Endocr Pract. 2024 Mar 13:S1530-891X(24)00083-1. doi: 10.1016/j.eprac.2024.03.005.Online ahead of print.

Brittany Bohinc Henderson ¹, Shelby P Smith ², Marlena E Mengelkamp ², Emma Kate Rhymer ², Kensi N Gray ², Abigail G Jackson ², Samantha F Henry ², Stacey Chuang ², Erin H Stavrakas ², Olivia M Blair ², Melissa Heaps ²

Author information

¹Charleston Thyroid Center, 1054 Johnnie Dodds Blvd, Suite A, Mount Pleasant SC 29464. Electronic address: charlestonthyroid@gmail.com.

²Charleston Thyroid Center, 1054 Johnnie Dodds Blvd, Suite A, Mount Pleasant SC 29464.

Abstract

Objective: 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.

Methods: 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.

Results: 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.

Conclusion: 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.