Hi Mary, Thanks so much for reading my article and commenting. I can see that I was not clear or complete in writing that sentence and will correct it, thank you for highlighting this to me.
I hope this can clear it up a bit more:
You are correct that levothyroxine is actually most commonly prescribed, and it is a synthetic version of T4. This medication can cause issues with people who have difficulties converting T4 to T3.
There are however medications – liothyronine (Cytomel) is a synthetic version of T3 – which are sometimes prescribed for those who are unresponsive to thyroxine (T4) or have reverse T3 issues.
Cytomel has more potential for toxicity due to the fast absorption of T3, and because many people don’t have issues with conversion of T4 to T3 I believe Thyroxine is preferred, However in the cases where cytomel is used, it mean that vitamin B2 is not converted as much to it’s active form, FAD.