Hi Bethan, welcome to Kidney Community! We haven’t funded any research into a link between CKD and hypothyroidism so unfortunately I can’t help there.
I really hope the biopsy goes well and you get some of the answers you’re looking for. Feel free to keep us updated if you wish 
There are some hints at tips from other people with stage 3 CKD on this topic: Advice on Kidney disease stage 3 - Uncategorised - Kidney Community