Available online 13 March 2024

Small intestinal bacterial overgrowth is common in hypothyroid patients with IBS

Gastrointestinal (GI) issues affect levothyroxine absorption and TSH optimization

Switching from levothyroxine tablet to solution normalizes TSH in this population

Switching to levothyroxine solution improves gastrointestinal and thyroid outcomes

Switching to levothyroxine solution resolves GI symptoms in 1 in 5 patients

Malabsorption of levothyroxine (LT4) is often seen in patients with hypothyroidism and gastrointestinal (GI) conditions. Our study was designed to establish the prevalence of small intestinal bacterial overgrowth (SIBO) in patients with hypothyroidism and irritable bowel syndrome (IBS), and to demonstrate that liquid LT4 is more consistently absorbed vs. tablet, leading to improvement in thyroid and GI symptoms.

This was a single-center, open label, prospective cohort study of liquid LT4 in 75 adult patients with hypothyroidism and IBS. Patients were transitioned from LT4 tablets to solution at equivalent dosing. Patients returned at 6 and 12 weeks for repeat thyroid levels and completion of validated questionnaires. A standard 2-hour SIBO breath test was administered at Week 6. Patients recorded daily stool appearance and frequency.

Prevalence of SIBO was 65.3%. Liquid LT4 normalized TSH in a higher percentage of patients vs. tablet (77.55% vs. 57.14%); significantly decreased TSH in subjects with SIBO; improved hypothyroid symptoms, IBS symptoms, and stool appearance in all groups; and significantly altered bowel frequency among those with SIBO.

SIBO is common in patients with hypothyroidism and IBS. Among SIBO patients, LT4 tablets were inefficiently absorbed, leading to suboptimal thyroid control; however, transitioning from LT4 tablets to solution normalized TSH and improved hypothyroid symptoms. Liquid LT4 also significantly improved GI symptoms in all patients with hypothyroidism and IBS, regardless of SIBO status. Additionally, 1 in 5 patients had complete resolution of IBS symptoms after switching from LT4 tablets to solution, independent of changes in TSH.

hypothyroidism

IBS

levothyroxine

malabsorption

SIBO

© 2024 Published by Elsevier Inc. on behalf of the AACE.