I've been on a search to find a book to explain all this thyroid stuff to me and I found a really informative book called The Thyroid Solution by Ridha Arem, M. D. I'm only 1/4 through this book and have learned so much already. It explains all about T4 and T3 and how it works in the body. It explains in detail why some people can or can't lose weight with thyroid issues. It is just loaded with very informative explanations as to what exactly is going on in our bodies regarding hypo and hyper thyroidism. I highly recommend it as it has answered a lot of the questions that I see posted here quite a bit.