggling with ongoing digestive issues? Then this post is for you.

In my practice, I see many women suffering with digestive issues that coincide with thyroid disease. These women are experiencing ongoing digestive issues, such as bloating, constipation, diarrhoea, indigestion, reflux, flatulence, low iron and vitamin B12. This coupled with fatigue, cold extremities, weight gain, hair loss, brain fog, low sex drive (if any) can become a pretty soul destroying picture. Believe me I know! Sadly, these women are not getting the right support. Just receiving an early diagnosis with hypothyroidism, appears to be a lucky start. I see many women with Irritable Bowel Syndrome (IBS), unaware that they have hypothyroidism or hashimotos.


What is hypothyroidism?

Your thyroid is the butterfly-shaped organ wrapped around your trachea (breathing tube) at the base of your neck. Your thyroid releases hormones that control metabolism and thereby regulate the way that your body uses energy. It is involved in many functions such as breathing, heart rate, body weight, temperature control, muscle strength, cholesterol and more.

Hypothyroidism is a common endocrine condition where your thyroid is either not producing enough thyroid hormone or not getting sufficient levels of thyroid hormone to the target tissues around your body. The thyroid’s involvement with other bodily systems, means that it can affect your gut, skin, hair, cardiovascular system, brain, female hormones, mood and more. Hashimotos, an autoimmune condition, can also cause hypothyroidism and present in the same way.

Key symptoms of Hypothyroidism

  • Fatigue
  • Constipation
  • Always feeling cold
  • Dry skin
  • Brain fog
  • Weight gain/difficulty losing weight
  • Hair loss
  • Muscle pain
  • Joint pain
  • Brittle nails
  • Decreased sex drive
  • Low mood

Hypothyroid Symptoms

What is SIBO?

In this post about SIBO and IBS, I explain that SIBO is a condition where non-pathogenic bacteria, that should reside in your large intestine, accumulate in your small intestine. A healthy small intestine should have significantly less microbes compared to the large intestine. There are many potential causes for SIBO and associated risk factors.

Key symptoms of SIBO

  • Bloating – physical swelling
  • Belching & Flatulence (gas exiting)
  • Pain – intestines sensitive to pressure
  • Diarrhoea (Hydrogen dominant bacteria)
  • Constipation (Methane dominant bacteria)
  • Both diarrhoea & constipation (both hydrogen and methane)
  • Gastrointestinal Reflux Disorder (GERD)
  • Nausea (where the food sits in the stomach)
  • Fatigue
  • Anxiety
  • Brain Fog
  • Underweight (for the majority) although some might be above normal weight
  • Loss of appetite
  • Uncomfortable feeling of fullness after eating
  • Anaemia
  • Fatty Stools
  • Bladder irritation


Hypothyroidism and SIBO

The connection between SIBO and the thyroid?

Gut and thyroid health are very closely connected. When your thyroid is not performing as it should, metabolic processes are often one of the first areas that can be affected. You start to notice changes in your bowel habits, changes in digestive symptoms as absorption becomes impacted and changes in your weight. Furthermore, you also start to experience the dreaded brain fog.  Anyone with hypothyroidism or hashimotos will know exactly what I mean. I can genuinely say that it is one of the worst aspects of living with thyroid disease! Brain fog all comes back to the gut. Yes, that’s right, we are back to that gut-brain axis thing again.

So why are people with hypothyroidism at greater risk for developing SIBO?

The answer is simple. Slow digestion, slow motility, slow migrating motor complexes, low stomach acid and poor bile flow and insufficient enzymes to digest food. These all put you at greater risk for having more bacteria in you small intestine than you should, which can result in SIBO.

In order to understand the connection between hypothyroidism and SIBO, I want to briefly discuss some key factors present in hypothyroidism that can lead to SIBO.

Factors in hypothyroidism that can lead to SIBO

Reduced Motility

Gastrointestinal (GI) motility is regulated by not just your autonomic and enteric nervous systems (ENS), but also by your endocrine system. As mentioned before, your thyroid is a key part of your endocrine system. Thyroid hormones actually influence gut motility by modulating the enteric nervous system and altering smooth muscle function.

It is therefore not surprising that people with hypothyroidism present with altered intestinal motility and transit time. In fact, abnormalities in GI motility is the primary cause for constipation in hypothyroidism and diarrhoea in hyperthyroidism (when the thyroid is making too much thyroid hormone). Studies in both humans and animals have shown that hypothyroidism is associated with reduced motility, delayed gastric emptying, decreased peristalsis and slower transit time.

One study evaluated the impact that hypothyroidism had on gastroesophageal motility. The study compared gastric motility in 30 women with hypothyroidism to a control group of 10 healthy women. The results proved that hypothyroidism reduced oesophageal and gastric motor activity.

This reduction in motility can ultimately result in GI dysfunction and is a major risk factor for developing SIBO.

I would also like to add that whilst the majority of people with hypothyroidism will present with constipation, some might actually present with diarrhoea. This could be the result of fluctuating between hyper and hypothyroidism or the presence of hydrogen dominant SIBO. Slow orocecal transit time, followed by rapid colonic transit time is a key indication of carbohydrate malabsorption and will present as diarrhoea.

This is one of many reasons why testing for SIBO is important when people with hypothyroidism present with diarrhoea.

Under functioning Migrating Motor Complexes

Your gut has a complex network of millions of neurons, that together with other nerve cells make up the enteric nervous system (ENS). During times of fasting the ENS send out electrical waves through the small intestine to move contents into the colon. These waves are called Migrating Motor Complexes (MMC) and are crucial for keeping bacteria out of your small intestine and in the colon where they belong. They are activated every 90-minutes during periods of fasting.

Various factors such as stress, antibiotics, medication, gastrointestinal surgery, anatomical abnormalities, intestinal diverticula or fistula, inflammation, autoimmune disease and poor thyroid function can affect MMC.

Numerous studies in both humans and animals show that hypothyroidism is associated with slower digestion and peristalsis, delayed gastric emptying and slower orocecal transit time (a key measure for SIBO).  This often results in constipation, which is one of the most common symptoms I see in clients with hypothyroidism.

The function of your MMC is one of the most important factors to consider when looking at the health of your gut and the development of SIBO. When your MMC doesn’t work effectively, you start to run into problems with not clearing out bacteria from the small intestine and this can result in SIBO. In hypothyroidism, the function of your MMC might already be compromised, which can put you at greater risk for developing SIBO.

Pancreatic enzymes and bile

The cause of pancreatic insufficiency in patients with hypothyroidism is not precisely known or understood. This is definitely something that I see in clinic and also something that I have personal experience with.

One rational is the role that thyroid hormones play in the transportation of amino acids (proteins) in several tissues. A decrease a thyroid hormone, as seen in hypothyroidism, can therefore lead to a decrease in pancreatic amino acid uptake. The thyroid also stimulates the pancreas to produce pancreatic enzymes, potentially leading to a reduction in thyroid hormone.

Another important aspect to consider especially in cases where SIBO has already established, is pH balance. Pancreatic enzymes actually require a high pH to work. This is why the pancreas secretes sodium bicarbonate with the digestive enzymes that it produces. If hydrogen producing bacteria are present in the small intestine, they can lower the pH in order to make their environment more acidic. In doing so these hydrogen bacteria create an environment with more undigested food for them to feed on, resulting in more overgrowth and a vicious cycle.

It appears that there is a link between hypothyroidism and the gallbladder, especially common bile duct stones. Some studies conclude that hypothyroidism results in changes in bile composition as well as bile excretion rate. Delayed emptying of the biliary tract can result in common bile duct stones. Bile is a really important way for the body to remove bacteria and yeast from your intestines. Slow bile flow can not only result in sluggish digestion and constipation, but a build-up of bacteria in the small intestine, which can lead to SIBO.

Stomach acid

Optimal stomach acid is a really important aspect of healthy digestion and immune function. Every time you eat, the parietal cells that line your stomach produce stomach acid. When food hits your stomach, your stomach acid will start to begin the digestion process and support the breakdown of proteins and most minerals and prepare for the absorption of key nutrients such as iron, B12, Vitamin D and more. Stomach acid also plays a key role in keeping your gut microbe free as the right acidity prevents parasites and other pathogens from residing in the stomach and gut.

So how does Hypothyroidism impact your stomach acid? Like with everything in hypothyroidism, things are slowed down and it can impact the number of parietal cells as well as their ability to produce stomach acid, which can then lead to hypochlorhydria, low stomach acid.

Whilst low stomach acid might sound like a good thing if you suffer from reflux, believe me when I say that reflux is actually mostly the result of LOW stomach acid.

I personally struggled with low stomach acid for years and it is still something that I need to closely monitor and work on from time-to-time. It is actually estimated that around 80+% of the population suffer with hypochlorhydria. Hypothyroidism can contribute to low stomach acid, and conversely, low stomach acid can lead to SIBO, hypothyroidism and autoimmune disease.

Conversion of T4 to T3

Bacteria in your gut play a key role in assisting with the conversion of inactive thyroid hormone (T4) to its active form (T3). Without sufficient gut flora, this conversion becomes challenging and can lead to lower levels of T3 available for your body to use. You might start to experience fatigue, bloating (imbalanced bacteria), intolerance to cold, weight gain and brain fog. These can all be signs that your T3 levels might be low. Working with the microbiome and gut bacteria is a key part in optimising thyroid function.

When dysbiosis, an imbalance in bacteria, is present, you are at greater risk for developing SIBO. When SIBO is present, you are more likely to experience problems with converting T4 to T3, which can further exacerbate your hypothyroid symptoms.

SIBO in Hypothyroidism

It seems pretty clear that there is a really strong connection between hypothyroidism and SIBO. In fact, it is estimated that SIBO may be present in more than half  of patients with hypothyroidism. This begs the question why are so few doctors or practitioners testing for it?

I myself struggled with SIBO and was completely unaware that my underactive thyroid was a key force behind my SIBO. The good news is that despite still having Hashimotos, my health is so much better. I now have energy, significantly less brain fog, my sleep is amazing (most of the time) and so is my digestive health. I no longer suffer with SIBO or digestive symptoms and continue to remain SIBO-free.

If you have hypothyroidism or hashimotos and feel fed-up of dealing with persistent fatigue, intolerance to cold, brain fog, hair loss and ongoing unresolved digestive symptoms, I can help. My own personal journey with SIBO and Hashimotos enables me to support women just like you to improve your symptoms and regain your quality of life.

If you want to know what steps to take next, get in touch.




Nutritional Therapist Cheshire, Health, Nutritionist Cheshire, Functional Medicine Cheshire, Rootcause Solution