Effects of probiotics assumption on serum thyroid hormone and TSH levels in hypothyroid patients on levothyroxine treatment.

Background. Probiotics are live microorganisms able to confer a health benefit to the host, when administered in adequate amounts. Despite the widespread use of probiotics, their pharmacological interference remains unclear. The relationship between probiotics and levothyroxine (LT4) requirement has not yet been investigated. Objective. To assess whether a mixture of highly charged Lactobacilli and Bifidobacteria (VSL#3®) is able to influence LT4 metabolism acting on the gut microbiota. Methods. A prospective, randomized, single-blind, controlled, investigator-started clinical trial was carried out. Patients with primary hypothyroidism were randomly assigned to the study (VSL#3®+ LT4) and the control group (LT4). A two months treatment phase was followed by two months of follow-up. Clinical examination, blood tests for thyroid function and for peripheral tissue markers of thyroid hormones effect were performed monthly for 4 months. LT4 dose adjustments were performed during the study when necessary. Results. Thirty-nine patients were enrolled in the study group and 41 in the control group. No difference in thyroid function (thyroid-stimulating hormone (TSH), free triiodothyronine (fT3) and free thyroxine (fT4)) and peripheral tissue markers was found between groups and among visits. FT3/fT4 ratio was directly correlated to TSH at each visit in the control and in the study group, with the exception of the first evaluation of subjects treated with probiotics. LT4 daily dose adjustments occurred 10 times in 8 patients, more frequently in the control than in the study group, despite no differences in the mean LT4 daily dose. Conclusions. VSL#3® does not directly alter thyroid functional compensation. A probiotics- mediated influence on thyroid hormones homeostasis is suggested since probiotics supplementation could be able to prevent serum hormonal fluctuations.