Researching toy breeds

Dear Dogs Today

I’m looking to buy a toy breed puppy but want to do some more research before I make a decision on what breed to get. A friend recently mentioned that a lot of toy breeds can get hyperglycemia. What exactly is hyperglycemia, how do I stop my future puppy from getting it and what do I do if he/she does get it?

I’ve been specifically looking at Chihuahuas but I’ve heard they’re prone to it, is this also true?

Hope you can help!

Josie, via email