When I am dead, I hope it is said;

Has anyone considered looking into the target organ (kidney and ion channels) and not just the endocrinology. (Per your analogy - checking out for hardware issues and not just software.) It is more rare for a channel to be defective than for endocrinologic changes, but when the endo doesn't look off, one should consider alternatives. The folks at DECODE in Iceland have been identifying a number of rare gene mutations common in Northern Europeans so there is a chance that the cause of your disease may have been identified by them. Have they checked to see if you might have one of the less severe mutations in the CFTR (cystic fibrosis) gene? That mutation is certainly common in whites and mutations can be mild or severe.