The Gut-Thyroid Axis

As science continues to explore gut health and its influence on non-digestive areas of health and wellbeing, there is strong evidence connecting the composition of the gut microbiome and the thyroid. In the following blog, we take a closer look at the studies connecting gut bacteria with thyroid health and what these links suggest.

What is the thyroid?

The thyroid is a small endocrine gland located in the anterior neck. The gland is responsible for iodine homeostasis, as well as thyroid hormone secretion. The thyroid gland produces 90% inactive hormone thyroxine (T4) and 10% active hormone, triiodothyronine (T3)¹. Its hormones induce effects on nearly all nucleated cells, and therefore the thyroid can affect almost all major organ systems in the body, including cardiovascular, respiratory, nervous, skeletal, and reproductive systems¹.

What does the thyroid do?

The thyroid gland releases hormones that the body needs. The central nervous system, for example, is particularly dependent on thyroid hormones for neural development and function, suggesting that thyroid disorders can be associated with an element of cognitive decline². The thyroid gland is perhaps best known for the role it plays in metabolism; how effectively the body can turn food and nutrients into energy. As a result, the gland plays a major role in growth and development of the human body

What are common thyroid disorders?

An overactive thyroid, or hyperthyroidism, involves excess secretion of thyroxine from the gland leading to increased metabolic activity. Symptoms of an overactive thyroid gland can include heat intolerance, weight loss, anxiety, hyperreflexia, and diarrhoea, as well as heart palpitations¹. In contrast, some individuals can have a reduced production of thyroxine, known as hypothyroidism, which slows down body metabolism. This condition is associated with fatigue, intolerance to cold temperatures, weight gain, constipation, dry skin and irregular menstruation³.

Hashimoto's thyroiditis (HT) and Graves' disease (GD) are examples of autoimmune thyroid disease (AITD) that are associated with hypothyroidism, and hyperthyroidism, respectively. Antibodies to thyroid peroxidase (TPO), the enzyme that catalyses thyroid-hormone production, are characteristic of HT. In contrast, antibodies to the receptor for the thyroid-stimulating hormone are significant in GD⁴. It’s likely that the interplay between the gut and the immune system is a contributing factor in how the gut and the thyroid interact. It’s important to note that thyroid disorders are often more prevalent in women compared to men. For example, hypothyroidism and hyperthyroidism is approximately 10 times more common in women than in men⁵.

What are the links between the gut and thyroid?

It appears there is an association between intestinal health and thyroid health, which may be underpinned by several key mechanisms. It is often seen that thyroid and intestinal diseases coexist, which provides further evidence for a link between these two metabolic organ systems. An altered gut microbiome composition can increase the prevalence of autoimmune diseases such as HT and GD, and these conditions often co-occur with celiac disease and non-celiac wheat sensitivity⁶.

The connection between the gut and the thyroid has been defined as the ‘gut-thyroid axis’; this is where the state of the gut microbiome can interact with thyroid function. A healthy gut microbiota may have a beneficial effect on the thyroid by supporting intestinal barrier function, which is important for nutrient availability. Healthy gut microbiota also help to eradicate potentially harmful opportunistic pathogens by dominating the gut and creating gastrointestinal homeostasis⁷. It is suggested that dysbiosis, a term used to describe a less favourable composition of bacteria in the gut microbiome, can lead to intestinal permeability, resulting in the thyroid being exposed to high levels of antigens and inflammation⁶.

The gut-thyroid axis and mineral absorption

Further evidence for the gut-thyroid axis is related to micronutrient status and the absorption of minerals, which is impacted by the gut microbiome. Synthesis of thyroid hormones is influenced by levels of copper, iron and iodine. Zinc and Selenium are important for conversion of T4 to T3, while vitamin D also plays an important role in immune response regulation. Sufficient levels of iodine and selenium are related to good thyroid health, and therefore a lack of these micronutrients may impact thyroid function and health. Deficiency of these nutrients is often prominent in patients suffering with autoimmune thyroid diseases⁶. Probiotics have been shown to have beneficial effects on trace elements and microbes are shown to prevent thyroid hormone fluctuations⁶.

Autoimmune condition GD causes the thyroid to become hyperactive, and cases prevail in regions with insufficient availability of iodine, which leads to disruption of thyroid function. It has been observed that beneficial gut bacteria levels were decreased in patients with severe GD⁸. Gut microbiome variations were detected in patients suffering with autoimmune thyroid diseases. The bacterial composition of the gut microbiome shows significant impact on absorption of vitamins and minerals and additionally affects interaction between host and gut microbes⁹. Lactobacilli and Bifidobacteria, shown to be a beneficial part of intestinal microflora, were significantly lower in Hashimoto’s Thyroiditis patients, which suggests a possibility of gut microbial dysbiosis¹⁰.

Prebiotics and dietary fibre help to promote growth of beneficial bacteria in the gut, that might have an impact on thyroid function through nutrients availability. However, food supplements should not be used in place of a balanced diet with high quality nutrients¹¹.

In conclusion

There are clear links between the gut microbiota and gut health, and thyroid health. This can be seen in relation to intestinal permeability, functioning of the immune system and micronutrient availability. However, there is a need for multifactorial therapeutic and preventive strategies to manage thyroid conditions that are more specifically adjusted to patients. This is particularly true considering gut microbiota composition and the manipulation of this, since the gut microbiome composition is unique to an individual and a ‘one-size-fits-all’ approach would not work in this regard. In the future, more well-powered human studies are required to evaluate the impact of alterations in gut microbiota on thyroid function and diseases.

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