Those of you who have trouble converting Levothyroxine

I’m curious about those of you who have issues with the conversion of T4 to T3 when taking Levothyroxine, after TT or RAI. I read a post from someone who said they were a "bad converter." How did you notice that the conversion wasn’t working? Is it measurable? How were you treated by your doctors? Were there any signs before you had TT or RAI? What alternatives are available if you have conversion issues? And does anyone know what causes problems with the conversion?