No known toxicities with soy protein have been reported to my knowledge. Soy estrogens are not known to interact with other medications, but it is always good to double check with your own physician who prescribed you the Prometrium. My Mom recently had a melanoma skin cancer removed and does not want to continue traditional HRT because of increase cancer risks. Animals fed a soy diet have lower rates and spread of melanoma. Soy is also associated with decreased risks of many cancers.
Soy protein is very safe because it is a food.
Of course, continue to regularly check your thyroid levels with your physician. Foxiladi4U, go ahead, please. Soy protein is not known to interact adversely with any prescription medications. Mood swings, anxiety, and headaches are often hormonally-related. We have seen good improvements with soy protein.