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The thyroid gland is an important part of the body found at the base of the neck and has a wide variety of roles. It is responsible for producing hormones that regulate the rest of the body and plays an essential role in maintaining regular homeostasis (i.e., keeping the remainder of the body in check).
The thyroid gland regulates many important processes in the body and is a key part of the endocrine system. The endocrine system handles the production and regulation of hormones.
In particular, the thyroid gland is responsible for taking iodine and turning it into thyroid hormones. This happens when the thyroid cells absorb iodine from the food we eat (as we cannot produce iodine ourselves) and combine it with the amino acid tyrosine.
Tyrosine is known as a non-essential amino acid, which simply means we can make it ourselves and don’t need to get it from our diet. Iodine can’t be synthesized, and the amount of iodine you absorb from your food can be a rate-limiting factor in the ability of the thyroid to make its hormones.
Specifically, the thyroid gland makes thyroxine (T4) and triiodothyronine (T3). These hormones are essential for the control of metabolism. Mostly, the thyroid produces T4 (80%), but it also makes a small amount of the much stronger T3 (20%).
When the level of thyroid hormones drops too low, the brain will signal to the thyroid gland through other hormones (thyroid stimulating hormone known as TSH) that it needs to produce more T3 and T4 and vice versa.
T3 and T4 work together in a complicated process to regulate the speed at which every cell in the body creates and uses its energy. This means that it controls how fast your heart beats and how quickly your food gets digested.
Because the thyroid is involved in the regulation of almost every single cell in the body, any issues in the thyroid will have widespread effects all the way across the body. This can include gastrointestinal problems.
Also, iodine has to be absorbed from the food we eat, which adds another layer of interaction between the gut and the thyroid. This means that if your gut is struggling to absorb food, you may end up with dysregulation of the thyroid hormones.
Research has found that problems with the motility of the gastrointestinal (GI) tract can cause GI symptoms of thyroid disease.¹
We know that they are connected, as the thyroid is connected to almost every process in the body and directly controls digestion and metabolism through T3 and T4 production.
What isn’t known is how these connections occur. Once you finish eating a meal, the GI tract will begin to release hormones to control digestive enzymes (the things that break down your food to allow absorption).
We know that the thyroid can cause gastrointestinal diseases because thyroid diseases and gastrointestinal diseases tend to occur together. The GI tract can also have an effect on thyroid diseases by influencing which micronutrients are available for the thyroid gland.
IBS stands for irritable bowel syndrome. It is a common but not very well understood disease. Essentially, people with IBS can struggle with cramping, bloating, diarrhea, or constipation.
We aren’t sure what causes IBS, so it is difficult to know if it is directly linked to thyroid disease. However, it has been linked to changes in the gut microbiota (the bacteria that live in your GI tract and help you to digest food).
Some types of thyroid issues, like autoimmune thyroid diseases (where the thyroid issue is caused by the body’s immune system going haywire), have been linked to atrophic gastritis.
Atrophic gastritis is a disease that causes the loss of glands in the lining of the stomach, leading to reduced hydrochloric acid production. This is a very strong acid that is used in the body to break food down and is essential for the digestive system to function.
Although we know there is a link between GI dysfunction and thyroid disorder symptoms, it is difficult to say for sure that thyroid problems are the direct cause of bloating and gas, as they are also noticed in other conditions like IBS. However, there has been an association between thyroid problems and GI dysfunction, particularly in hypothyroidism, so there may be a connection between them.
Digestion issues are the most likely cause of bloating and gas. These issues can be caused by the reduction or increase in thyroid hormones. But it is difficult to determine if the dysregulation of thyroid hormones came first or whether it was a result of abnormal digestion and absorption from the digestive issues.
In essence, we have a chicken-egg problem. The digestive symptoms of bloating and gas can indicate that there are issues with the absorption of nutrients in the GI tract. A lack of nutrients can stop the thyroid from working properly, and a lack of thyroid hormones can cause more digestive issues.
Recent studies have found that patients with autoimmune thyroid disorders have differences in their gut microbiota compared to people without thyroid disorders. However, so far, no studies have been able to discover whether these differences are due to the thyroid disorder or if they instead have a role in creating it in the first place.²
When the thyroid gland generates too much thyroid hormone, the condition is known as hyperthyroidism. This can be caused by a variety of issues but is usually a result of Graves' disease, where the body creates antibodies to its own receptors and stimulates the excessive production of T3 and T4.
Because thyroid hormones help to regulate the digestive system, hyperthyroidism can cause problems with gut health. In particular, hyperthyroidism is known to speed up your bowel activity, forcing food too quickly through the gut.
This can cause someone with hyperthyroidism to experience too frequent or loose bowel movements (diarrhea) and can cause them to feel tired even after eight hours of sleep.
Hypothyroidism is when the thyroid gland doesn’t produce enough thyroid hormone. It is generally caused by Hashimoto’s thyroiditis (inflammation of the thyroid).
Hypothyroidism does the opposite of hyperthyroidism and can lead to constipation. This is because the thyroid gland’s function of stimulating the digestive system to convert food into energy is reduced without a healthy level of thyroid hormones.
Hyperthyroidism can lead to several digestive issues because of the poor regulation of the speed of bowel movement. These issues often include upper abdominal (epigastric) pain, vomiting, weight loss, and diarrhea.
Sometimes, thyrotoxicosis (the state of having too much thyroid hormone in the body) can also cause pain in the throat or trouble swallowing. This is because thyrotoxicosis can cause muscle weakness, triggering dysphagia (swallowing difficulties).
People with hyperthyroidism can also have an increase in appetite and food intake, but this is rarely enough to make up for the increase in metabolism. As a result, it is common to experience unexplained weight loss with hyperthyroidism.
Additionally, people with hyperthyroidism caused by Graves' disease may be more likely to experience gastrointestinal diseases such as ulcerative colitis or celiac disease.
You should see a doctor if you have noticed any long-lasting changes to your digestive habits or are rapidly gaining or losing weight without explanation. You should also see a doctor if you notice a nodule in your thyroid or have stomach pains.
Your doctor will conduct a physical examination at their office if you ask them to check you for a thyroid disorder. This examination will usually involve looking at the thyroid, heart rate, and weight.
Your doctor will also ask you questions about your medical history, such as whether you or your family members have had thyroid problems in the past.
A blood test can help to assess thyroid function and determine whether thyroid activity is within the usual range.
This test checks the levels of:
Thyroid-stimulating hormone (TSH)
Aside from the management options listed above for digestive issues, thyroid issues can be treated medically. This can include the use of medications or surgery.
To treat an underactive thyroid, a medication called levothyroxine is used to supplement the thyroid hormone levels of the body.
An overactive thyroid can be treated through thioamides (medicine to stop your thyroid-producing hormones), radioactive iodine treatments, or surgery. These treatments are best prescribed through consultation with an endocrinologist (a hormone specialist).
If you are experiencing digestive issues as a result of a thyroid issue, there are several things you can try to manage these issues. These include:
Eliminating gluten. Autoimmune thyroid conditions have been linked to some GI diseases, such as celiac disease. Eliminating gluten for these diseases from your diet may provide some relief, although the evidence on how a gluten-free diet affects thyroid disorders is mixed.
Diet modulation. You can also attempt to modulate the amount of fiber, carbohydrates, and water in your diet to help either increase or decrease the softness of your stool. These factors can help to manage the symptoms of GI disease.
Eliminate trigger foods. This can be done using an elimination diet. This is where you have only a few foods you know you can tolerate before slowly adding back foods to see which ones are causing your issues.
Probiotics. If your digestive issues are caused by issues with the bacteria in your microbiota, taking probiotic (good bacteria) supplements can be useful to help re-establish a healthy gut microbiome.
Low FODMAP diet. FODMAPs are short-chain carbohydrates that are fermentable and poorly absorbed. A diet that reduces FODMAPs may reduce digestive symptoms such as abdominal pain and bloating.
The thyroid is an important regulating organ that produces and releases hormones that control metabolism. Although it is clear there are some links between thyroid dysfunction and GI tract issues, it isn’t sure exactly what those are at this point in time.
If you think you may have a thyroid disorder, it’s important to see your doctor. They can give a diagnosis and commence treatment for the thyroid disorder and any complications, such as gastrointestinal problems.
Irritable bowel syndrome (IBS) | NHS Inform
IBS and your gut | About IBS
The thyroid and the gut (2010)
Thyroid tests | National Institute of Diabetes and Digestive and Kidney Diseases (NIDDK)