@mrainne welcome to Mayo Clinic Connect.
First I’ve never heard of Nephrogenic Diabetes Insipidus. I’ve got some learning to do!
Second I’m somewhat familiar with poly cystic kidney disease (PKD), but I have chronic kidney disease (CKD) and lowering eGFR.
Third being bipolar and on Lithium can be tricky as you probably already know. Are your Lithium level labs and dose still monitored by the prescribing Dr.? Sometimes when you’ve been on a medication a long time, an adjustment becomes necessary. Sometimes meds can be flushed out of your system with too much hydration. Ask your provider to make sure nothing like that is happening. I don’t know if that has anything to do with declining eGFR but it’s worth asking about.
Fourth your weight and blood pressure are good so you are doing a good job taking care of yourself.
Fifth going to Mayo is a good thing. You are in great hands there. Their collaborative team approach will get to the bottom of this.
My hope and prayer for you is a thorough diagnosis and comprehensive plan to help restore you to as good as you can be! Keep us posted?