Too much soy is also known to supress thyroid function. I think a little soy in the diet is probably healthy, because is is low-fat protien. But, like everything else, it should be taken in moderation.
More info: Soy isoflavones are phyto-endocrine disrupters. At dietary levels, they can prevent ovulation and stimulate the growth of cancer cells. Eating as little as 30 grams (about 4 tablespoons) of soy per day can result in hypothyroidism with symptoms of lethargy, constipation, weight gain and fatigue.