Synthetics are Synthroid, Levoxyl, Cytomel.
Naturethroid, Armour are both animal derived from porcine (pigs) ......
In addition, levothyroxine contains only T4. This inculdes, as Laura said, Synthroid, Levoxyl, generic levo, etc. Cytomel contains only T3. T4-only and T3-only meds can be combined to offer maximum symptom relief. 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.