For what it is worth ( I don't have the study in front of me ) there was a clinical study that measured the composition changes of men who were bed-ridden, on 50mcg T3 and 200mg Testosterone per week.
The subjects didn't lose any muscle mass. I distinctly remember this study because it eradicated my fears of losing mass while on T3.
Also, while I was running less than 400mg/wk of various androgens during contest prep, I ran 37.5-50mcg of T3 per day and added 8lbs of lean mass while losing 16lbs of fat mass. This was over the course of about 7-8 weeks though. Used in conjunction with insulin or boat loads of carbs, T3 actually becomes solely anabolic. The catabolic nature of T3 can easily be reduced or removed with insulin or carbs. Unlike other hormones, T3 is both catabolic AND anabolic - just depends on the metabolic environment (fasted or fed state).
Just remember that T3 is strong but insulin is stronger and can overcome T3's catabolic tendencies. The best way I can explain it is using a warehouse full of goods that has a steady stream of incoming supplies that equals a steady stream of shipments. We'll call the supplies amino acids. In this scenario, you'd have zero change in composition and weight.
T3 increases the income supplies (receipts) and the shipments. But the shipments would increase in greater amounts. So we will eventually have a net deficit inventory of supplies (amino acids). We're losing inventory basically.
Insulin and AAS basically slow down the orders and therefore the shipments out of the warehouse, while T3 is still increasing the incoming re-stocking supplies on the other end. The more insulin (carbs) and AAS we use, the slower the shipments (muscle protein breakdown) we see going out. You end up with gaining inventory in that environment because T3 is still increasing re-stock receipts while insulin and AAS are slowing down the loss of inventory on the other side.
Understand? This is why T3 is awesome. It increases protein turnover but when you slow down the use of proteins via anabolics of various sorts, you just get a hyper nutrient-dumping environment from the T3. This is where cheat meals and higher calories really pay off with T3 usage. Combining T3 and AAS, you can really do things that are quite unfair in terms of what the body would be able to do naturally. You can literally transform your body and do it significantly faster to boot.
