SIBO (small intestinal bacterial overgrowth) is one of the most common underlying causes of IBS. And is often overlooked. Symptoms that may indicate SIBO are: bloating, abdominal cramping, gas, diarrhea, constipation, chronic illness (such as fibromyalgia, chronic fatigue syndrome and autoimmune conditions, food intolerances, vitamin deficiencies, and, of course, if you have previously been diagnosed with IBS [1].

Symptoms of SIBO occur when bacteria that are present in the large intestine begin to populate the small intestine. One of the main causes of this phenomenon is a decrease in gastrointestinal motility [2]. And motility is where the thyroid comes in.

Thyroid hormones act on the intestinal nervous system to control the pace of digestion. When there is less thyroid hormone, digestion slows, and SIBO is able to manifest [3].

Studies conducted over the last decade have shown a significant comorbidity between a history of hypothyroidism and SIBO.

A study [4] that tested for SIBO prevalence in patients with a history of hypothyroidism. Based on thyroid levels, glucose breath test to evaluate for bacterial presence, and symptom reporting resulted in 54% of patients with a history of autoimmune (hashimoto’s) thyroiditis and low thyroid hormone having SIBO, versus only 5% in the control group. Another study [5] evaluated levels of aTPO, the marker for hashimoto's thyroiditis, and found that patients with SIBO had significantly higher levels of aTPO, especially in SIBO patients with constipation.

Interestingly, levothyroxine (Synthroid) (a T4 thyroid hormone replacement), use was highly correlated with increased SIBO [6]. Showing that giving T4 thyroid hormone may not contribute to the correction of SIBO in hypothyroid patients and more comprehensive treatment should be explored.