Thyroid Problems

The Gut Side of an Underactive Thyroid

The gut connection here is well documented, but it works differently from what most people expect. Thyroid hormones set the pace of many processes in the body, including how fast food moves through the intestines. When thyroid hormone levels are low, that movement slows. Bacteria that should be swept along the small intestine linger and multiply instead. The result is bloating, wind and discomfort, and it can also affect how much thyroid medication gets absorbed. What the evidence does not show is food or supplements shifting thyroid hormone levels themselves. The gut's role here is about movement and absorption.

Slow gut, misplaced bacteria

The small intestine normally holds relatively few bacteria compared with the large intestine. When too many take up residence there, the condition is called small intestinal bacterial overgrowth (SIBO), meaning an abnormally large bacterial population in a stretch of gut that should stay sparse.

Researchers compared 49 adults with a history of an underactive thyroid against 323 people without one. Fluid was sampled directly from the small intestine during an endoscopy. Overgrowth was present in 33% of the thyroid group. In the comparison group the figure was 15%.

A much larger review of medical records supported this. People with a history of an underactive thyroid were 2.2 times as likely to develop overgrowth over ten years. For those with autoimmune thyroiditis, where the immune system attacks the thyroid gland, the figure was 2.4 times.

The kinds of bacteria differed too. Klebsiella species were common in the thyroid group with overgrowth. A different family of bacteria dominated in people who had overgrowth but no thyroid history.

Source: Relationship Between Hypothyroidism, Risk of Small Intestinal Bacterial Overgrowth, and Duodenal Microbiome Alterations

Why this matters for the tablet

That link has a practical consequence. Levothyroxine, the standard replacement for a missing thyroid hormone, is absorbed in the small intestine. Conditions in that stretch of gut affect how much of each dose gets through. An earlier study first flagged the link between an underactive thyroid and overgrowth. It also noted that bacteria sitting in the small intestine could interfere with absorption of the medication.

The more recent work found something encouraging. Among people taking levothyroxine, the raised risk of overgrowth was largely gone. Treating the thyroid appears to restore normal movement through the gut, and normal movement keeps bacteria where they belong.

Source: Association between Hypothyroidism and Small Intestinal Bacterial Overgrowth

What the supplement trials actually found

It would be neat if all this meant a probiotic could help thyroid function. Pooled trial evidence says it does not. Researchers combined eight randomised controlled trials testing probiotics, prebiotics or a mixture of the two in people with thyroid conditions. Across those trials there was no effect on thyroid hormone levels. There was a modest reduction in one antibody linked to an overactive thyroid, but the hormones themselves did not move.

That is a clear finding and worth taking at face value. Gut supplements are not a route to changing thyroid function.

Source: Effect of probiotics or prebiotics on thyroid function: A meta-analysis of eight randomized controlled trials

What this means for you

Taken together the case is narrow but solid. Low thyroid hormone slows the gut, slow movement lets bacteria build up where they should not, and that affects both comfort and how well medication is absorbed. Thyroid problems need diagnosis and treatment from a doctor, and taking levothyroxine as prescribed does more for the gut side of this than any dietary change will. It is usually taken on an empty stomach well before food, since fibre and other foods can reduce how much of the dose is absorbed. Away from that window, a varied plant-rich diet supports regular movement through the gut, which is the mechanism that keeps bacteria in the right place. Fibre from whole grains, pulses, vegetables and fruit is what the bacteria of the large intestine feed on, and a well-fed community there is part of what keeps things moving. That is the idea GutLab is built on, where everyday gut health starts with regularly feeding the microbiome, the community of microbes living in the gut, a broad range of plant-rich ingredients.