Keto dieting is the best thing since sliced bread! LOL.
But really, I am a big keto diet advocate. Let me just say that a keto diet can not be wrong. There is absolutely no such thing as an essential carb!
As far as losing muscle while on the diet, it should not be a concern at all if your healthy fats are in the right ballpark. If you are not taking in enough fats(healthy fats), you will appear flat, and eventually start to notice muscle loss...considering your are taking in plenty of protein.
The very first time I used the Palumbo Diet, I got leaner than ever. Easier than ever before. I felt GREAT the entire time. And I gained muscle while in the cut. The key for me wasn't more protein. It was messing with my fat intake until I got it up to where it needed to be.
And for T3, as said above, it is an individual thing. Mostly a decision! Those horror stories of messing up your thyroid are just that, stories. In this community where excess are abuse are so prevalent, you just simply don't see the issue of thyroid shutdown. There is a study showing long time use(years) with relatively high doses, and the patients stopping cold turkey and it only taking up to 2 weeks for the thyroid to rebound. The thyroid is very resilient.
However, there is absolutely no need to go much higher than 50mcg per day. 62.5mcg should be the absolute highest anyone should ever try. And that is on gear. It can be very catabolic in high doses. And for fatloss, high doses are not needed.