Soy contains plant estrogens (phytoestrogens), which may cause harm to a cat's thyroid gland. Many pet food manufacturers use soy to add bulk and a relatively inexpensive source of protein. According to Dr. Lisa Pierson, "Given how common hyperthyroidism is in the cat, soy has no business being in cat food."
SO WHAT? If you're feeding commercial food, look for animal-based, rather than plant-based, sources of protein for your carnivore.