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By: Rania Al-Dulaimi, dietetic student-volunteer University of Guelph, reviewed by Lyndsay Hall, Registered Dietitian and the JM Nutrition Team

Introduction: Thyroid

What is the Thyroid and What Does It Do?

According to the AACE, the thyroid is a “small, butterfly-shaped gland in the neck that makes hormones to control your body’s energy use, heart rate, and temperature”.

It affects almost every organ in the body.

Despite its size, being a small gland located in the middle of our throats, the thyroid has a big impact on how much energy is generated in the body, metabolism and healthy digestion.

The thyroid gland makes three hormones: Calcitonin, Tetraiodothyronine (T4) and Triiodothyronine (T3) (Institute for Quality and Efficiency in Health Care, 2024).

Too much or too little thyroid hormone can impact many of the body’s functions.

Thyroid complications can cause many problems including sensitivity to hot or cold temperatures, unexplained weight gain or loss, anxiety and/or irregular heartbeats.

During the growing phase or pregnancy, the thyroid gland makes more thyroid hormones as the body needs more energy. Being aware of the symptoms that indicate thyroid dysfunction can help you get treatment earlier and avoid further complications (Office on Women’s Health 2020).

Thyroid and Metabolism: The Energy Connection

When T3 and T4 hormones are more active in the body, a person’s basal metabolic rate will increase. This will make the body’s cells use more energy and burn calories at a faster rate. As a result, the energy that is stored in our muscles and liver will start to break down (Shahid et al., 2023).

The thyroid hormone receptor isoforms α and β are expressed differently across tissues and have different roles in Thyroid Hormone (TH) signalling. The thyroid hormone receptor regulates cholesterol and carbohydrate metabolism by direct actions on the expression of the gene, and also as crosstalk with other nuclear receptors (Mullur et al., 2014).

Cholesterol levels can increase with an under-active thyroid because the body can’t break down and remove cholesterol properly. Even when thyroid levels are slightly low, cholesterol levels can still be impacted. When thyroid levels are high, this will cause cholesterol levels to drop very low (Watson 2017).

Adrenergic signalling

Thyroid hormones work with the nervous system to regulate metabolism.

Adrenergic signalling is the most relevant pathway that comes into play in the regulation of metabolism by TH. TH production is centrally regulated by Thyroid-releasing-hormone/Thyroid stimulating hormone, which is influenced by nutritional feedback and the adrenergic nervous system.

Fasting and sickness serve as additional models illustrating how thyroid hormones can adapt to protect energy storage during times of stress.

The regulation of basal metabolic rate (BMR) and adaptive thermogenesis is influenced by TH, and this has a great effect on body weight (Mullur et al., 2014). If the thyroid hormones are high, the BMR rises, and the body burns more calories, resulting in weight loss.

However, low levels of thyroid hormones lower the BMR, which means fewer calories are burned and possibly results in weight gain (TH Medical, 2016).

What reduces Levothyroxine absorption?

Levothyroxine is a replacement for one of the thyroid hormones made naturally in the body, called thyroxine, which is also called T4.

It brings your hormone levels back to normal levels, so your thyroid can work properly (Stephanie Thurrott, 2026).

Poor tablet dissolution or mal-digestion can cause reduced uptake of levothyroxine. A study showed that there is a connection between poor intestinal absorption and hypochlorhydria-related conditions, such as the use of proton pump inhibitors, Helicobacter pylori infections, and atrophic gastritis. It was determined that the sodium salt preparation of levothyroxine is poorly soluble under hypochlorhydric conditions in the stomach and remains hydrophilic. Because of this, it won’t be absorbed in the gut.

In cases of hypochlorhydria, patients may also suffer from iron and vitamin B12 deficiency if they experience symptoms of dyspepsia (indigestion). Serum gastrin and anti-parietal-cell antibodies, along with a carbon-14 urea breath test and a gastroscopy and subsequent biopsy, can be useful in the diagnosis of this condition (Ramadhan & Tamilia, 2011).

When Thyroid Function is Low (Hypothyroidism)

Hypothyroidism occurs when the thyroid gland is unable to produce thyroid hormones at an adequate level. This causes the metabolism to slow, which results in your body burning energy (calories) at a slower rate.

Hypothyroidism can be caused by Hashimoto’s thyroiditis, an autoimmune disease, thyroiditis, iodine deficiency, thyroid surgery (i.e. having part of, or all of your thyroid gland removed), or being treated with medication/iodine therapy for hyperthyroidism (Cleveland Clinic, 2022).

When Thyroid Function is High (Hyperthyroidism)

Hyperthyroidism occurs when too many hormones are made and released by the thyroid gland. This causes the metabolism to speed up and burn energy (calories) at a very fast rate. Hyperthyroidism can be a result of Graves’ disease (an autoimmune disorder), thyroid nodules, thyroiditis, iodine toxicity and treatment for hypothyroidism (Cleveland Clinic,2022).

Thyroid and Digestion: More Connected Than You Think

Furthermore, thyroid hormones do not only affect metabolism. They also have a big impact on the digestive system.

Hypothyroidism and Digestion

Due to its effect on metabolism, hypothyroidism also causes digestion to slow, which can lead to constipation and bloating.

This occurs because stool takes longer to move through the large intestine when metabolism is slowed. When stool remains in the colon for a longer time, more water will be reabsorbed back into the body, which causes the stool to become dry and hard.

In addition, thyroid hormones are needed to produce gastrin, which is a hormone that helps with stomach acid secretion. Stomach acid is needed for digestion, and it helps to kill harmful bacteria that break down proteins (Blue Horizon Team, 2026).

If thyroid hormone levels are low, the stomach produces less acid. This can cause feelings of fullness, heartburn, and can affect the absorption of vitamins and minerals. Ghrelin is a peptide hormone that stimulates appetite; it is gut-derived, which impacts how much energy is used and hunger cues. Ghrelin is shown to increase the gastric emptying rate (Xu, 2024).

Hyperthyroidism and Digestion

Individuals who deal with hyperthyroidism can experience diarrhea and increased bowel movements. This occurs because the increase in metabolism associated with hyperthyroidism speeds up digestion. There isn’t enough time for the colon to absorb water, which causes more watery stool.

Malabsorption is also common because the small intestine is unable to extract the necessary vitamins and minerals, which can lead to steatorrhea. Steatorrhea is the term used for oily and pale stools that result from fat not being absorbed correctly (Vanier, 2025).

Additional symptoms of hyperthyroidism may include unintentional weight loss despite having an increased appetite, an accelerated heart rate, even at rest, and some even experience tremors (i.e. shaking hands) (Blue Horizon Team, 2026).

Common Thyroid Disorders

Specific digestive diseases may be associated with autoimmune conditions that impact thyroid function, such as Hashimoto’s thyroiditis and Graves’ disease. Among these digestive diseases are celiac sprue and primary biliary cirrhosis, although a clear common mechanism has never been proven (Daher et al., 2009).

Signs Your Thyroid Might Be Affecting You: (Mayo Clinic, 2022).

Hypothyroidism

Some symptoms you can experience are:

feeling more sensitive to cold temperatures
constipation
weight gain
fatigue

Hyperthyroidism

Some symptoms you can experience are:

irregular and/or accelerated heartbeats
anxiety
heat intolerance
unintentional weight loss

Thyroid and Nutrition

Nutrition and gut microbiota affect thyroid functions in several ways.

They regulate immune responses, produce microbial metabolites, affect the absorption of nutrients, and contribute to epigenetic changes.

Therefore, these factors can influence the development and prevention of thyroid disorders. Thyroid function and optimal thyroid health are important and are influenced by balanced nutrition.

Furthermore, gut bacteria play a role in regulating selenium, iodine, and other nutrient levels required for the production of thyroid hormones. An unhealthy gut microbiome can result in inflammation and may contribute to the development of autoimmune-related thyroid diseases (Shulhai et al., 2024).

Additionally, the composition of the gut microbiota affects the availability of essential micronutrients needed for thyroid function (Knezevic et al., 2020).

Weight and Thyroid

People with hyperthyroidism–an overactive thyroid gland–can lose weight unintentionally, as they often burn calories at a faster rate than normal. The degree of weight loss is related to how overactive the thyroid is.

However, because hyperthyroidism increases appetite as well, some patients may even put on weight unintentionally depending on how much more they eat.

The opposite occurs among people with hypothyroidism–an under-active thyroid gland. They may burn fewer calories than usual, which can cause unintentional weight gain. The degree of weight gain can be greater in more severe cases of hypothyroidism, but the weight change in hypothyroidism is usually not as noticeable as in hyperthyroidism. The bulk of the weight gain that occurs is related to an accumulation of salt and water (water retention).

Depending on the degree of hypothyroidism, an average of 5-10 pounds of weight gain may be caused by an under-active thyroid among the average patient; however, there are individual differences (ATA, 2016).

Supporting Thyroid Naturally

To support the thyroid, a balanced diet is needed, and this is partly achieved by eating foods that are rich in iodine, such as fish and dairy products.

Selenium and zinc are additional minerals that support thyroid hormone production.

Selenium is obtained from legumes and Brazil nuts, while zinc is found in lean meats and whole grains.

To keep your thyroid gland in good health, maintaining adequate vitamin and mineral levels is important.

For example, vitamin D can help prevent autoimmune-related thyroid conditions by supporting the immune system. On the other hand, Vitamin A supports thyroid hormone metabolism. Moreover, iron is needed to produce thyroid hormones, which can be incorporated in your diet by eating foods like spinach and lean meats (Inspira Health, 2024).

Get Regular Testing

The primary thyroid hormone found circulating in the blood is T4 (thyroxine). Both bound, and free forms of T4 can be measured in a Total T4 blood tests. The total T4 may be affected by changes in the thyroid hormone transport proteins. A Free T4 test examines the non-bound form of the hormone, which is biologically available to enter tissues throughout the body.

High TSH and low FT4 levels are usually indicative of primary hypothyroidism, caused by a disease of the thyroid gland. High TSH and low FT4 may be due to a disorder of the pituitary gland that can cause hypothyroidism.

Hyperthyroidism is diagnosed when levels of TSH are low, and levels of FT4 are elevated. T3 (triiodothyronine) tests can be used to diagnose hyperthyroidism and the severity of it, as people with hyperthyroidism will have high T3 levels (American Thyroid Association, 2024).

Final Thoughts: Thyroid: Metabolism and Digestion, the Drivers

When the thyroid is not functioning well, it can cause symptoms like fatigue, low mood, constipation, diarrhea and weight fluctuations. These symptoms happen because thyroid hormones support healthy digestion and metabolism. Recognizing the symptoms of thyroid disorders and getting medical advice early on will help you prevent long-term health complications and manage your condition effectively.

In the event of a thyroid disorder, a healthy diet, thyroid testing and medication when needed, will help support normal thyroid functioning and good health.

Conclusion

Should you feel you require personalized nutritional sessions and support of a dietitian for thyroid health or a related health matter, book a free consultation or contact us for an appointment. As always if you have comments or questions, we encourage you to let us know.

References

Xu, G.-M. (2024b). Thyroid disorders and gastrointestinal dysmotility: an old association. Frontiers in Physiology, 15. https://doi.org/10.3389/fphys.2024.1389113.

Vanier, D. (2025). Eating with Gut Problems (Part 3) — the Thyroid Connection. Dr. Dominique Vanier | Naturopathic Doctor. https://www.drvaniernd.com/blog/2018/7/29/eating-with-gut-problems-part-3-the-thyroid-connection.

Shahid, M. A., Sharma, S., & Ashraf, M. A. (2023, June 5). Physiology, Thyroid Hormone. National Library of Medicine; StatPearls Publishing. https://www.ncbi.nlm.nih.gov/books/NBK500006/.

Ramadhan, A., & Tamilia, M. (2011). Treatment-refractory hypothyroidism. Canadian Medical Association Journal, 184(2), 205–209. https://doi.org/10.1503/cmaj.110994.

Mullur, R., Liu, Y.-Y., & Brent, G. A. (2014). Thyroid Hormone Regulation of Metabolism. Physiological Reviews, 94(2), 355–382. https://doi.org/10.1152/physrev.00030.2013.

Mayo Clinic. (2022). Hypothyroidism (underactive thyroid). Mayo Clinic. https://www.mayoclinic.org/diseases-conditions/hypothyroidism/symptoms-causes/syc-20350284.

Institute for Quality and Efficiency in Health Care. (2024b, May 28). How does the Thyroid gland work? National Library of Medicine; Institute for Quality and Efficiency in Health Care (IQWiG). https://www.ncbi.nlm.nih.gov/books/NBK279388/.

Cleveland Clinic. (2022). Thyroid: What It Is, Function & Problems. Cleveland Clinic. https://my.clevelandclinic.org/health/body/23188-thyroid.

Blue Horizon Team. (2026a). Can Thyroid Issues Cause Diarrhoea? Blue Horizon Blood Tests. https://bluehorizonbloodtests.co.uk/blogs/thyroid-health/can-thyroid-issues-cause-diarrhoea.

Blue Horizon Team. (2026b). Can Thyroid Issues Cause Digestive Problems? Blue Horizon Blood Tests. https://bluehorizonbloodtests.co.uk/blogs/thyroid-health/can-thyroid-issues-cause-digestive-problems?srsltid=AfmBOoqyyWFXXtCYsYrHRyeRjGbhQxIJNCjYnLE-y7EJhO1BM6Wmcg3T#section4.

American Thyroid Association . (2024). Thyroid function tests. American Thyroid Association. https://www.thyroid.org/thyroid-function-tests/.

Inspira Health. (2024, February 21) 8 Proactive Steps To Maintaining a Healthy Thyroid and Preventing Complications. Www.inspirahealthnetwork.org. https://www.inspirahealthnetwork.org/news/healthy-living/8-proactive-steps-maintaining-healthy-thyroid-and-preventing-complications.

ATA. (2016). Thyroid and Weight | American Thyroid Association. American Thyroid Association. https://www.thyroid.org/thyroid-and-weight/.

Knezevic, J., Starchl, C., Tmava Berisha, A., & Amrein, K. (2020). Thyroid-Gut-Axis: How Does the Microbiota Influence Thyroid Function? Nutrients, 12(6), 1769. https://doi.org/10.3390/nu12061769.

Shulhai, A.-M., Rotondo, R., Petraroli, M., Patianna, V., Predieri, B., Iughetti, L., Esposito, S., & Street, M. E. (2024). The Role of Nutrition on Thyroid Function. Nutrients, 16(15), 2496–2496. https://doi.org/10.3390/nu16152496.

Office On Women’s Health. (2020). Thyroid disease | Office on Women’s Health. OASH | Office on Women’s Health. https://womenshealth.gov/a-z-topics/thyroid-disease.

Watson, S. (2017, December 14). What Effect Does My Thyroid Have on My Cholesterol? Healthline; Healthline Media. https://www.healthline.com/health/thyroid-issues-and-cholesterol.

TH Medical. (2016). How Does the Thyroid Affect Metabolism? [Review of How Does the Thyroid Affect Metabolism?]. Carondelet.Org. https://www.carondelet.org/healthy-living/corporate-content/how-does-the-thyroid-affect-metabolism.

Kim, H. S., MD, Shon, H. S., MD, PhD, Dal Jung, E., & MD. (2026). Serum Ghrelin Levels in Hypothyroidism Patients. Korean Thyroid Association, 2(2), 114–119. https://www.ijthyroid.org/journal/view.html?uid=147&page=&pn=mostread&sort=publish_date%20desc&spage=&vmd=full.

About Author:

Lyndsay Hall is a Mississauga and Toronto-area based dietitian who provides personalized support for weight loss, various digestive health concerns, prenatal nutrition and more. Lyndsay is registered with the College of Dietitians of Ontario.

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Thyroid: Metabolism and Digestion, the Drivers