I don't know whether the percentage would be higher or not; in other words, I don't know if the oral bioavailability is 100% or nearly so.
But, what would it matter if instead of getting a given effect from so many mcg (say 50 mcg) of T3 orally, one could instead inject 40 mcg or 30 mcg? Cost savings?
Probably not after paying for vials, submicron filters, and syringes.
Certainly not IM or sub-Q injection: that would generate an enormous local overdose.
So IV then; but that would require an aqueous solution, in which T3 has very poor long term stability.
Not worth it for the sake of being able to inject a few less mcg to get the same result (you can always get as much effect as you want, or more, with oral.)