Could your thyroid be the cause of your digestive woes?

The thyroid is the butterfly-shaped gland in the lower part of the neck. It produces hormones that regulate many body functions, including metabolism, heart function, digestion, muscle control, brain development, mood regulation, and bone maintenance. It’s essential to your overall function and wellness — which makes it no surprise that thyroid health also contributes to digestive health.

What healthy digestion looks like

In our modern times, digestion issues are very common. But as we often learn in functional medicine, “common” does not always define the way a body should optimally function. Many people think they’re eliminating waste regularly when they’re not. You’ll often hear it’s “normal” to have a bowel movement anywhere from three times a week to three times a day — but what’s “normal” isn’t necessarily ideal.

Your liver clears your body of toxins and moves them to your intestines — so if you’re not emptying waste regularly, those toxins are reabsorbed.^1,2 On the other hand, if you’re having frequent loose stools or diarrhea, that’s also problematic, because your body isn’t absorbing necessary nutrients. This brings us to the thyroid and the digestive trends of both hyperthyroidism (overactive) and hypothyroidism (underactive).

Two directions: hyper vs. hypothyroidism

Digestion and hyperthyroidism

In hyperthyroidism, an overactive thyroid produces too many hormones and increases the body’s metabolism. Because thyroid hormones help break down LDL cholesterol, the excess can push LDL abnormally low. Causes may include Graves’ disease (an autoimmune condition with a genetic component), thyroiditis (inflammation of the thyroid), and toxic nodular goiter. Patients may have an increased appetite, yet in severe cases increased calorie intake isn’t enough to maintain a healthy weight. They often have mild to moderate diarrhea and frequent bowel movements — intestinal hypermotility decreases small-bowel transit time — and fat malabsorption may contribute to excess fat in the stool.³

Digestion and hypothyroidism

In hypothyroidism, an underactive thyroid isn’t producing enough hormone and your whole body may feel like it’s slowing down. With decreased hormone production, the body can’t break down LDL cholesterol, so LDL builds up in the blood. Causes may include Hashimoto’s (an autoimmune condition in which the body attacks the thyroid) and thyroiditis.

Constipation is a common complaint — motility is significantly slower than in people without the condition,⁴ possibly due to intestinal swelling from mucopolysaccharide accumulation in gastrointestinal tissue. When thyroid hormone is replaced, transit time can decrease significantly, improving constipation.⁵

Though constipation is the most frequent complaint with hypothyroidism and SIBO, patients sometimes alternate between constipation and diarrhea — or have diarrhea as a primary symptom — due to bacterial overgrowth.⁷ Thyroid dysfunction is also prevalent in patients with irritable bowel syndrome (IBS), specifically those with subclinical hypothyroidism.⁸ When a patient has IBS, the health of the thyroid should be kept in mind. See our overview of SIBO and how it’s tested for more.

Chicken or the egg?

So thyroid health can affect gut health — but does gut health affect thyroid health? Yes. When the gut is inflamed, it can impair nutrient absorption, including malabsorption of nutrients like iodine and selenium, which are crucial for optimal thyroid health.⁹

A significant portion — about 20% — of the conversion of T4 into the active form of thyroid hormone (T3) occurs in the gut. When there’s an unhealthy balance between beneficial and pathogenic bacteria, this conversion decreases. Lipopolysaccharide (LPS), a component of bacterial cell walls, can also wreak havoc on T4-to-T3 conversion. With increased intestinal permeability (“leaky gut”), LPS can leak through the intestinal walls into the bloodstream, where it inhibits iodothyronine deiodinase — the enzyme responsible for converting T4 into T3¹⁰ — and can decrease the expression of thyroid receptors, so the thyroid’s messages aren’t communicated effectively.¹¹

As covered in why ditching gluten and dairy may help autoimmune thyroid disease, a leaky gut also allows large particles to seep through the gut lining into the bloodstream, which can increase the chances of an autoimmune reaction such as Hashimoto’s.

A comprehensive approach

Gut/digestive health and thyroid health are intertwined, so it’s important to heal both. That may include:

• Replacing inflammatory foods with nutrient-dense options
• Taking probiotics and prebiotics (both in foods and supplementation)
• Managing stress
• Testing for and treating SIBO
• Testing for and treating an underlying thyroid condition

To address many of these factors, we recommend a comprehensive stool test to accurately assess and address gut health. We use these tests to look for infections, enzyme status, whether fat is properly digested, gliadin tolerance, inflammation in the colon, intestinal permeability, and the need for pre/probiotics.