Review: Plant-Based Estrogen Effects Still Not Understood

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....At the same time, there are areas where phytoestrogens do show promise, the team emphasized. For example, in breast cancer, a 2013 analysis of 40 randomized controlled trials, 11 uncontrolled trials, and 80 observational studies led to the conclusion that soy consumption may be associated with reduced risk of breast cancer incidence, recurrence, and mortality.

Even in this case, however, the authors indicated that there is not enough evidence to confirm the safety of using soy isoflavones in doses that are high enough (generally >100 mg/day) to obtain the anti-cancer benefits.

Conversely, some of the more widely discussed harmful effects of phytoestrogens also remain unproven. Rietjens and colleagues highlighted a 1997 study in which the soy isoflavones daidzein and genistein were found to inhibit thyroid peroxidase, but no other adverse effects on thyroid functions had been observed.

Human trials on soy isoflavones and thyroid function are entirely inconclusive, Rietjens and colleagues found.