Interference or non-interference between soy and levothyroxine: that is the question. A narrative review of literature

Objective: To evaluate the effect of soy intake on L-T4 absorption among different L-T4 formulations. Methods: A PubMed/MEDLINE, Web of Science, and Scopus research was performed. Case reports, case series, and original studies written in English and published online up to 30 November 2022 were selected and reviewed. The final reference list was defined based on the relevance of each paper to the scope of this review. Results: Few data, mainly case reports, seem to suggest a possible interference of soy products on L-T4 tablets absorption. However, the only prospective randomised cross-over study showed no differences in L-T4 absorption when L-T4 and soy isoflavones were assumed concomitantly.The very little data available on liquid L-T4 formulations do not allow for any conclusions to be made, even if a double blind placebo controlled trial showed no impaired L-T4 absorption. Conclusion: The inference of soy products on L-T4 absorption, if present, seems to have little clinical impact. Considering this fact, the Hamlet-like question whether soy milk interferes or not with levothyroxine absorption remains unanswered.