What causes protein in piss?

Answer 1

Proteinuria, protein in urine, can be caused by many, many things. Only way to know what is causing your problem is for your doctor to do further testing. Some causes are:
cystitis
UTI (urinary tract infection)
cirrhosis of the liver
diabetes mellitus
infectious mono
Epstein-Barr virus
rheumatoid disease
pre-eclampsia (in pregnant women)
polycystic kidney disease

Those are only a FEW of the things that cause it. The most common causes are the first two I listed...cystitis and UTI.