@rotate You would want to look at the eGFR numbers over a range of time. Do the value changes correlate at all with treatment times of your prostrate cancer? That might show a side effect of the medication. Have you changed your diet around, perhaps leaning more to the "eat what feels right/tastes good to me", rather than "this is the healthier option", in the face of your diagnosis? Are you hydrating enough with liquids?

As you can see, there are several factors to consider. I will be interested to hear more from you, and the thoughts of your PCP.
Ginger