I'm sorry to hear about Ben too.
Scott, there are lots of websites that give insight into hyperthyroidism and other cat health issues. www.sniksnak.com has some useful info on hyperthyroidism and so did www.winnfelinehealth.org
My Cleo was diagnosed with it at the end of August (see my thread of today's date) and has responded well to medication. Fortunately she eats well and has gained weight. However now she has responded to medication it has unmasked the true state of her kidneys which are starting to fail.
So if your cat is losing such a lot of weight it could be the hyperthyroidism in conjunction with other things, not always evident from blood tests because hyperthyroidism gives false results for other issues e.g renal failure.
Once Ben's thyroid is under control a bit, blood tests might tell another story, as in Cleo's case.
I wish you and Ben well, it's not easy, is it?