Soy left thyroid hormones alone and nudged one signal up
Pooling 18 randomised trials, soy left the thyroid's own hormones unmoved. Thyroid stimulating hormone, the pituitary's signal to the gland, rose 0.248 milliunits per litre.
What the study found
Soy left thyroid hormones alone and nudged TSH up a fraction
Pooling 18 randomised trials, soy left free T3 and free T4, the thyroid's own hormones, unmoved. Thyroid stimulating hormone, the pituitary signal telling the gland to work, rose by 0.248 milliunits per litre. The authors call the clinical meaning of a change that small unclear.
The caveat · Untested where iodine runs short
The rise in thyroid stimulating hormone only just cleared significance, its interval reaching down to 0.001. The pooled trials disagreed sharply with one another. Everyone studied was thyroid-healthy and presumed iodine-replete, so the worry that soy matters more when iodine is scarce goes untested.