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

Given the prevalence of gut issues reported here, often (but not always) in those taking levothyroxine, I hope the existence of this paper helps.

At least it is something to quote!

Very interesting to know how adding liothyronine (T3) - or liothyronine-only - would be assessed.

World J Gastroenterol. 2017 Feb 7; 23(5): 842–852.

Published online 2017 Feb 7. doi: 10.3748/wjg.v23.i5.842

PMCID: PMC5296200

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

Thorsten Brechmann, Andre Sperlbaum, and Wolff Schmiegel

Thorsten 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.

Keywords: Bacterial overgrowth syndrome, Hydrogen breath tests, Immunosuppression, Intestinal motility, Hypothyroidism

Full paper freely available here:


7 Replies

helvella thanks for posting this, I'm pondering your question of whether the addition of T3 makes a difference or in fact whether taking NDT makes a difference? Wouldn't we all like to know?

"Multivariate analysis confirmed that levothyroxine therapy is a stronger predictor of SIBO than hypothyroidism. The underlying mechanism is unclear. One might speculate that hypothyroidism leads to hypomotility, but, surprisingly, levothyroxine therapy was even more associated to SIBO and not able to reverse the effect of hypothyroidism."



At least with this paper available someone, somewhere who has facilities available, might also ask the question. :-) And yes, of course, include desiccated thyroid.

I have long questioned the whole concept of oral administration of thyroid hormones. Whilst we do have some recycling/recirculation capability, in healthy people that is only a modest amount - not our full daily dose.

It doesn't seem at all surprising that some oral medicines are likely to have significant effects on our guts.

I'd very much like to see a proper comparison between a slow, injected dose and oral administration.


I find it no coincidence that as soon as I started NDT the stomach problems that were plaguing me and getting worse, just disappeared. That along with a good probiotic has made a huge difference.

This does not surprise me at all.


Maybe that's why, although I haven't got autoimmune thyroid disease, I'm a lot better when I don't eat carbs or gluten?? I've always thought that levothyroxine messed up my gut which in turn imbalanced everything else. I'm wondering whether the fact that B12 and levothyroxine both are processed in the illium is a reason for lack of B vitamins absorption?

If levothryoxine actively increases the wrong kind of intestinal bacterium, could it be a cause for disruption of processing vitamins? Getting levothyroxine through the intestines or the terminal illium is not a natural way for our bodies to recieve it so there is bound to be some consequences. Could it be the reason why levothyroxine works so poorly for some people?

Do you think our bodies adapt to the levothyroxine formulation and then even a small change causes a big disruption to our internal bacteria and this is what causes the problems or inability to adapt to new formulations?


Whenever I've read scientific papers or research comparing LT4/T3 combination therapy against LT4 alone I've questioned the indices the scientist measured to prove or disprove superiority or efficacy. I think the indices patients would wish to measure would be different from the ones scientist compare.

So for example, how it affects the gut, breathing, incisive and interpretive reasoning, calmness and I'm sure other people would have a lot of other ideas. As I don't take T3 due to its increasing unavailability and cost I only have other's people's experience to judge by so I really hope someone does more research using different parameters.

I think research should be much more patient led than it is at the moment. Afterall we're the ones with the experience of how it affects us.


I've been taking T3 for about 15 months, which I added to, then replaced the NDT I started taking a little over two years ago. With both medications, solely or together, I found a marked improvement in my digestive health.

I'd like to refer anyone interested to Chris Kresser's website for potentially useful advice re digestive disorders, gut health, hypothyroidism, & general wellbeing. He's pro-paleo, if you're interested in this, & offers what seems to be very good, free advice, that I've foumd helpful.


Thanks for that helvella . I must admit that SIBO has crossed my mind recently . Pp

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