The warning on the label is about soy flour, not isoflavones
Levothyroxine's labelling lists soybean flour alongside cottonseed meal, walnuts and dietary fibre as things that cut absorption, and warns against soy-based infant formula.
What the study found
The thyroid drug label names soybean flour, never isoflavones
Levothyroxine's current labelling lists soybean flour alongside cottonseed meal, walnuts and dietary fibre as things that bind the drug and cut how much of it is absorbed, and it warns against mixing a crushed tablet into soy-based infant formula. Isoflavones appear nowhere in it.
The caveat · No figure for how much is lost
The label states the interaction without citing the studies behind it or saying how much absorption is lost. It is one manufacturer's label for one brand, and generic levothyroxine carries its own separate labelling. A drug label is a regulatory summary rather than a study.
What we don't know
The label names soybean flour and soy formula but gives no figure for how much absorption they cost and cites no study, so it cannot say how much a soy meal matters to the dose.