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The Estrogen Effect
Soy contains plant estrogens called "soy isoflavones." These chemical compounds have biological effects similar to human estrogen hormones but are mostly of lesser potency. Soy-based foods are often recommended to women to blunt the effects of menopause.
Bodybuilders don't like excess estrogen because, they theorize, too much may inhibit the effects of testosterone or make them store fat. They want to maximize muscle and strength and minimize fat. As a result, many bodybuilders, weight trainers, and athletes won't use soy protein foods or supplements because they fear this negative effect on body and performance.
This concern is controversial and the research shows mixed results. One small study on 10 resistance-trained men published in 2013 demonstrated that 14 days of supplementation with soy protein did appear to partially blunt testosterone following an acute bout of heavy resistance exercise. But a 2018 study on 47 previously untrained men that resistance trained for 12 weeks found that soy supplementation didn't appreciably affect either male or female hormones. Two earlier studies reported similar findings.
In light of the fact that soy doesn't seem to be as effective as other types of protein supplementation, if you're concerned about effects on testosterone, you may want to consider supplementing with a different form of protein.
Thyroid Disease
Anyone with thyroid disease or a predisposition to thyroid dysfunction should limit his or her intake of soy-based protein, due to its potential to affect hormone balance and inhibit the absorption of thyroid hormone medication. No soy product should be consumed within 2-4 hours after taking thyroid medications. If you choose to eat a soy product it, should be done as far apart from thyroid hormones as possible. *
