Armour is a T3/T4 combination, where synthroid is only T4, with the idea that the body converts T4 to T3. And Armour is a natural form, it's derived primarily from pig thyroid. I have been taking Armour for almost 3 years. There have been issues lately with availability, and if you contact Stella5349 on this forum, she can share more on that. Or, go on the "Save Natural Thyroid" page on Facebook, where you can find a ton of info on the shortages.