Probiotics Ingestion Does Not Directly Affect Thyroid Hormonal Parameters in Hypothyroid Patients on Levothyroxine Treatment

PURPOSE: The relationship between probiotics and levothyroxine (LT(4)) requirement has not yet been investigated. The aim of this study was to assess whether a mixture of highly charged Lactobacilli and Bifidobacteria (VSL#3(®)) is able to influence LT(4) 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(®) + LT(4)) and the control group (LT(4)). A 2-month treatment phase was followed by 2 months of follow-up. Clinical examination, blood tests for thyroid function and for peripheral tissue markers of thyroid hormones (PTM) were performed monthly. LT(4) dose adjustments were performed 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 (fT(3)), and free thyroxine (fT(4))] and PTM was found between groups and among visits. FT(3)/fT(4) ratio was directly correlated to TSH at each visit in both groups, with the exception of the first evaluation of probiotics-treated subjects (rho = 0.287, p = 0.076). LT(4) daily dose adjustments occurred more frequently in the control than in the study group (p = 0.007), despite no differences in the mean LT(4) daily dose. In particular, LT(4) doses were increased six times in the control group and decreased four times in the study group. CONCLUSION: 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. CLINICALTRIALS.GOV ID: Registration number NCT03095963.