Thyroid Health: The Importance of The Gut

While there are several thyroid hormones in the body, the main three are TSH (thyroid-stimulating hormone), T3 (thyroid hormone 3) and T4 (thyroid hormone 4, better known as thyroxine).

Following signals from the hypothalamus, the pituitary gland produces TSH  which travels to the thyroid and instructs it to produce hormones (mainly T4).  T4 is an inactive form of the thyroid hormone which has to be converted to an active form (T3) to be useable.  When there are sufficient amounts of T4 and T3 circulating in the blood, the hypothalamus signals to the pituitary gland to produce less TSH, therefore reducing the thyroid’s hormone production.  The effectiveness of this feedback loop assumes that all the organs and glands are working well.

If you display symptoms of thyroid issues your doctor is likely to run tests for TSH and T4.  If they suspect an autoimmune condition, antibodies may also be checked.  It is unlikely that an NHS GP will check your T3 levels as your body works hard to keep it within a normal range, and so an imbalance here will show later than a TSH or T4 imbalance.  To test your T3 level to help understand how well your body is converting the T4, you would most likely require a private test.

You are unlikely to be tested for your levels of all the nutrients detailed in this article.  Of those listed, iron is the most likely to be checked by a GP.  Take a look to make sure you include plenty of the nutrient-rich sources in your diet to support you thyroid function.

The gut includes the liver and the colon, which are important for the conversion of T4 to T3.  The liver converts around 60% of the T4, and the colon around 20%.

Within the liver the thyroid hormones are bound up ready to be released into the blood by thyroid binding globulin.  If your liver is not functioning well, you may not make the appropriate amount of this substance.  Too much may result in insufficient T3 being released, and too little may result in too much T3 being released.  In addition the liver makes reverse T3 (another inactive hormone) in response to high T3 levels in the bloodstream.  If the liver isn’t working well the levels of these hormones may not be balanced.

Interestingly, the thyroid hormones play a role in balancing our cholesterol levels.  They increase the activity of enzymes that help reduce LDL cholesterol levels, and increase the production of a protein that forms part of HDL cholesterol.  This protein (apolipoprotein A1) is formed in the liver and intestines, and higher levels are associated with a lower risk of plaque build-up in the arteries.

In terms of thyroid health, the colon is believed to have a variety of roles:

• Gut bacteria and enzymes convert the T4 into T3
• Gut bacteria metabolise bile acids into secondary bile acids which support another enzyme required for the T4 to T3 conversion.
• We absorb nutrients important to the functioning of the thyroid, liver and other relevant parts of the body through the different parts of the intestine.

It is also thought that gut health may have a role in supporting the cell proteins that thyroid hormones bind to throughout the body.  So, managing inflammation in the gut may be helpful for this role too.

The kidneys, brain and other organs also convert small amounts of T4 to T3.

As always a varied, healthful diet is important.  As you can see below, helpful nutrients can be found in both animal foods and plant foods.  Here are details of some key thyroid supporting nutrients, including an explanation of their role and example food sources.

It is important to ensure that you get enough iodine in your diet.  This is needed by the thyroid to produce T4.  But, take care not to get too much as a high intake may also contribute to thyroid problems.  High-dose iodine supplements are therefore generally NOT recommended.

Some of the best food sources include dairy, fish, eggs, prunes, seaweed and butter beans.  Iodised salt can also be purchased in the UK if you need to increase your intake further.

Iron helps us to use the iodine.  Good food sources include red meat, liver and organ meats, shellfish, tuna, leafy greens, lentils, beans, pumpkin seeds and dark chocolate.  If you get your iron mainly from plant sources, make sure you include vitamin C-rich foods with your meals to help you absorb the iron.

We need zinc to make the thyroid hormones, and the thyroid hormones help us to absorb zinc.

Zinc can be found in shellfish, meat, oats, lentils, beans, nuts and seeds, dairy, eggs and wholegrains.

Low zinc levels may contribute to excess copper in the body.  Meanwhile, high levels of zinc (for example, through long-term high dose supplement use) may result in copper deficiency.

While we do not want excess levels of copper in the body, we do need some.  Copper is believed to support thyroid hormone production and prevents our blood cells over-absorbing the hormones.  This happens because it helps to control our calcium levels.

Good sources include liver, shellfish, seaweed and algae (for example spirulina powder), nuts and seeds, leafy greens, dark chocolate and shiitake mushrooms.

The thyroid gland has a high concentration of selenium.  It has a role in thyroid hormone metabolism, and contributes to the antioxidant activity that takes place within the thyroid gland.  Again, excess selenium may also be harmful.

Good sources include meat, shellfish, fish, eggs, brazil nuts (limit to three a day to reduce the risk of excess levels), cottage cheese, brown rice, sunflower seeds, oats and lentils.

Vitamin D deficiency has been associated with autoimmune-related thyroid conditions (association means that it has not been confirmed as either a cause or consequence of the conditions).  Vitamin D is absorbed in the small intestine and so supporting your overall gut health will support the nutrient’s absorption.

We can get vitamin D via skin exposure to the sun between late spring and early autumn (please remember sun safety).  There are a few food sources of this nutrient too: include oily fish, eggs and mushrooms grown with UV light exposure.

Vitamin A is thought to support our iodine utilisation.  Vitamin A deficiency has been associated with poor thyroid function and iodine deficiency.  It is also possible that low vitamin A affects the metabolism of thyroid hormones in our cells.  Good sources of vitamin A include: Red, yellow and orange fruit and vegetables, green leafy vegetables, beef liver, milk and eggs.

Remember that both vitamin A and D are fat-soluble vitamins so ensure you include some fat in your diet (avoid hydrogenated and trans fats found in butter replacements, processed foods, fried foods, etc where possible!).

The anti-inflammatory properties of omega-3 fatty acids helps to support the thyroid.  Inflammation in the thyroid may reduce its ability to function and may result in the release of the antibodies associated with autoimmune conditions.

These can be found in oily fish, flaxseeds, chia seeds, walnuts, algal oil and olive oil.

It is also important to support your microbiome as it plays a part in the T4-T3 conversion and the gut-thyroid axis.  The microbiome supports the absorption of nutrients required by the thyroid to function.  As you’ll already know from my other posts, the gut bacteria in the microbiome like fibre, including resistance starch, and a variety of whole foods.  You can also include probiotic foods to give your microbiome extra support, such as yoghurt, tempeh, fermented foods and some cheeses.

Try to avoid or limit foods that have an inflammatory effect on the gut.  If you are aware of foods that increase symptoms of IBS, IBD or other sensitivity within your body, these are a good place to start.  Being aware of your sugar and gluten intakes and reducing them where possible may also help, as could limiting your intake of ultra-processed foods.

Taking general good care of your liver will also support the conversion.  For some tips on how to do this, take a look at my previous blog post about liver health.  If your liver is working more slowly than is considered ‘normal’, it may not breakdown hormones, such as oestrogen, properly and return them into the bloodstream.  This can result in high levels of the hormones which may negatively affect your health.  For example, increased levels of oestrogen can affect the T4-T3 conversion.

Unfortunately, if you have low thyroid activity the liver functions will also slow which may inhibit some of the T4-T3 conversion activity.

Stress can also cause interfere with the thyroid hormones.  Chronic stress (long-term or severe stress) may result in excess levels of the hormone cortisol in the body which may slow the thyroid function down and reduce its production of hormones.  Stress may inhibit the conversion of T4 to T3.  While it is not proven that stress itself causes thyroid disorders, it can exaggerate symptoms of existing conditions, including undiagnosed conditions.

Looking after our mental health is so important for many aspects of our lives.  But, mental health is only a part of the stress that we experience – stress on our body can also come from inflammation (see intestine nutrition support, above).

Individuals with thyroid problems may find they have a lower tolerance to stress than others.

Don’t forget that all the nutrients detailed here also have other roles in supporting our health.  It is important to make sure you support your whole body so that it can support itself.

• The relationship between thyroid disorders and vitamin A: A narrative minireview (Capriello, et al.  2022) – https://www.ncbi.nlm.nih.gov/pmc/articles/PMC9592814/
• Zinc deficiency associated with hypothyroidism: an overlooked cause of severe alopecia (Betsy, et al.  2013) – https://www.ncbi.nlm.nih.gov/pmc/articles/PMC3746228/#:~:text=Zinc%20and%20other%20trace%20elements,result%20in%20acquired%20zinc%20deficiency.
• Selenium and thyroid disease: from pathophysiology to treatment (Ventura, et al. 2017) – https://www.ncbi.nlm.nih.gov/pmc/articles/PMC5307254/#:~:text=In%20fact%2C%20it%20was%20found,conversion%20of%20T4%20to%20T3.