At the 13 minute mark he says we simply do not know. My own single data point: I use only T3 for my non-working thyroid with zero T4 showing on labs. Since starting reta I have had to raise my T3 by over ten percent and might need more. My guess is reta sped up my metabolism so I now need extra T3 to match it. Even after the increase I still get cold easily, even living in Florida. I have dropped seventy pounds to my target weight, something that never happened before, and that loss started before I raised the T3. The giveaway was feeling freezing all the time plus labs showing T3 at the bottom of the normal range. Diet has stayed the same, carnivore OMAD for years.
It looks like pairing tirz with another compound that raises calorie burn, such as clen, might turn out better than reta when the priority is holding on to muscle.