Naturethroid,
Armour, etc. are dessicated porcine
thyroid and contain both T3 and T4, along with other
thyroid hormones and trace elements. Some people do well on T4-only meds, some do well on synthetic T3/T4 combos, and some only do well on dessicated. It's all an experiment to see what works best for you...what makes most of your symptoms go away. T3-only meds are sometimes used temporarily, but only in special cases.