NP Thyroid is a natural form of thyroid med. It contains both T3 and T4, unlike Synthroid, which is only T4. Your body has to convert T4 to T3. NP Thyroid is made from pig thyroid and has both T4 and T3. Natural dessicated thyroid medicine predated Synthroid. I also took Synthroid from 1995. I was afraid to switch, but have been on NP for a few weeks now and have had no issues. Maybe a bit calmer. I would like to see more studies on it. Synthroid , if over dosed, can cause bone loss.