No. 18 Gut x Thyroid

Jun 19

Written By Elle Cole

**What Is the Thyroid, and Why Does It Matter?**

Before diving into the thyroid-gut connection, it’s important to understand the role of the thyroid itself.
The thyroid is a small, butterfly-shaped gland located at the base of your neck. Don’t let its size fool you—it’s one of the most powerful regulatory organs in your entire body.

The thyroid produces hormones, primarily thyroxine (T4) and triiodothyronine (T3), that help control your:

Metabolism (how your body uses energy)
Heart rate
Body temperature
Digestion
Mood and mental clarity
Reproductive health
Muscle and bone function

It does all this by interacting with virtually every cell in your body, acting like a master thermostat.

The Hormonal Chain of Command

Your thyroid doesn’t act alone. It's part of a tightly controlled feedback loop known as the hypothalamic-pituitary-thyroid (HPT) axis. The hypothalamus in your brain releases TRH (thyrotropin-releasing hormone), which tells the pituitary gland to release TSH (thyroid-stimulating hormone), which in turn tells your thyroid to produce T3 and T4.

These hormones then circulate through the bloodstream and regulate vital processes. When their levels drop or rise too much, your brain adjusts the signaling in response—unless something disrupts the system.

Common Thyroid Disorders (and How Common They Really Are)

Thyroid dysfunction is incredibly common—especially among women. According to the American Thyroid Association:

More than 12% of the U.S. population will develop a thyroid condition during their lifetime.
An estimated 20 million Americans currently have some form of thyroid disease.
Up to 60% of those affected are unaware of their condition.
Women are 5 to 8 times more likely than men to have thyroid problems.

The two most common thyroid disorders are:

Hypothyroidism (Underactive Thyroid)

This occurs when the thyroid doesn’t produce enough hormones. It slows down metabolic processes, which can lead to:

Fatigue
Weight gain
Depression
Cold intolerance
Constipation
Brain fog
Dry skin
Menstrual irregularities

The most common cause of hypothyroidism in developed countries is Hashimoto’s thyroiditis, an autoimmune condition where the immune system attacks the thyroid.

Prevalence: Hypothyroidism affects about 4.6% of the U.S. population age 12 and older (NHANES data).

Hyperthyroidism (Overactive Thyroid)

This occurs when the thyroid produces too much hormone, speeding up the body's processes. Symptoms include:

Anxiety or irritability
Rapid heartbeat or palpitations
Unexplained weight loss
Heat intolerance
Insomnia
Frequent bowel movements or diarrhea
Muscle weakness

The most common cause is Graves’ disease, another autoimmune condition where the immune system overstimulates the thyroid.

Prevalence: Hyperthyroidism affects approximately 1.3% of the U.S. population.

Other Common Complications

Goiter: Enlargement of the thyroid, often from iodine deficiency or autoimmune disease.
Thyroid nodules: Lumps in the thyroid, which are common (up to 65% of people over 60 may have them).
Thyroid cancer: Though relatively rare, it's the most common endocrine cancer, with a rising incidence over the past two decades.
Infertility and miscarriage: Thyroid imbalance can significantly disrupt reproductive hormones.
Depression and cognitive issues: Low thyroid function is closely tied to mood disorders and memory problems.

From these numbers, it’s clear: thyroid dysfunction is widespread, often underdiagnosed, and can affect almost every aspect of physical and mental health.

Now, let’s explore how this tiny gland has an even deeper connection to your gut—and why understanding that relationship might be the key to resolving stubborn symptoms.

The Thyroid-Gut Axis: Unraveling the Bidirectional Link Between Your Thyroid and Digestive Health

Introduction: Your Body’s Most Overlooked Relationship

The human body functions as a network of deeply connected systems. When something is off in one area, chances are it’s affecting another. One of the most powerful—and often under-recognized—connections is the one between your thyroid and your gut, known as the Thyroid-Gut Axis.

This axis operates in both directions. Your thyroid hormones regulate digestive speed, enzyme production, and even the diversity of gut bacteria. Meanwhile, your gut controls the absorption of nutrients your thyroid needs and helps convert thyroid hormones into their active forms. When one is disrupted, the other often follows. Understanding this connection can be the missing piece in resolving lingering symptoms like bloating, fatigue, mood swings, or thyroid imbalance.

How Your Thyroid Affects Your Gut

Slowing (or Speeding Up) Your Digestion

Thyroid hormones influence the pace at which food travels through your digestive system—a process called gut motility. In hypothyroidism, everything slows down. This can lead to constipation, bloating, delayed gastric emptying, and even serious conditions like bowel obstruction. Research shows up to 40% of people with low thyroid function experience significant constipation.

In contrast, hyperthyroidism speeds everything up. The result? Frequent bowel movements, diarrhea, and nutrient malabsorption. Food moves so quickly that the body doesn’t have time to extract and absorb what it needs, especially fats and fat-soluble vitamins.

Functional medicine practitioner Dr. Will Cole notes that hypothyroidism can also weaken muscle function in the digestive tract, contributing to SIBO (Small Intestinal Bacterial Overgrowth), as the slowed movement creates the perfect environment for bacterial overgrowth.

Impaired Nutrient Breakdown and Absorption

Thyroid hormones help your stomach produce acid and enzymes needed for digestion. When thyroid levels are low, acid production can drop, causing incomplete breakdown of food and symptoms like gas, bloating, and indigestion.

Even though hypothyroidism can prolong the time food spends in the gut—potentially increasing surface-level absorption—it still often results in deficiencies of critical thyroid-supporting nutrients like iron, zinc, selenium, iodine, and vitamin D. As Dr. Cole emphasizes, these deficiencies can become both a result of and a contributor to thyroid dysfunction, creating a self-reinforcing loop.

Changing the Microbiome

Thyroid hormones also influence the balance of bacteria in your gut. Hyperthyroidism has been associated with a loss of beneficial microbes like bifidobacteria and lactobacilli, while hypothyroidism is frequently linked with microbial imbalance and overgrowth (particularly in the small intestine).

Because the microbiome regulates inflammation, immunity, and even hormone signaling, changes to microbial diversity can amplify thyroid-related symptoms and hinder recovery.

Damaging the Gut Barrier

One of the most significant downstream effects of thyroid dysfunction is its impact on gut barrier integrity. Low thyroid function is associated with increased intestinal permeability—commonly known as “leaky gut.” When this barrier becomes compromised, toxins and undigested food particles can escape into the bloodstream, triggering immune reactions and systemic inflammation.

This can lead to autoimmune conditions, including Hashimoto’s thyroiditis and Graves’ disease. Dr. Will Cole frequently highlights this relationship, emphasizing that repairing the gut lining is essential for calming autoimmune responses and supporting thyroid recovery.

How Your Gut Affects Your Thyroid

Nutrient Absorption as Thyroid Fuel

The gut is responsible for absorbing the nutrients your thyroid depends on. Even a nutrient-dense diet can fall short if your gut isn’t properly absorbing what you eat.

Nutrients critical to thyroid function include:

Iodine, necessary for hormone synthesis
Selenium and zinc, which convert inactive T4 into active T3
Iron, which supports hormone production and immune defense
Vitamin D, which modulates the immune system and reduces inflammation

Gut issues such as SIBO, IBS, celiac disease, or gastritis can dramatically reduce the absorption of these nutrients. For example, autoimmune gastritis—commonly co-occurring with Hashimoto’s—can impair iron and B12 absorption. As Dr. Cole notes, even subtle gut imbalances can lead to widespread nutrient deficiencies that compromise thyroid function.

Hormone Conversion in the Gut

Your thyroid produces mostly T4, which needs to be converted into the active T3 hormone. While this conversion happens in the liver and peripheral tissues, about 20% occurs in the gut, with the help of the microbiome.

When the gut is inflamed, imbalanced, or undernourished, this conversion can falter—leading to low T3 levels and persistent hypothyroid symptoms, even when bloodwork appears “normal.” Dr. Cole explains that this "low T3 syndrome" often stems not from thyroid failure, but from poor gut-based conversion capacity.

Triggering Autoimmunity

The gut is ground zero for immune education. If your gut lining is damaged or your microbiome is out of balance, it can send confused signals to the immune system. Over time, this can lead to molecular mimicry—where your immune system begins attacking your own tissues, including the thyroid.

This is a known mechanism behind Hashimoto’s and Graves’ diseases, both autoimmune thyroid disorders. Dr. Cole underscores that repairing the gut barrier and rebalancing the microbiome can be key steps in calming the immune system and halting autoimmune progression.

Common Digestive Symptoms That May Point to Thyroid Imbalance

People with thyroid dysfunction often experience gut symptoms—even if their labs are in the “normal” range. Some of the most common include:

Constipation, especially with hypothyroidism
Diarrhea or loose stools, often in hyperthyroidism or SIBO
Bloating, due to slowed digestion or bacterial overgrowth
Acid reflux, linked to low stomach acid in hypothyroidism
Abdominal pain and cramping, often tied to IBS or SIBO
Nutrient deficiencies (iron, B12, D, zinc), especially in autoimmune cases

If you’ve addressed these issues in isolation without resolution, consider the possibility that thyroid and gut dysfunction are working together.

How to Support the Thyroid-Gut Axis

Eat to Nourish Both Systems

A whole-foods, anti-inflammatory diet supports both gut and thyroid health. Prioritize:

High-fiber foods for microbial diversity
Anti-inflammatory fats (like omega-3s)
Nutrient-dense sources of iodine, selenium, zinc, and iron
Gluten-free options if you have autoimmune thyroid disease or sensitivity

Dr. Will Cole recommends eliminating known gut irritants (gluten, dairy, processed foods) and focusing on root-cause nutrition—not just symptom management.

Probiotics and Prebiotics

Restoring microbiome balance can improve hormone conversion and reduce inflammation. Some research suggests that probiotic supplementation may reduce TSH levels and improve energy in hypothyroid patients. Dr. Cole supports this approach and often incorporates gut-repairing protocols that include targeted probiotics, prebiotics, and microbiome-friendly foods.

Address Underlying Gut Imbalances

Resolving conditions like SIBO, IBS, or gastritis is essential for long-term thyroid health. This often involves testing (stool analysis, breath tests), eliminating gut irritants, and restoring barrier integrity with nutrients like L-glutamine, zinc carnosine, and omega-3s.

According to Dr. Cole, treating gut issues can sometimes resolve thyroid-like symptoms entirely—even reducing the need for hormone medication.

Take a Root-Cause, Not Hormone-Only, Approach

If you’re treating thyroid issues without looking at your gut—or vice versa—you may be missing the bigger picture. The thyroid-gut axis isn’t just a theory; it’s a clinical reality. Integrating both systems into your healing approach can lead to better outcomes, fewer symptoms, and greater resilience.

Final Thoughts: Two Systems, One Conversation

The thyroid and the gut are in constant communication, shaping metabolism, immunity, energy, and digestion. When one struggles, the other responds. For lasting health, we need to treat them as the deeply connected systems they are.

Whether you’re facing unexplained fatigue, bloating, weight changes, or autoimmune conditions, exploring the Thyroid-Gut Axis can provide powerful answers. Supporting both systems together—through diet, targeted supplementation, and gut healing—offers a path toward deeper, more sustainable wellness.