Interesting research, although inconclusive, examining the effect of probiotics on thyroid hormone. It appears there may be some possible effect on thyroid hormone stability but further study is needed.
"These data generate the hypothesis of a “stabilization” of LT4 treatment after probiotics ingestion, suggested by the reduced dose adjustments needed in the treatment group, as well as the need of a dose reduction observed only in subjects assuming probiotics. In particular, VSL#3® seems to be able to prevent hormonal fluctuations, perhaps modulating THs enterohepatic recycle and justifying the absence of differences in serum hormonal levels. The effect, however, is limited and no major clinical consequences can be proven at this stage.
The possible probiotics role as “LT4 therapy stabilizers” could be an endearing research starting point for both endocrinologists and investigators concerned with probiotics, in order to clarify the interaction between intestinal environment and endocrine system. This association was not evaluated so far and further studies should explore the relationship between gut microbiota and THs, considering other probiotic strains or longer treatment."