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

Purpose: The relationship between probiotics and levothyroxine (LT4 ) 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 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 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. LT4 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 (fT3 ), and free thyroxine (fT4 )] and PTM was found between groups and among visits. FT3 /fT4 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). LT4 daily dose adjustments occurred more frequently in the control than in the study group ( p  = 0.007), despite no differences in the mean LT4 daily dose. In particular, LT4 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.

ClinicalTrialsgov ID: Registration number NCT03095963.