Senior Dog and Parsley

About 4 months ago, I started feeding my 15 year-old mini dachshund a pre-mix with various lightly cooked meats--on this diet she is healthy and lively for her age. However, around that time she also started drinking and peeing more than ever before, and I wondered whether this is attributable to the parsley (as diuretic) in the pre-mix, and whether it's a good idea to include the parsley in this way in her daily diet. I am concerned about overworking the kidneys, especially as she has been on cortisone meds in past.

It would be most useful to know exactly what was in the pre-mix... when you say pre-mix are you referring to a mixture of foods that comprises her entire diet or is this an adjunct to other foods? If the latter, what else is she receiving? If the former, please let me know what is in the mix.
Parsley does have duiretic properties but this alone shouldn't cause the symptoms you describe above, hence wanting to know what else she is consuming. At the same time, have you had any blood work done on her recently? Increased urination and thirst can also be implicated in a number of conditions; it would be best to have those ruled out whilst looking at her food.