Liver disease e.g. cirrhosis (most common)
Excess renal excretion e.g. nephrotic syndrome, glomerulonephritis
Excess bowel loss e.g. protein-losing enteropathy in Crohn's
Burns (via plasma loss through the skin)
Hemodilution e.g. in pregnancy
Capillary leak e.g. in hantavirus
Malnutrition
Genetic mutation (rare)
Any acute disease state e.g. sepsis, pancreatitis, Kawasaki
Hypoalbuminemia
Liver disease e.g. cirrhosis 
Proteinuria
Excess renal excretion e.g. nephrotic syndrome, glomerulonephritis
Abnormally colored urine
Excess bowel loss e.g. protein-losing enteropathy in Crohn's
Coagulopathy 
Burns 