Lady has been getting milk thistle for almost ten years to protect her liver from her seizure medication. It really is great stuff.
If your Yorkie has liver disease, though, you need find out first. She needs to get a bile acids test. It's a very easy and inexpensive test that all Yorkies should have anyway.
Depending on the results, you would then decide if milk thistle is appropriate for her.
Here is a great link about milk thistle:
MILK THISTLE