I am taking both,,
The thyroid produces T4 and T3, T4 is considered the storage hormone and T3 is considered the active hormone, normally the body converts T4 to T3.

Synthroid and generics are a synthetic T4 only hormone.

Cytomel is a synthetic T3 only hormone.

Armour / dessicated thyroid is a natural T4/T3 hormone ( pig sourced not = to human but as close as we are going to get)

In a perfect thyroid world the body produces a signal called TSH which is like the gas pedal and the thyroid would release T4 and a little T3 to wake up the body and then the body would convert the T4 to T3 ( thyroid hormone with 4 atoms attached/ vs thyroid hormone with 3 atoms attached) on an as needed response and all would be fine.

Throw in thyroid disease, different types, different states and different responses in individuals and the combination of that and people will be on different meds.

For me, I have graves but I also have blocking hormones which means I am currently hypo and on Synthroid, but I also have a painful goitre ( inflammation of the thyroid) and nodules.

My FT4 labs were at the higher end of the range but my TSH was over 2, so in order to lower the TSH to rest the thyroid and not raise the T4 any more we lower the Synthroid a little and added in Thyroid ( Canadian version of Armour).

Thyroid( Armour) falsely lowers the TSH because of the T3, I am still doing labs every 2-4 weeks but the FT4 has only dropped a point and the TSH is back to under 1.

I have gone from taking Tylenol 3-4 times a day to 2-3 times a week, so it is working, but slowly.