Probiotics do nothing for your thyroid hormones

The gut microbiome has become a popular target for improving almost everything, and thyroid function is no exception: probiotic and prebiotic products are increasingly used by both healthy people and those with thyroid disorders in the hope of optimising hormone levels. The thyroid sets the pace of the body's metabolism, so the idea that a daily capsule could tune it is understandably appealing. Doubts have recently been raised about whether these products deliver on that promise. This review set out to test the claim against the randomised trial evidence.

The authors searched PubMed, Scopus, Web of Science and Embase up to April 1st, 2023, without any language restriction, for randomised controlled trials testing probiotics, prebiotics or synbiotics against thyroid measures. The outcomes were free T3, free T4, thyroid stimulating hormone and TRAb, an antibody involved in Graves' disease. Eight articles met the inclusion criteria. Results were pooled using a random-effects model and reported according to PRISMA guidance.

Supplementation made no significant difference to any thyroid hormone: TSH, free T4 and free T3 all sat essentially where they started. The one significant finding was a reduction in TRAb levels, the antibody that drives Graves' disease, and the included trials broadly agreed with each other on that point. The authors found no indication of publication bias. Their conclusion is blunt: probiotics and prebiotics do not influence thyroid hormone levels, though they may modestly reduce TRAb in people with Graves' disease.

This pooled analysis directly answers the doubts it opens with, and the answer for healthy thyroids is a clean null across all three hormone measures. The TRAb signal is more interesting and fits the broader idea that the gut microbiome interacts with the immune system, but it concerns a diagnosed autoimmune condition rather than general thyroid health. With only eight trials in the pool, questions remain about specific strains, doses and durations. Larger trials in Graves' disease would be needed to firm up the antibody finding.

If you have a healthy thyroid and are taking, or considering, a probiotic for thyroid support, this is a reason to be sceptical of that particular promise: the hormones simply did not move in the pooled trials. Probiotics may have other merits, but tuning thyroid output does not appear to be one of them. The antibody reduction is a matter for people with Graves' disease and their clinicians, not a signal for supplement shoppers. Knowing where a supplement's evidence ends is as useful as knowing where it begins.