Thyroid & Gut Health: The Underrated Connection You Should Know About

Our thyroid and our gut may seem like two completely different and unrelated organs at first glance: one sits in the throat and controls numerous bodily functions through its hormones, while the other is a long tube in the abdomen responsible for digestion. But behind the scenes, there is a lively exchange taking place – mediated by the gut microbiome. And this exchange plays an important role in health, metabolism and well-being.

What does the thyroid gland actually do?

The thyroid gland is a small, butterfly-shaped organ in the neck. It produces important hormones that are essential for many bodily functions. You can think of it like a gas pedal that determines how fast or slow the body works.

Thyroid hormones affect, among other things:

• how much energy we use
• whether we feel balanced and productive or tired and exhausted
• how high our pulse and blood pressure are
• how well digestion and metabolism function
• whether we feel warm or cold
• how stable our mood, concentration and mental health are

An overview of the most important thyroid hormones

• T4 (thyroxine): The “storage hormone”
  It is produced in larger amounts and mainly serves as a precursor for the active hormone T3.
  
• T3 (triiodothyronine): The “active” thyroid hormone
  It ensures that our cells produce energy, that digestion runs smoothly, that our heart beats faster or slower, and that we feel energetic and capable.
  

💡 The thyroid mainly produces the hormone T4. This is a kind of precursor and has only a weak effect in the body. Only when T4 is converted into the active hormone T3 can it exert its full effect. This conversion does not only take place in the thyroid itself, but especially in other organs such as the liver, the gut, and other tissues. This way, the body can regulate how much active thyroid hormone is actually available according to its needs.

• TSH (thyroid-stimulating hormone): TSH does not originate in the thyroid gland, but in the pituitary gland. It is the control signal for the thyroid gland.
• Low T3/T4 levels → TSH rises → thyroid should produce more
• Sufficient T3/T4 → TSH falls → production is reduced.

💡 Only when TSH, T4 and T3 are well balanced does metabolism run at the right pace.

• Calcitonin: Produced in the C cells of the thyroid gland, it is involved in calcium and bone metabolism. It supports bone stability and counteracts bone loss.

The thyroid gland is a vital organ that controls many different bodily functions.

What is the gut microbiome and how does it affect our thyroid gland?

The gut is home to trillions of microorganisms (mostly bacteria) that together form the gut microbiome – also known as gut flora. These microbes help us digest food, produce vitamins, regulate the immune system, and influence our hormonal balance and our psychological and mental health.

A diverse and balanced gut flora is an important pillar of overall health and is connected to all organ systems. The less diversity there is, the higher the risk may be for metabolic problems, infections, or chronic inflammation.

When the microbiome becomes unbalanced—for example, when potentially harmful bacteria proliferate and displace important, beneficial bacteria—this is called dysbiosis. Dysbiosis is frequently found in patients with autoimmune thyroid diseases and thyroid cancer. But how can this be explained?

Leaky gut and the thyroid gland

Dysbiosis can lead to leaky gut – a disruption of the intestinal barrier. When the important barrier function of the intestinal mucosa is impaired, unwanted substances can more easily pass from the gut into the body. This can overactivate the immune system, potentially leading to chronic inflammation and even autoimmune diseases.

Impact on nutrient supply

Furthermore, the composition of gut bacteria influences the availability of important micronutrients for the thyroid gland. Iodine, iron, and copper are crucial for the production of thyroid hormones. Selenium and zinc are needed to convert the hormones T4 to T3, and vitamin D helps regulate the immune response. These micronutrients are often deficient in thyroid disorders, which can impair the thyroid gland's ability to perform its vital functions (1).

Understanding the microbiome composition and improving it intentionally

Whether gut dysbiosis is a cause or a consequence of thyroid disorders remains unclear. However, since the gut and thyroid are closely linked, targeted modulation of the microbiome could play an important role in the future treatment of thyroid diseases. Initial studies on the use of probiotics to support thyroid function are also showing promising results.

Studies show that the bacterial composition in the gut is altered in both hyperthyroidism and hypothyroidism (2–4). With the myBioma Microbiome Test Ultimate, you can find out how well your microbiome supports your thyroid activity.

Note: These are possible correlations based on scientific studies and the constantly growing myBioma database. The results do not constitute a diagnosis and do not replace medical treatment, but can provide valuable information.

How are the intestines and thyroid gland connected?

The gut-thyroid axis describes the close, reciprocal communication between these two organs. As mentioned earlier, the gut microbiome plays an important role through its influence on the immune system and nutrient availability. Another example of the significance of this axis is that thyroid and gut disorders often occur together: the autoimmune diseases Hashimoto’s thyroiditis and Graves’ disease are the most common thyroid disorders and frequently co-occur with celiac disease or gluten sensitivity, or non-celiac wheat sensitivity (NCWS) (1).

Let's now take a closer look at how the gut and thyroid gland interact:

How the gut affects the thyroid gland

• Nutrient absorption: For your thyroid to produce hormones, it primarily needs iodine, selenium, iron, and zinc. A healthy gut microbiome indirectly supports the absorption and utilization of these micronutrients, thus influencing how well the thyroid is supplied. If gut health is compromised, the absorption of these micronutrients can be limited – with consequences for hormone production.
• Hormone signaling conversion: The hormone T4 is converted into active T3 in the body. Studies suggest that probiotic gut bacteria such as bifidobacteria and lactobacilli support this process.
• Immune system & autoimmunity: Around 70% of our immune cells reside in the gut. A disrupted microbiome can trigger or exacerbate inflammation and misdirect the immune response, which can promote autoimmune diseases such as Hashimoto's thyroiditis or Graves' disease (1,4).

How the thyroid gland affects the intestines

• Digestion: Thyroid hormones regulate intestinal motility – that is, the speed at which food is digested and moves through the intestines. In hypothyroidism, digestion can slow down (constipation), while in hyperthyroidism it can speed up (diarrhea) (5).
• Microbiome composition: Thyroid hormones indirectly influence the gut environment and thus which bacteria can proliferate in the gut. For example, people with hypothyroidism are more likely to suffer from SIBO (Small Intestinal Bacterial Overgrowth). Studies show that properly adjusted thyroid therapy can reduce the risk of SIBO and stabilize the microbiome (6).

The gut microbiome and the thyroid gland influence each other through various mechanisms.

How can you support your microbiome and thyroid gland?

Proper nutrition and a healthy lifestyle promote gut health and support your thyroid in hormone production.

Micronutrients for the thyroid gland (1)

• Iodine
• It is essential for the production of thyroid hormones and is stored in the thyroid gland.
• Sources: Primarily iodized table salt, seafood, and other marine products such as seaweed. The iodine content of foods varies considerably, depending on the iodine content of the soil and the iodine supply to livestock. Accordingly, milk and eggs, for example, can also contribute to meeting iodine requirements.
• Iron
• It is essential for the efficient utilization of iodine and the formation of thyroid hormones.
• Certain intestinal bacteria, such as lactobacilli, can increase iron bioavailability.
• Sources: Animal sources such as red meat and organ meats provide iron that is most most easily absorbed by the body. Plant sources such as legumes, whole grains, nuts, seeds, and leafy greens are best consumed with a source of vitamin C to increase bioavailability.
• Zinc
• Supports the conversion of T4 to active T3 and has an antioxidant effect.
• Sources: Oysters, beef, offal, cheese, whole grain products, oats, legumes, nuts and seeds.
• Tip: The bioavailability of plant-based zinc sources can be increased by consuming them with animal protein such as yogurt or eggs. Fermentation (e.g., sourdough bread) or germination also increases bioavailability.

• Selenium
• Is a component of important enzymes that protect the thyroid gland from oxidative stress and is involved in the activation of thyroid hormones.
• Sources: Primarily fish, meat, eggs, and Brazil nuts. For other plant-based sources such as mushrooms, cabbage, onions, lentils, and asparagus, the selenium content depends heavily on regional soil quality. 
  

• Vitamin D
• Is important for the normal function of the immune system and a deficiency is frequently observed in thyroid diseases.
• In cases of proven deficiency and after consultation with a doctor, targeted supplementation may be advisable.

The right diet for a robust microbiome

• Rich in fibre, diverse and colourful: vegetables, fruits, legumes, whole grains, nuts and seeds provide food for beneficial gut bacteria and supply important micronutrients and secondary plant compounds such as polyphenols.
  
• Fermented foods: Raw sauerkraut, kimchi, kefir, yogurt and kombucha enrich the microbiome with lactobacilli and bifidobacteria.
• Healthy fats: Omega-3 from fish or algae oil, avocados, high-quality olive oil, flaxseed oil, nuts and seeds.
• Sufficient protein from various sources (more information can be found here: Protein and gut health: How protein affects your microbiome)
• Avoid highly processed foods, sugar, sweeteners, alcohol and cigarettes - they can disrupt the intestinal barrier and the balance of the microbiome.

Furthermore, lifestyle plays an important role in gut health and thyroid health: Pay attention to good stress management, sufficient relaxation and sleep, and regular exercise.

Conclusion

The connection between the thyroid gland and the gut microbiome is a fascinating field of research that could lead to better treatment options for thyroid diseases in the future. While not all mechanisms are fully understood, one thing is clear: the microbiome influences the thyroid gland directly or indirectly – for example, via the immune system, the availability of important nutrients, and metabolism.

If you support your gut health, you’re also doing something good for your thyroid!

Do you want to know more about what happens when the thyroid gland becomes unbalanced or autoimmune diseases occur, and how the microbiome can be involved? Then read our second blog article on the gut-thyroid axis: Thyroid: Hypothyroidism, Hyperthyroidism, and what the gut microbiome has to do with autoimmune diseases. 

Carina Gurtner BSc, BA

Nutritionist

As a nutritionist and media & communication scientist, Carina uses her expertise to communicate complex health topics in an understandable way. Her aim is to inspire others to lead a conscious, healthy and sustainable lifestyle.

More from this author