OR06-05 Patients with Hypothyroidism Have a Higher Prevalence of Small Intestinal Bacterial Overgrowth Based on Duodenal Culture, with a Unique Microbial Profile

Abstract Disclosure: M. Wei: None. G. Leite: None. G.M. Barlow: None. A. Hosseini: None. M. Rashid: None. D. Brimberry: None. D. Flor: None. I. Rivera: None. M. Sanchez: None. S. Weitsman: None. B. Taylor-Harding: None. M. Pimentel: None. R. Mathur: None. Hashimoto’s thyroiditis is a common autoimmune condition, but its etiology has not been fully elucidated. Recent studies suggest that small intestinal bacterial overgrowth (SIBO), a disorder of excessive bacteria in the small bowel, may be associated with hypothyroidism (HT). For the first time we investigate the relationship between SIBO and HT by evaluating the small bowel microbiome by culture as well as sequencing. Methods: Subjects >18 years old who underwent upper endoscopy without colon prep and reported HT via survey were identified. Once identified, a detailed chart review was conducted and HT subjects were confirmed to be taking levothyroxine replacement at the time of endoscopy. Duodenal aspirates were cultured on MacConkey agar, and SIBO was defined as ≥103 CFU/mL. Aspirate DNA underwent 16S sequencing for microbiome analysis. The core microbiome was defined as genera present in 51% of subjects. Results: 49 HT subjects (F=40, age=58.6±14.4yrs, BMI=26.3±7.3kg/m2) and 323 controls (F=165, age=54.4±15.6yrs, p=0.08; BMI=25.4±6.5kg/m2, p=0.4) were compared. All HT subjects were receiving thyroid replacement, so there were no differences in thyroid-stimulating hormone (TSH) levels between groups (HT: 2.07±1.97 uIU/mL vs. controls: 1.94±1.17 U/mL, P=0.5). There was a significantly increased prevalence of SIBO in HT subjects, independent of sex (OR=2.71, CI=1.39-5.30, P=0.005). As SIBO is a major driver of duodenal microbiome changes, subjects were divided into four groups for microbial analysis (HT-/SIBO- n=274, HT-/SIBO+ n=49, HT+/SIBO- n=33, and HT+/SIBO+ n=16). There was a trend towards difference in alpha diversity (P=0.05) and a significant difference in beta diversity among all 4 groups (P=0.001) (PERMANOVA). Genus Neisseria was part of the core microbiome in HT subjects, but not in non-HT subjects. Further, the Gram-negative profile of the core microbiome appeared to be different in HT+ and HT- subjects. In HT+/SIBO+ the core microbiome contained Klebsiella (a genus associated with SIBO), while in the HT-/SIBO+ group, Escherichia (a genus associated with SIBO) was part of the core duodenal microbiome. Conclusion: This is the first study to look at duodenal microbial culture as a gold standard for SIBO in patients with and without hypothyroidism. Using this technique along with microbial sequencing, our study confirms an association between SIBO and HT. Interestingly, sequencing reveals that the microbes associated with SIBO differ in those with and without hypothyroidism. Presentation: Saturday, July 12, 2025