Soy is a food, but when you start down the "the more the better" route with the supposed "health food," you transform soy into a drug. Overconsumption of soy has been linked to increased risk of thyroid disease. In particular, the craze for soy powders, smoothies, soy patties, and potions -- everything from Revival Soy to Isoflavone pills -- has been problematic, as too much of these unnatural forms of soy may put a strain on your immune system and trigger thyroid problems in susceptible people.
Stick to natural forms of soy -- tofu, tempeh, miso soup -- in moderation, and you should be fine, but stay away from the pills, powders, smoothies, creams and other or your "health food" may actually ruin your health.
For more information on the soy/thyroid connection, see:
Soy and the Thyroid.