Objectives: Small intestinal bacterial overgrowth is defined as an abnormally high bacterial population level in the small intestine. Intestinal motor dysfunction associated with hypothyroidism could predispose to bacterial overgrowth. Luminal bacteria could modulate gastrointestinal symptoms and interfere with levothyroxine absorption. The aims of the present study were to assess the prevalence and the clinical pattern of bacterial overgrowth in patients with a history of overt hypothyroidism and the effects of bacterial overgrowth decontamination on thyroid hormone levels.

Methods: Fifty (50) consecutive patients with a history of overt hypothyroidism due to autoimmune thyroiditis were enrolled. Diagnosis of bacterial overgrowth was based on positivity to hydrogen (H₂) glucose breath test. Bacterial overgrowth positive patients were treated with 1200 mg of rifaximin each day for a week. A glucose breath test, gastrointestinal symptoms and thyroid hormone plasma levels were reassessed 1 month after treatment.

Results: Twenty-seven (27) patients with a history of hypothyroidism demonstrated a positive result to the breath test (27/50, 54%), compared with 2 in the control group (2/40, 5%). The difference was statistically significant (p <0.001). Abdominal discomfort, flatulence and bloating were significantly more prevalent in the bacterial overgrowth positive group. These symptoms significantly improved after antibiotic decontamination. Thyroid hormone plasma levels were not significantly affected by successful bacterial overgrowth decontamination.

Conclusion: History of overt hypothyroidism is associated with bacterial overgrowth development.

Excess bacteria could influence clinical gastrointestinal manifestations. Bacterial overgrowth decontamination is associated with improved gastrointestinal symptoms. However, fermenting carbohydrate luminal bacteria do not interfere with thyroid hormone levels.