I've seen a few Yorkies on here that are therapy dogs but I do think shihtzu's are more laid back in general. Eddie, alaskayorkie's Yorkie is a therapy dog and seems to do amazing at it!
I always wished Jackson could be a therapy dog but I don't really know if it's something you can necessarily "train" a dog to be, ya know? Jackson is such a wonderful dog, and to the people that he knows (even if he's met you only a few times) he will love you forever. But brand new strangers he is just very shy. He won't just allow most strangers to come up and pet him w/o knowing them. He has gotten better so I often wonder if I worked hard enough at it, if it'd be possible, but I just dunno. I've looked at parts of the tests, like walking through crowd, etc... he would be absolutely fine with other dogs, crowds etc but when it comes to strangers petting him... he's just too shy.
Anyways, I'm rambling, lol. I think it'd be wonderful if you adopted a second dog, whether yorkie or not, to become a therapy dog!