Common bedfellows

Approximately 50% of hypothyroid patients have been found to have concurrent small intestinal bacterial overgrowth (SIBO),¹ with these patients often presenting with bloating (indicative of low stomach acid) and constipation (delayed gastric emptying); the two main drivers of SIBO. Research further demonstrates that patients with a history of hypothyroidism, including those on levothyroxine, are at an increased risk of developing this gastrointestinal complication.² As such, reduced thyroid function provides the ideal breeding ground for SIBO development, leading to a perpetual cycle of disrupted gastric emptying, microbiome dysfunction and reduced nutrient absorption, ultimately limiting the availability of nutrients essential for adequate thyroid hormone production. As Practitioners, keeping the hypothyroid-SIBO connection on your radar may therefore help improve clinical outcomes in hypothyroid patients presenting with gastrointestinal symptoms.

Slow thyroid, slow gut motility

Hypothyroidism is a common clinical presentation where the thyroid is unable to produce enough thyroid hormone to initiate the required systemic endocrinological effects.¹ This results in a hypometabolic state characterised by slowed digestion, decreased energy expenditure, reduced gluconeogenesis, increased cholesterol, fatigue, and weight gain.³

With regards to their impact on the gastrointestinal tract, thyroid hormones interact with the enteric nervous system (ENS) initiating the migrating motor complex (MMC), which is responsible for sweeping excess bacteria out of the intestinal lumen.⁴  During fasting states, the MMC promotes peristaltic waves that function to keep gastric contents moving through the bowel, and bacteria out of the small intestine.⁴ Reduced gastric motility and lack of smooth muscle contraction via the MMC therefore reduces the clearance of food through the intestines, resulting in common symptoms such as constipation and anorexia,^5,6 in addition to increased intestinal bacterial numbers. ^2,6,7 Further, hypothyroidism decreases oesophageal and gastric motor activity, resulting in lowered gastrointestinal motility,^1,8 with patients often presenting with some degree of dysbiosis.⁹ Hypothyroidism primes SIBO development.

Hypothyroidism primes SIBO development

SIBO is a condition where an unusually high number of bacteria accumulate in the small intestine.² It is characterised by symptoms of abdominal pain, bloating, flatulence, diarrhea, and constipation.² A positive hydrogen breath test is commonly used to identify the presence of SIBO.²

Results from a 2017 retrospective cohort study demonstrated that the most common factors for the development of SIBO include hypochlorhydria, impaired intestinal clearance, immunosuppressive therapies (including corticosteroids), and hypothyroidism; with those on levothyroxine at an even greater risk.⁷ It is unclear as to why those on levothyroxine therapy have a stronger risk of SIBO, however, research has shown that once SIBO is established in the initial hypothyroid phase, it does not clear when a euthyroid picture has been established, suggesting the drug itself may predispose an individual to SIBO persistence.²

Bacteria - friend and foe

Commensal bacteria are an important part of this clinical picture, as they maintain the sensory and motor function of our gastrointestinal tract also by way of the MMC, which as previously discussed, is fundamental to preventing bacterial overgrowth in the small intestine. In conditions where gastric emptying is delayed, such as hypothyroidism, there is an increased risk of dysbiosis, which negatively impacts the function of the MMC, compounding the effects of reduced gastric motility and microbiome dysfunction. This in turn negatively influences the uptake of iodine, selenium, zinc, and iron, which are essential to the production of thyroid hormones.⁹

In the case of SIBO, iron is a double-edged sword, on the one hand, it is essential for bacterial growth and microbiota composition, however, the composition of the microbiome affects the uptake of iron.⁹ An abundance of Gram-negative bacteria can sequester iron for their requirements and further colonisation, which may impact the host’s ability to absorb this essential mineral.⁹ Iron is important for iodine utilisation and thyroid hormone function, with a deficiency directly impairing thyroid hormone synthesis, storage and secretion.⁹

As such, reducing the dysbiotic bacterial load and supporting MMC function in hypothyroid patients is essential for symptomatic relief of SIBO, as well as improving nutritional status for thyroid hormone production.

Treatment focus:

The use of herbs and nutrients to support thyroid hormone production in these patients will not only support the function of the thyroid but also play a role in re-establishing gastrointestinal health. Treatments aims, as outlined in Table 1, will focus on improving thyroid health, in addition to improving gastric acid production, reducing bacterial overgrowth, improving gastric motility, and restoring a healthy microbiome.

Table 1: Treatments aims and considerations in hypothyroid patients with SIBO

Treatment aim		Herb or supplement
Support thyroid hormone conversion and thyroid function		Withania, bacopa, bladderwrack Iodine, selenium, iron, zinc, vitamin D, vitamin A
Reduce bacterial overgrowth		Phellodendron, golden seal, barberry, pomegranate, myrrh, oil of oregano/thyme, garlic
Improve gastric acid production		Gentian, dandelion root, chen pi, andrographis
Improve gastric motility		Chen pi, ginger, cinnamon, fennel, rosemary, peppermint, lemon balm
Restore a healthy microbiome		Bifidobacterium bifidum W23 Bifidobacterium lactis W51 Bifidobacterium lactis W52 Lactobacillus acidophilus W37 Lactobacillus brevis W63 Lactobacillus casei W56 Lactobacillus salivarius W24 Lactococcus lactis W19 Lactococcus lactis W58

^10,11

When two become one becomes none

Patients with a history of hypothyroidism, including those on levothyroxine therapy, should be considered for diagnosis and treatment of SIBO if presenting with persistent gastrointestinal symptoms. Conversely, those patients presenting with chronic SIBO may be considered for thyroid assessment due to the role that gastric motility plays in the prevention of SIBO and the bacterial influence on the MMC, microbiome function and nutrient uptake. These factors may be significant in the ongoing management of these patients in achieving better health outcomes.

References

1. Patil AD. Link between hypothyroidism and small intestinal bacterial overgrowth. Indian journal of endocrinology and metabolism. 2014 May;18(3):307.
2. Lauritano EC, Bilotta AL, Gabrielli M, Scarpellini E, Lupascu A, Laginestra A, Novi M, Sottili S, Serricchio M, Cammarota G, Gasbarrini G. Association between hypothyroidism and small intestinal bacterial overgrowth. The Journal of Clinical Endocrinology & Metabolism. 2007 Nov 1;92(11):4180-4.
3. Cicatiello AG, Di Girolamo D, Dentice M. Metabolic effects of the intracellular regulation of thyroid hormone: old players, new concepts. Frontiers in endocrinology. 2018 Sep 11;9:474.
4. Barbara G, Stanghellini V, Brandi G, Cremon C, Di Nardo G, De Giorgio R, Corinaldesi R. Interactions between commensal bacteria and gut sensorimotor function in health and disease. Official journal of the American College of Gastroenterology| ACG. 2005 Nov 1;100(11):2560-8.
5. Nakazawa N, Sohda M, Ogata K, Baatar S, Ubukata Y, Kuriyama K, Hara K, Suzuki M, Yanoma T, Kimura A, Kogure N. Evaluation of changes in gastrointestinal motility caused by altered thyroid function in conscious dogs.
6. Thyroid UK [Internet]. Essex England 2021. [Reviewed: 2021 April 21. Cited 2021 July 20]. Available from: https://thyroiduk.org/related-conditions/small-intestinal-bacteria-overgrowth-sibo/
7. Brechmann T, Sperlbaum A, Schmiegel W. Levothyroxine therapy and impaired clearance are the strongest contributors to small intestinal bacterial overgrowth: results of a retrospective cohort study. World journal of gastroenterology. 2017 Feb 7;23(5):842.
8. Yaylali O, Kirac S, Yilmaz M, Akin F, Yuksel D, Demirkan N, Akdag B. Does hypothyroidism affect gastrointestinal motility?. Gastroenterology Research and Practice. 2009 Oct;2009.
9. Thyroid-Gut-Axis: How Does the Microbiota Influence Thyroid Function? Knezevic J, Starchl C, Tmava Berisha A, Amrein K. Thyroid-Gut-Axis: how does the microbiota influence thyroid function?. Nutrients. 2020 Jun;12(6):1769.
10. Bone K. The Ultimate Herbal Compendium: a desktop guide for herbal prescribers. Phytotherapy Press; 2007.
11. Van Hemert S, Ormel G. Influence of the multispecies probiotic Ecologic® BARRIER on parameters of intestinal barrier function. Food and Nutrition Sciences. 2014 Sep 22;5(18):1739.