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Small intestine in detail illustration

Small Intestinal Bacterial Overgrowth (SIBO)

What is SIBO?

Small Intestinal Bacterial Overgrowth (SIBO) is essentially the presence of excessive bacteria in the small intestine.  This increased number of bacteria can cause symptoms on a varying scale, from very mild to more severe cases that present as chronic diarrhoea, weight loss and malabsorption.1  Displaying symptoms similar to those of Irritable Bowel Syndrome (IBS) can cause confusion during diagnosis and treatment. In fact, some people with IBS can also have SIBO.1

There are thought to be 2 main types of SIBO2

  • Hydrogen-predominant SIBO
  • Methane-predominant SIBO

Treatment can vary depending on the type of SIBO identified in the patient and SIBO can recur.2

What causes SIBO?

 SIBO is not widely recognised or particularly well understood and research can be hard to come by.

However, the presence of excessive bacteria in the small intestine has been linked to a lack of movement of the intestine such as anatomy abnormalities; diverticulosis; surgery (such as gastric bypass or surgery on the colon); adhesions after radiation therapy or multiple abdominal surgeries or amyloidosis (build-up of amyloid protein deposits in the small intestine).2

Excessive use of certain medicines has also been linked to SIBO.  These include narcotics, anti-spasm medications often prescribed for IBS, medications that are used to decrease stomach acid and heartburn (proton pump inhibitors) and the frequent use of antibiotics.2

SIBO is thought of as a medical phenomenon by many experts,3,4 but in the late 1990s, more research was put into this condition.1 This has influenced a better understanding of the gut microbiome and its roles in human health and disease.

What are the symptoms of SIBO?

As mentioned, symptoms displayed can be similar to those associated with IBS. Symptoms of SIBO include:1,2

  • constipation
  • diarrhoea (extreme in some cases)
  • bloating
  • increased flatulence
  • abdominal pain
  • nausea
  • fatigue
  • malabsorption
  • weight loss
  • nutritional deficiencies
  • osteoporosis

What is the connection between hypothyroidism and SIBO?

Hypothyroidism decreases the gastroesophageal motility.  Reduced gastroesophageal motility is one of the factors considered in the diagnosis of SIBO.5

Decreased gastric motility results in a slower passage of food through the intestines, enabling more bacteria to build up in the small intestine, resulting in some or all of the symptoms listed above.5,6

Thyroid hormones, especially T3, are used to support the proper movement of food through the intestines and therefore people with hyperthyroidism tend to suffer from diarrhoea, whereas people with hypothyroidism tend to suffer from constipation.7


There has been some testing carried out using a hydrogen and/or lactulose breath test to diagnose SIBO.6 This test involves taking breath samples that will then be analysed. There are very specific instructions to follow ahead of this testing process, including fasting and avoiding certain foods and medications prior to your test date.8

The testing of breath samples is used to identify the levels of hydrogen or methane in the breath.1,2


The most common treatment for SIBO is an antibiotic called rifaximin.1,6 This is also used to treat traveller’s diarrhoea and tummy bugs as well as other gastric conditions.9

Rifaximin is usually used to treat Hydrogen-predominant SIBO but treatment for Methane-predominant SIBO may include rifaximin and neomycin – a bacteria-fighting antibiotic medicine.2

Treatment is aimed to control the overgrowth of bacteria.


In addition to prescribed medications, specific changes to diet may also benefit the symptoms of SIBO, particularly the reduction in fermentable products.10

Whilst there is no definitive cure, symptoms can be managed by medications and dietary changes11

You can obtain a SIBO test from:

Genova Diagnostics using our code – A42AQ and this link:

Regenerus Labs using this link:

For more support check out our online community:


  1. NCBI
    Small Intestinal Bacterial Overgrowth
    A Comprehensive Review
    Andrew C. Dukowicz, MD, Brian E. Lacy, PhD, MD, and Gary M. Levine, MD
    Gastroenterol Hepatol (N Y). 2007 Feb; 3(2): 112–122.
  2. Johns Hopkins Medicine
    Small Intestinal Bacterial Overgrowth (SIBO)
  3. Is SIBO a real condition?
    Dr. Alan Christianson,
  4. IBS or Small Intestinal Bacterial Overgrowth
    GI Society – Canadian Society of Intestinal Research
  5. Does Hypothyroidism Affect Gastrointestinal Motility?
    Olga Yaylali, Suna Kirac, Mustafa Yilmaz, Fulya Akin, Dogangun Yuksel, Nese Demirkan, and Beyza Akdag
    Gastroenterol Res Pract. 2009; 2009: 529802.
  6. NCBI
    Link between hypothyroidism and small intestinal bacterial overgrowth
    Anant D. Patil
    Indian J Endocrinol Metab. 2014 May-Jun; 18(3): 307–309.
  7. Dr Hedberg
    The Hashimoto’s Disease and SIBO Connection
  8. NHS
    Hydrogen Breath Test
    Royal United Hospital Bath
  9. Rifaximin for the treatment of acute infectious diarrhea
    Kyoung Sup Hong; Joo Sung Kim
    Therap Adv Gastroenterol. 2011 Jul; 4(4): 227–235.
  10. ACG Clinical Guideline
    Small Intestinal Bacterial Overgrowth
    Pimentel, Mark MD, FRCP(C), FACG; Saad, Richard J. MD, FACG; Long, Millie D. MD, MPH, FACG (GRADE Methodologist); Rao, Satish S. C. MD, PhD, FRCP, FACG
    The American Journal of Gastroenterology – February 2020 – Volume 115 – Issue 2 – p 165-178
  11. SIBO (Small Intestinal Bacterial Overgrowth) Causes, Symptoms, Treatments, and Diet
    Benjamin Wedro, MD, FACEP, FAAEM

Date updated: 28.04.21 (V1.1)

Review date: 31.03.22

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