Hello, I'm Dr. Deb and will do my best to help you today.
I'm sorry that Lexie has been diagnosed with liver issues but glad that your vet has suggested liver protectant supplements for her.
There's quite a wide range of dosages for Milk Thistle (Silymarin)--- 20–50 mg/kg per day ---which would mean anywhere from 100 mgs to about 300 mgs. I tend to dose in the middle of the range would would be about 200 mg for a 12 lb dog.
I also recommend the following for my patients:
1. Vitamin E--d-Alpha-tocopherol (not d-l) at a dose of 10 IU/kg/day which would be 50 IU for a 12 lb dog
2. SAMe (S-adenosyl-methionine) at a dose of 10 mg/lb once daily which would be about 100 mg
SAMEe and Milk Thistle can be purchased separated as human supplements but they are combined in one veterinary product called Denamarin (which I wanted to mention if you're not aware of it).
4. Many dogs with liver disease will benefit from antibiotics since bacterial colonization can easily take place. If he's not already taking them, Clavamox,Cephalosporin or Metronidazole are good choices. I usually prescribe them for several weeks at least.
5. And, sludgy bile is frequently a component of liver issues so I recommend Actigall (Ursodiol) for my patients, too, although this is a drug which would have to be prescribed by your vet.
I hope this helps. Deb