Almost forgot - thanks for the PM. I also agree with your SQ fluid statement! More is not always better!
Also I did not go into the treatment or evaluation of secondary hyperparathyroidism which may be an issue as well. I would talk to your vet about it. It may be a complicating issue in the reasons why your pet is losing weight. However if the phosphorus is above 6 you may not be able to treat at this point until the level is brought down. So... for another discussion.
Keep us posted.