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The Gut-Thyroid Connection

As scientists more deeply explore the gastrointestinal environment, otherwise known as the microbiome, they are gaining awareness that it is connected to many other bodily functions. Through their research, we’re learning that the state of the microbiome influences not only digestive function, but also the brain and cognitive function, neurotransmitters, the immune system, and possibly thyroid function. When the microbiome is lacking healthy bacteria in balanced proportions, due to damage from toxins, medication use or poor diet, inflammation ensues. Over time, that inflammation becomes systemic and can affect all the other systems of the body which the gut is connected to.

SIBO, small intestinal bacterial overgrowth, can significantly alter the microbiome due to excessive bacteria in the small intestines. It is not easily diagnosable or treatable, but is believed to more likely occur in patients with chronic conditions such as IBS (irritable bowel syndrome) and fibromyalgia. Some  of the potential causes of SIBO include poor digestion due to low gastric acid or digestive enzymes, or GI motility disorders.  SIBO symptoms can include abdominal pain, diarrhea, and bloating.

SIBO sets the stage for systemic inflammation, which can trigger a host of chronic conditions, particularly autoimmune reactions. Inflammation can cross react with thyroid tissue to cause an autoimmune thyroid condition. There hasn’t been a lot of research on this topic, but there was one small study that investigated the relationship between hypothyroidism and SIBO.

The goal of the study was to determine if patients with hypothyroidism and GI motility disorders were more vulnerable to GI bacterial overgrowth. 50 patients with autoimmune thyroiditis and hypothyroidism were tested for bacterial overgrowth through a H2 breath test. If they were positive, they were treated with an antibiotic and symptoms were monitored. All patients were also being treated ongoing with thyroid medication.

It was determined that slightly over half the hypothyroid patients tested positive for bacterial overgrowth, as compared to a health control group in which only 5% were positive. This indicated that there may be a significant positive correlation between SIBO and thyroid dysfunction. The antibiotic treatment did successfully resolve the bacterial overgrowth and related symptoms, however it did not alter thyroid hormone levels.

It’s interesting to note that the symptoms related to microbiome imbalances overlap with some of the symptoms of hypothyroidism, such as constipation, diarrhea, brain fog and fatigue. It’s important for physicians to keep this in mind, as this overlap could cloud the clinical presentation and prolong the diagnosis of either of the conditions.

We know that low body temperature is often linked to thyroid dysfunction. It would be helpful if future research investigated the role of body temperature on the microbiome. It’s possible that normalizing body temperature helps balance the microbiome more effectively when done in conjunction with other gut supportive treatments.


Reference: Association between Hypothyroidism and Small Intestinal Bacterial Overgrowth, The Journal of Clinical Endocrinology & Metabolism, Volume 92, Issue 11, 1 November 2007, Pages 4180–4184,


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