Nephrotic syndrome is a group of problems caused by damage to the kidneys. They are: Too much protein in the urine Low level of protein in the blood High levels of fat and cholesterol in the blood Swelling Normally, your kidneys have tiny filters in small groups of blood vessels. These filters let your body get rid of waste products. You are able to keep the protein and other cells your body needs. If your kidneys are damaged, these filters may not work well. They may start to let proteins leak into your urine. Then, not as much is in your blood and you become swollen.