Levothyroxine is the generic for synthroid, which is a T4 med only - those 2 can be interchangeable, but some people do better on one or the other.
Cytomel is a T3 only med. Armour is a natural
thyroid hormone which has both T4 and T3. T3 is a very short lived hormone - only lasts a few hours and often works best to split the dosage over the day.
The body has to convert T4 into T3 in order to benefit. A lot of people do not have a problem making this conversion but others do.