The Thyroid-Gut Connection: How Gut Health Affects Thyroid Health

Thyroid disorders are common in the United States, though they frequently go undiagnosed. 

Treatments for common thyroid disorders like hypothyroidism and hyperthyroidism generally aim to normalize hormone levels, but the root cause of the dysfunction often goes undiagnosed.

A growing body of research is finding that gut issues, including intestinal permeability (sometimes called “leaky gut”) and dysbiosis (an imbalance between beneficial and pathogenic bacteria within the gut), may often underlie thyroid disorders. 

The gut-thyroid connection constitutes a bidirectional relationship, where poor gut health may lead to poor thyroid function, and poor thyroid function may lead to poor gut health.

Restoring your gut health, chiefly through reducing intestinal permeability triggers and supplementing with probiotics, may be an important factor when it comes to improving thyroid health. 

An overview of thyroid disorders 

The thyroid, a small gland found at the base of the neck, is a vital component of the endocrine system, regulating metabolism, digestion, and mood, among many other functions. 

Disorders related to thyroid function are common in the United States, affecting approximately 12% of the population at some point in their lives(1). 

However, because the thyroid and the hormones it produces are involved in so many different functions throughout the body and can mimic the symptoms of other conditions, thyroid disorders frequently go undiagnosed. 

In fact, an estimated 60% of those with thyroid dysfunction are unaware of the issue(1). 

The two most common types of thyroid disorder are hypothyroidism (underactive thyroid or insufficient thyroid hormones) and hyperthyroidism (overactive thyroid or excess thyroid hormones). 

Common symptoms of hypothyroidism: 

• Fatigue 
• Sensitivity to cold temperatures
• Dry skin 
• Constipation 
• Dry or thinning hair 
• Unexplained weight gain 
• Depression 
• Muscle weakness 

Common symptoms of hyperthyroidism: 

• Nervousness or anxiety 
• Unexplained weight loss
• Irregular or rapid heartbeat; heart palpitations 
• Frequent bowel movements 
• Sensitivity to heat 
• Excessive sweating 
• Swelling of the thyroid gland 
• Fatigue 

The symptoms listed above are just a sampling, and individuals may experience any combination of these or other symptoms. 

Treatment of thyroid disorders 

The standard treatment approach to hypothyroidism, the most common type of thyroid disorder, is to take thyroid medications or synthetic hormones(2). Similarly, hyperthyroidism is often treated with antithyroid medications to lower thyroid hormone levels(3).

Thyroid hormones, including T4, TSH, and sometimes T3, are tested regularly, and medications are often adjusted until levels fall within a normal range.

While this approach may help normalize lab results, medication alone does not address the root of the problem, as it does not consider what may be causing the impaired thyroid function in the first place. 

The gut-thyroid connection 

Healing the gut may be a key piece of the puzzle when it comes to improving thyroid function(4). Similarly, addressing impaired thyroid function may help improve digestive health. 

One thing that is not always understood about thyroid disorders is that they are generally autoimmune in nature(5). In other words, the thyroid itself is not the problem; the immune system is the problem. 

When we look back at what may be causing the chronicinflammation and autoimmunity that led to an underactive or overactive thyroid, it’s often there that we find gut issues(6). 

By restoring gut health, we may be able to remedy chronic inflammation and autoimmunity, in turn improving thyroid health. 

Leaky gut and Hashimoto’s hypothyroidism 

Hypothyroidism, a condition in which the thyroid does not produce sufficient hormones, is the most common thyroid disorder in the United States. 

The vast majority of hypothyroidism cases (approximately 90%) are caused by an autoimmune condition called Hashimoto’s(7). 

In autoimmune disorders, the immune system attacks the “self,” or the body’s own tissues. In the case of autoimmune hypothyroidism (Hashimoto’s disease), the immune system is attacking the body’s thyroid tissue, mistaking it for a threat. 

This helps explain one of the most prominent links between gut health and thyroid health, as autoimmunity is often accompanied by poor gut health(6). 

The gut and the immune system   

To understand how gut health can contribute to autoimmunity, it’s important to first examine the role a healthy digestive system plays in immune system function. 

Approximately 70% of the body’s immune tissue resides in the gut(8). This tissue is known as Gut-Associated Lymphoid Tissue or GALT. Your GALT stores immune cells involved in launching attacks and producing antibodies when faced with potential threats or antigens. 

In a healthy digestive system, the gut lining allows nutrients to enter the body but stops foreign substances (including undigested food particles and bacteria) from entering the bloodstream. 

Intestinal permeability and autoimmunity 

Things start to go wrong when the lining of the intestine becomes compromised. This is known as intestinal permeability or “leaky gut.” 

Several factors can contribute to intestinal permeability, including: 

• Diets high in sugar and refined carbohydrates(9) 
• Low fiber diets(10) 
• Chronic stress(11)
• Use of NSAIDs (e.g., Advil, Aleve)(12)
• Use of antibiotics(13)

When the intestinal lining is compromised, foreign proteins and substances can escape into the body and bloodstream(14). These substances may include harmful bacteria, infectious agents, and other pathogens, but even seemingly innocuous food particles can be problematic. 

Outside of the gut, undigested food particles in the bloodstream are foreigners, just as pathogenic bacteria are foreigners. The immune system, therefore, will respond by producing antibodies and launching an attack. 

The immune system will create antibodies specifically designed to destroy a new foreign substance. If the foreigner returns, the antibodies will recognize it as a known threat and flag it for an attack. Over time, this may lead to sensitivities or intolerances to certain types of food. 

This is also where chronic inflammation and autoimmunity may emerge. The immune system may confuse the body’s cells and tissues with the foreign substances it has learned to attack. 

Connecting leaky gut and autoimmune thyroid disorders 

As described above, intestinal permeability can lead to the development of Hashimoto’s and other autoimmune conditions(15). Grave’s disease, the most common type of hyperthyroidism, is another autoimmune condition that may develop in the same way.  

The connection appears to go both ways, as the thyroid hormones T3 and T4 may also play a role in protecting the gut lining(16,17). 

Molecular mimicry 

It may come as a surprise that the immune system can mistake the body’s tissues for something as seemingly different as a food particle, but their molecular structures can be quite similar. This phenomenon is known as molecular mimicry(18). 

When it comes to molecular mimicry and autoimmune thyroid disorders, gluten may be of particular concern, given its structural resemblance to thyroid tissue(19). 

For this reason, some practitioners may recommend eliminating gluten in all cases of autoimmune thyroid disease. 

Research has also shown that individuals with Celiac Disease are more likely to have an autoimmune thyroid disorder, and vice versa(20). 

Gut dysbiosis and the thyroid 

Patients with thyroid disorders often present altered gut microbiomes compared to healthy individuals(4). In particular, those with hyperthyroidism tend to have lower levels of beneficialBifidobacteriaandLactobacillispecies in the gut, and higher levels of potentially harmfulEnterococcusspecies(21). 

Gut bacteria are also an identified factor in the development of Hashimoto’s(22). 

There is also a connection between thyroid disorders and Small Intestinal Bacterial Overgrowth (SIBO), a condition in which bacteria that should primarily reside in the large intestine colonize within the small intestine, leading to digestive symptoms including bloating and gas. 

Those with autoimmune hypothyroidism are significantly more likely to test positive for SIBO than those with healthy thyroid function(23). 

The role of gut bacteria in T3 conversion 

The thyroid itself primarily produces the hormone T4, which is inactive(24). Once released, it relies on other tissues to convert T4 into T3, the active form of the thyroid hormone. Active T3 is what we need for healthy metabolism, energy, body heat, and many other functions. 

Approximately 20% of the conversion from T4 to T3 depends upon the supply of healthy gut bacteria(25). If there is an imbalance between beneficial and harmful bacteria within the gut microbiome (known as dysbiosis), T3 production may slow down. 

This is another example of how an individual may have symptoms of hypothyroidism, even if the thyroid itself is otherwise healthy. 

Because hypothyroidism can contribute to digestive issues including constipation, this can turn into another vicious cycle(26). 

How thyroid function affects digestive health  

So far, we have focused primarily on how gut health affects thyroid function. However, we know that the relationship between the gut and the thyroid is bidirectional. So, how does thyroid function affect digestive health? 

Thyroid hormones play an important role in digestion, and many people with thyroid conditions experience digestive symptoms. Hypothyroidism slows down many of the body’s regular functions, including digestive motility, which is why many individuals with low thyroid function experience constipation(26). 

In the case of hyperthyroidism, the opposite is true: many systems within the body speed up, and you may experience more frequent bowel movements(27).

Hypothyroidism and low stomach acid

There is also a link between hypochlorhydria (low stomach acid) and hypothyroidism(28). An underactive thyroid may contribute to low stomach acid, and low stomach acid can, through its impact on digestion, lead to hypothyroidism. 

The gallbladder connection

The gallbladder, which stores bile for digestion, is an integral part of the digestive system. Research shows that hypothyroidism can lead to a sluggish gallbladder and reduced bile flow(29). A sluggish gallbladder can interfere with the liver’s ability to detoxify(30), which may perpetuate a vicious cycle by reducing the conversion of T4 to T3, a process in which the liver plays an important role. 

Tips for optimal gut-thyroid health  

Supporting a healthy gut may help to improve thyroid function, and there are several steps you can take to improve the health of both the gut and the thyroid at the same time. Here are a few suggestions:

• Remove or reduce “leaky gut” triggers. Chronic stress, sugar, refined carbohydrates, antibiotics, and NSAIDs (including Advil and Aleve) are among the factors that can lead to increased intestinal permeability. Such permeability can contribute to autoimmune thyroid conditions like Hashimoto’s. Where possible, aim to reduce or remove as many of these triggers as you can.
  
• Take probiotics. Although studies looking at probiotics specifically for thyroid function are limited, the gut microbiome plays a demonstrated role in immune system and thyroid function(31,4). Therefore, supporting the gut microbiome with high-quality probiotics may be one way to improve both gut health and thyroid health.
  
• Consider gut microbiome testing. If you suspect a gut imbalance is causing a thyroid issue, you may consider taking a gut microbiome stool test to pinpoint the root of the problem.

In summary 

Thyroid disorders, including hypothyroidism (an underactive thyroid or low levels of thyroid hormones) and hyperthyroidism (an overactive thyroid or high levels of thyroid hormones), are generally treated with medications that help normalize lab results. 

However, medication alone does not necessarily help determine or treat the root cause of thyroid conditions. 

Thyroid disorders are often autoimmune conditions in which the immune system, rather than the thyroid itself, is the problem. As autoimmunity is closely linked to poor gut health, healing the gut may help manage or resolve thyroid disorders. 

The gut-thyroid connection goes two ways, and thyroid dysfunction can also have a negative effect on digestion and gut health, leading to a vicious cycle. 

Restoring gut health through diet, lifestyle modifications, and high-quality probiotics may improve thyroid function and put an end to the cycle. 

Ellie Ellias -Contributing Writer, Physician’s Choice 

Up next: Understanding the Gut-Hormone Connection