How Does Thyroid Dysfunction Affect My Digestive System?

Thyroid dysfunction — whether hyper or hypothyroidism — exerts far-flung effects on every cell of the body. Digestive challenges may be an early warning sign of thyroid dysfunction. An overactive thyroid may cause nausea, vomiting, and diarrhea, while an underactive thyroid may cause constipation.

Research is also beginning to describe a bidirectional relationship between gut microbiota and autoimmune conditions like thyroid dysfunction. Let's explore how thyroid dysfunction can affect your digestive system.

What are Graves’ disease and Hashimoto’s thyroiditis?

Graves’ disease and Hashimoto’s thyroiditis are the most common autoimmune conditions affecting the thyroid gland. People living with Graves’ disease have an over-active thyroid, while people living with Hashimoto’s have an under-active thyroid.

• Graves’ disease. Characterized by weight loss, rapid or irregular heartbeat, nervousness, irritability, trouble sleeping, shaky hands, muscle weakness, heat intolerance or sweating, more frequent bowel movements and an enlarged thyroid gland.
• Hashimoto’s. Symptoms include weight gain and constipation, cold intolerance, mood swings, hair and nail changes, joint pain, and brain fog.

How does thyroid dysfunction affect my digestive system?

Thinking about the two conditions—overactive vs underactive—is helpful to understanding how digestive symptoms might present differently. While managing thyroid dysfunction with medication may help resolve some symptoms, it may not fully resolve all.

Symptoms of thyroid dysfunction

The first symptoms of thyroid dysfunction may well be related to digestion, but the signs are subtle.

• Digestive issues such as constipation or diarrhea.
• Neck discomfort and dysphagia (difficulty swallowing).

Issues in the neck are common due to an enlarged thyroid, but disruptions in how quickly food moves through the entire gastrointestinal tract may also be noted.

The gut microbiota may also be affected by thyroid dysfunction

A recent study of people with Graves’ disease and Hashimoto’s thyroiditis found that the gut microbiota in both groups was different from healthy controls.

In addition, an important metabolic pathway might be affected. Our body converts nutrients like carbohydrates and proteins into smaller molecules like glucose and amino acids. These metabolic pathways may be influenced by thyroid dysfunction.

The study found that the gut microbiota of people with hyperthyroidism and hypothyroidism had similar diversity and abundance compared to normal controls, but they were not balanced. In other words, the makeup of the microbiota was different. This opens the possibility of not only identifying Graves' disease and Hashimoto's thyroiditis through fecal samples, but potentially modifying one's nutrition to rebalance the microbiome.

The past decade has seen an explosion of research describing how important the microbiome is to our overall health. This research is beginning to shed light on interactions between the gut microbiota and the immune system. (The distinction between “microbiome” and “microbiota” will be important to understand shortly: the “microbiome” includes the microbes and their intestinal environment, while “microbiota” refers to the microbes alone.)

An overgrowth of unhealthy bacteria can lead to developing a “leaky” gut, alerting the immune system to be on guard. This can, in turn, develop into persistent inflammation and eventually autoimmune conditions. It has been suggested that altered gut microbiota may play a role in triggering thyroid dysfunction.

Microbiota's role in preventing thyroid dysfunction

While thyroid dysfunction can cause strikingly divergent effects on digestive motility, we may need to consider a bidirectional pathway causing symptoms. This two-way street may help us understand what causes disease, and how we can prevent or reverse it.

It has been recently suggested that the gut microbiota may target the thyroid-stimulating hormone (TSH) receptor. This receptor is important because it is what tells the thyroid to secrete T4, which is converted to the active form of T3. Most of the T4 to T3 conversion happens in the liver and kidneys, but the gut is also involved.

The gut microbiota influences the availability of micronutrients, such as selenium and zinc, which are needed to convert T4 to T3. Probiotics can have a positive influence on thyroid hormones and function, supporting the importance of the gut-thyroid connection.

Gut motility symptoms can help with diagnosis

Thinking about gut motility is useful because it could be an important but subtle symptom that—combined with other complaints—points your doctor toward a diagnosis of thyroid dysfunction. However, diagnosing thyroid dysfunction is not always as simple as it might appear. It is important to consider thyroid function tests when digestive complaints are noted.

One case report in the literature illustrates how important it is to be well-informed, know your symptoms, and what tests might make sense to inform a diagnosis. A 45-year-old woman came to her doctor with vomiting, diarrhea, nausea, and abdominal pain. She was later found to have hypothyroidism but only after going through gastroscopy and duodenal biopsy, colonoscopy, and computed tomography scanning. An earlier thyroid function test would have saved a lot of time and money — as well as lessened the physical trauma of various invasive medical procedures.

Gut dysbiosis can also affect younger people. Two case reports illustrate how paying attention to gut motility and getting the right tests can help with diagnosis. The first is a 21-year-old woman with recurrent nausea, vomiting, loose stool, abdominal pain, and weight loss. A second case is a young man, 25, with the same symptoms.

While the symptoms sound more like Graves’ disease (over-active gut motility), these two cases were found to have hypothyroidism. It is important to consider thyroid function tests when digestive complaints are noted.

What tests should be done to check for thyroid dysfunction?

Two tests are used for thyroid function:

• TSH. Measuring thyroid stimulating hormone.
• T4. Measuring thyroid hormone in the blood.

The results will then indicate if thyroid function is over or under-active.

• High TSH suggests the thyroid is not making enough thyroid hormone.
• Low TSH conveys the opposite, that too much thyroid hormone is being produced.

Combining a “Free T4” test to measure the amount of circulating T4 hormone in the blood plus the TSH provides the most accurate picture of how the thyroid gland is functioning.

When should I see my doctor?

Digestive issues often come and go, but if you notice that you are having recurring digestive complaints along with the other symptoms common to thyroid dysfunction, it would be wise to check in with your doctor.

Your primary care doctor will want to know how long they have been going on and if there are any other symptoms, so grab a notepad and list the symptoms and keep an eye on them. If they persist, make an appointment.

If you are considering getting pregnant or have already conceived, it is important to get seen quickly so that your thyroid hormone levels can be monitored and medication prescribed if appropriate.

Talk to your doctor about your concerns if you suspect thyroid dysfunction, and read widely about the relationship between your diet, the health of your microbiota, and the complex interactions between gut health and overall health.

We are living in an exciting time when science is able to shed light on how good nutrition and exercise habits can help prevent or reverse chronic health conditions. This is very reassuring — there are steps we can take each day to improve our health.