Levothyroxine therapy and impaired clearance are the strongest contributors to small intestinal bacterial overgrowth

Minding your own business, thinking about not very much, pretty mindless television program on, click a button and WOW!

A new paper, which is freely accessible, pointing its finger very clearly at levothyroxine as one of the strongest contributors to a specific gut issue - SIBO. So many people here identify gut issues as a major problem.

World J Gastroenterol. 2017 Feb 7;23(5):842-852. doi: 10.3748/wjg.v23.i5.842.

Levothyroxine therapy and impaired clearance are the strongest contributors to small intestinal bacterial overgrowth: Results of a retrospective cohort study.

Brechmann T1, Sperlbaum A1, Schmiegel W1.

Author information

• 1Thorsten Brechmann, Andre Sperlbaum, Wolff Schmiegel, Department of Gastroenterology and Hepatology, Berufsgenossenschaftliches Universitätsklinikum Bergmannsheil GmbH, Ruhr-University Bochum, 44789 Bochum, Germany.



To identify a set of contributors, and weight and rank them on a pathophysiological basis.


Patients who have undergone a lactulose or glucose hydrogen breath test to rule out small intestinal bacterial overgrowth (SIBO) for various clinical symptoms, including diarrhoea, weight loss, abdominal pain, cramping or bloating, were seen as eligible for inclusion in a retrospective single-centre study. Clinical data such as co-morbidities, medication, laboratory parameters and other possible risk factors have been identified from the electronic data system. Cases lacking or with substantially incomplete clinical data were excluded from the analysis. Suspected contributors were summarised under four different pathophysiological pathways (impaired gastric acid barrier, impaired intestinal clearance, immunosuppression and miscellaneous factors including thyroid gland variables) and investigated using the χ2 test, Student's t-test and logistic regression models.


A total of 1809 patients who had undergone hydrogen breath testing were analysed. Impairment of the gastric acid barrier (gastrectomy, odds ratio: OR = 3.5, PPI therapy OR = 1.4), impairment of intestinal clearance (any resecting gastric surgery OR = 2.6, any colonic resection OR = 1.9, stenosis OR = 3.4, gastroparesis OR = 3.4, neuropathy 2.2), immunological factors (any drug-induced immunosuppression OR = 1.8), altered thyroid gland metabolism (hypothyroidism OR = 2.6, levothyroxine therapy OR = 3.0) and diabetes mellitus (OR = 1.9) were associated significantly to SIBO. Any abdominal surgery, ileocecal resection, vagotomy or IgA-deficiency did not have any influence, and a history of appendectomy decreased the risk of SIBO. Multivariate analysis revealed gastric surgery, stenoses, medical immunosuppression and levothyroxine to be the strongest predictors. Levothyroxine therapy was the strongest contributor in a simplified model (OR = 3.0).


The most important contributors for the development of SIBO in ascending order are immunosuppression, impairment of intestinal clearance and levothyroxine use, but they do not sufficiently explain its emergence.


Bacterial overgrowth syndrome; Hydrogen breath tests; Hypothyroidism; Immunosuppression; Intestinal motility

PMID: 28223728

DOI: 10.3748/wjg.v23.i5.842


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59 Replies

  • What about T3 & NDT?

    Is it not a hypo issue due to low stomach acids & enzymes?

  • Good question about T3 and desiccated thyroid.

    Thought: T3 is better absorbed than levothyroxine. Perhaps it is as simple as there being less thyroid hormone of any sort sloshing around in the gut because most has been absorbed?

    In desiccated thyroid, the simple fact of it being in a large amount of thyroglobulin (and other subsatnces) reduces the availability of thyroid hormones to the gut bacteria?

  • Would this be a case for taking NDT sublingually too?

  • Maybe - but I still suspect that a lot gets swallowed.

    Again, please some proper investigation of what really happens.

  • Leverette, It would be interesting to know about NDT and T3. However, by far the majority of thyroid patients are started on levothyroxine, and that's when the IBS develops/manifests. So, whilst it may improve or go away once on NDT or T3 (and improved stomach acid, dietary changes etc.), it was the levo that started the ball rolling.

  • Oh my gosh that comment has made me think this is the reason I have suffered ibs like symptoms since a year or so after starting levothyroxine!

    Certainly something for me to read more on before I get to see the endocrinologist.

  • I self-diagnosed secondary hypothyroidism, & as my tests results are always in "normal" range, I self-medicate with success after 40 years of hypo symptoms. My TRH was never tested. I think, wrongly diagnosed with FM as most symptoms vanished when I'm medicated. A common FM symptom is IBS & digestive disorders.

    I have always had a weak digestive system & had issues with meat, hence being healthier on a vegetarian diet, though I feel it's been more robust, as has the rest of me, since starting THs. My friend who is now hypo & on T4 only following RAI has just had a norovirus, for the first time to my knowledge in 40 years. Is this the T4 or weakened stomach acid? I also know a few people from my FM pain clinic (who have hypo symptoms) with Helicobacter pylori, which people with weak stomach acid are more prone to. Is there a connection with this & the bacteria that cause SIBO?

  • What has the BTA to say about this report or the RCoP?

  • Am tempted to suggest that, if based on previous experience of what the BTA say, it will not add much to the discussion. :-)

  • Probably haven't read it either, I should think and it will go the way of other studies - in the bin.

  • @helvella An erudite observation!

  • @shaws I suspect their line would be:

    T4 monotherapy is nevertheless still the only treatment we recommend as being effective for hypothyroidism and there are very good drugs GP's can prescribe to control SIBO

  • Correct, just as they prescribe additional tablets for insomnia, pain relief, palps, etc. etc. etc. So how much more would these cost - probably the same as NDT now that T3 must cost more than an amount of gold ie. $7 per ounce.

  • It must depend on ones diet as well I would think. Or would like to think 😕

  • amala57, Given that SIBO flourishes on sugar and carbohydrates, and given that this was a retrospective study, and given that for far too many years we have been brainwashed into eating low fat high carb diets...

    That said, it doesn't change the fact that ingestion of levothyroxine through the digestive system triggered the SIBO in so many patients. Very many of us suddenly and inexplicably 'acquired' IBS around the very same time that we started on levothyroxine. But of course, our GPs/gastros are adamant that the two conditions are not related.

  • Oh I didn't know that lots of you aquired ibs! That's scary! I hope you have it under control now.

  • a poll?

  • Yes, please.

  • @crabapple seconded!

  • TSH110 , when you mention someone it should come up blue like your name in this post and they get a mention under notifications. Are you not clicking on the name when it comes up?

  • crabapple sometimes it gives me the names but sometimes if does not give me anything at all! Hence it not working last time but oddly allowing me to choose you this time. It seems a bit glitchy at times

  • Sometimes it takes f-o-r-e-v-e-r to display the names. But it won't do what you expect if you don't wait and click on the name in the list.

  • @helvella In my case it sometimes never displays the names ever! I have subconsciously clocked that the @+namedoes not turn blue I presume when the feature for some reason is not working. Like this time - I have waited several minutes...it is definitely not going to work...I thought it was misspellling but it isn't or a gap between @ and name. At least I know if it don't go blue it ain't going to work

  • If it isn't turning blue, post what you have done so far, then go back in and edit it. Often works.

  • Sometimes the names are below the text if you are on a phone - so scroll down a bit :-)


  • Marz the names do appear ok when it is working. I will try Helvella's fix when it happens again my posts are typo rich so editing will be needed 😂😂😂

  • Marz gosh now I see what you mean the name was right below the post - that is probablythe answer 😉 Ta 👍🏽

  • helvella

    I will try that of course it worked this time 🙄😊

  • Sxd's law.

  • crabapple

    Especially with the seive brain hypo has given me anything new requires much practice and repetition before finally entering my long term memory - a once finely tuned rapid fire And reliable - none of the above now vague glacial and flakey more accurate descriptors!

  • I need a run-up getting used to my new phone before I'm willing to have internet access on it. So you're ahead of me there. ;)

  • crabapple i remember thinking I would never get the hang of texting it took me ages to learn it! You see these kids rattling away like machine guns txting faster than the speed of light! Guess they used computers before they could walk 😳

  • Medics seem happy to think they've fixed one problem, whilst refusing to acknowledge they've just caused an equally bad health issue that somebody else has to fix!

  • I don't understand, it starts by saying it picked patients who had been ruled out for SIBO. So what made them conclude that they did have it after all? It's late and I am sure I am missing something obvious!

  • Thanks so much for this it's an area I'm interested in from an academic viewpoint. I have not read this yet, but a quick comment. SIBO is strongly associated with hypothyroidism probably a consequence of reduced motility. It is likely that the association with levothyroxine is due to residual hypothyroidism, some patients not given enough. It is quite unlikely that NDT or L-T3 would fare any better as too much levothyroxine is associated with diarrhoea, i.e. excess motility. As I say I haven't studied the paper yet and it's past my bedtime!

  • Excellent points jimh111.

  • now theres a surprise

    i could have told them that many years ago

    both levo and t3 are synthetic and things like methylated spirits are used in production

    thus its no wonder that anyone who is chemically sensitive will have very bad effects from

    levo or t3

  • I didn't know and doubt many do on this forum that methylated spirits are used in the production. No wonder I was so very, very unwell on it.

  • We have no knowledge how they de-fat pig thyroid to make desiccated thyroid. In the early days, this was done with some form of solvent such as a petroleum distillate. No idea what is used now.

    Also, the really bad side of methylated spirit might be the dye and the bitter substance! The sort used in pharmaceuticals could be colourless and without the bittering agent. The rest is just common or garden ethanol (drinking alcohol, ethyl alcohol) and a bit of methanol (methyl alcohol). Although significant amounts of methanol are injurious to health, extreme trace amounts are present in quite a lot of foods and drinks.

    Even screenwash doesn't often cause problems - despite many products containing methanol:

    Most patients (71.9%) ingesting automotive screenwash did not develop features. The implication is that the amount of screenwash ingested was very small. Skin and eye exposure produced either no features or only minor toxicity.


  • Just the thought!!!! Good job we don't know what is in most of the stuff we take.

  • I think I got SIBO. I did not know levo causes it.

    I thought I have IBS-c or IBS-d. These days, I got loose stool and diarrhea for no reason. I quit taking magnesium but problem persisted. Diarrhea came with dehydration so I had hard time and I need to see a doctor. Maybe I need to take Rifaximin to fix it.

    Anyhow I guess SIBO and IBS are related or SIBO caused IBS.

    Levo has a lot of side effects and is ruining my health condition as I have problems all parts of my body now.

    Thank you for your information.

  • My husband was quite literally falling apart when on levo and or t3 he was in horrendous pain and life was not worth living now having been on NDT for about 12 yrs he is fit and well and age 74 he had hashis and graves running together from childhood will not describe the hell on carbimazole or the years after thyroidectomy and no thyroid meds thanks to reliance on tsh and t4 tests

  • May I ask what NDT your husband has been taking?

  • Is hypothyrodism itself a known cause of SIBO I wonder it certainly played havock with my guts they have never been quite right since but seem a bit better on NDT - sugary coffee is my weakness and that is not helping the old digestives I'd say

  • TSH110 Yes, hypo itself can contribute or cause SIBO because of the actual hormone imbalance, potential for slowed digestive motility and lowered stomach acid.

    Add to that the fact that many people get prescribed antibiotics by GPs who think they simply have a tummy upset or something. The antibiotics then exacerbate the SIBO and it becomes a vicious cycle :(

  • Maybe someone should come up with thyroxine patches or sub lingual? Would that solve the problem if it's the thyroxine?

  • Silver_Fairy , how about a thyroid hormone dosing implant that automatically delivers the hormone/s into the blood stream in the way a normal thyroid would.

  • Love it! At the very least give it a small-scale trial to see whether it acts (as we would hope) more in the way a working thyroid does - pushing out small amounts of levothyroxine into the bloodstream. Perhaps that should be a T4+T3 combination... ? :-)

  • It seems perfectly feasible to invent such a thing, and yes, mimic the thyroid by delivering both T4 and T3. Add to that an App to take your hormone readings every day so that you can fine tune your dose all by yourself (shock, horror, put patients in control of their dosing... hmm, now that's maybe asking too much? ;) )

  • Funny how self-adjusting doses is so readily accepted in some areas - such as pain control.

    (Just wait until somehow dreams up a blood test that is regarded as "the TSH of pain" - see how long patient control lasts. :-( )

  • helvella hope I have shed my mortal coil before the TSH of pain test arrives - sounds like a fate worse than death!

  • That's the best idea I've heard about thyroid care. It's always made me cross that Diabetic patients are encouraged and required to take control of their condition but Thyroid patients are treated like recalcitrant children.

  • Well this is an interesting and frustrating paper. Frustrating because they do not define hypothyroidism or levothyroxine therapy. I would assume that with just a few exceptions all the hypothyroid patients would be on levothyroxine therapy. The numbers indicate this is not so. I have e-mailed the author for an explanation and will let you know when I get a response.

    Generally the paper points to reduced motility leading to SIBO, so perhaps the links with hypothyroidism and levothyroxine use is a consequence of patients being a little under-medicated on average. Reference 27, the Lauritano paper, relates to levothyroxine patients that appear to be a little under-medicated with 1.39 and 1.44 mcg/Kg compared with the standard recommendation of 1.6 mcg/Kg. Of course patients believe this is sometimes too little. The stronger link between SIBO and L-T4 therapy may be because the patients receiving higher L-T4 doses are experiencing hypothyroid symptoms but the doctors are reluctant to fully treat them because of 'guidelines'. These more severe cases may also need T3 therapy to recover.

    A confounding facor is that hydrogen breath tests are not very accurate at diagnosing SIBO. They measure a spike of hydrogen in the breath within two hours caused by bacteria in the small intestine. However, this release of hydrogen can also occur if there is unusually rapid small bowel transit, these patients usually suffer from diarrhoea. I guess this is less likely in IBS patients. There is an article on this in the September 2016 issue of Gut Reaction, you have to be a member of the IBS Network to read it. Alternatively, this rather horrid (to understand) paper ncbi.nlm.nih.gov/pubmed/248... demonstrates how unreliable the breath tests are. (I have only read the abstract).

    So, it seems hypothroidism leads to SIBO by slowed intestinal transit and on this basis some patients are under-treated. But we can't be certain because the breath tests are not totally reliable. We can say the breath test results correlate with hypothryoidism and levothyroxine treatment.

  • Look forward to your update when you hear back!

  • To clarify the difference between the numbers of patients who are hypothyroid and those receiving levothyroxine. Some patients are receiving levothyroxine for struma (goitre) or after thyroid surgery, these would not be included in the hypothyroid group. Also, the study was retrospective and some of the information may have been lost, such as the initial diagnosis. I believe this is the major cause of the discrepancy, the accounts of some of the initial diagnosis are not on record in the data the study used.

  • I'm thinking it might not necessarily be about impaired motility because I'm on a high dose of levothyroxine and still have stomach problems and bad absorption problems. I'm wondering if it could actually be the levothyroxine's effect on the small intestine?

  • Remember this study reports on hydrogen breath tests which are less accurate that culture studies where they pass a tube into the intestine. If the transit from the stomach is rapid it can cause a hydrogen spike and falsely indicate SIBO. Maybe your gut problems are nothing to do with SIBO. Also thyroxine enters the gut via the biliary tract so levothyroxine should not cause problems although of course fillers, especially lactose, might do so.

  • Perhaps, I'd never heard of SIBO. Absorption of critical nutrients is problematic for quite a few thyroid patients and intestinal overgrowth is one of the possible causes isn't it?

  • Levo seems to contract blood as mentioned above. My blood pressure increased and blood contract (bad blood circulation) and beats. It got more serious when I wan on Levo 3 years and have been on more than 5 years since I got my thyroid gland removed due to cancer.

    So I quit it for 3 weeks recently, but I feel severe hypothyroidism so I started levo again. This time with T3. On the first day of resume, I felt blood vessels constricted (pressed)and beats fast & strong, reducing blood flow, especially my arms & leg and my head including bronchus(I have asthma). That was why I got stiffness and got my arms& legs asleep and nerve pain due to weaken nerves (I was informed bad blood circulation makes nerves weak)

    So I better try NDT.

    My doctors did not agree side effect of levo and just say too much levo cause hyperthyroidism but I now get very sensitive ad I get more hypothyroidism and feel levo is the problem. I don't know why they do not listen to patients and insist that there is no problem with levo.

    I tried a few other levo's so it is not the filer problem but levo itself.

    When dosage is low or your are healthy side effect is less but I am very weak now after 5 years of levo and can feel it more. After all the treatment I failed and came to the conclusion of levo is the problem for me.

    Also I got IBS or SIBO as you posted.