I've taken lithium for 4 years (admittedly not always at the dose prescribed due to hating the side effects).
My kidney function has always been ok (sometimes low sodium and quite a few doctors calls to check I'm OK following this). My kidney function has, however, started to deteriorate - my GP just says it's fine, and everyones will get lower over a four year period.
Apart from some low sodium results, I've also got low iron, and high WBC and high eosinophils (may be unrelated).
The big problem I have is my genetics. My dad had to have a kidney removed this year due to cancer, and his kidney function has always been low, even before this. My grandad also had this.
I know I' probably being stupid, but I've wanted to come off lithium for a while (it's really not working and making me feel 'flat', hence lowering the dose to feel more 'alive'), but the psychiatrist says as I've been stable on it, I have to keep on taking it, and they'll deal with kidney failure, if and when it happens.
I've seen some research that links kidney tumours and long-term lithium use, and am at the point of just stopping.