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In healthy people, there are two functioning kidneys, each doing about 50 percent of the work of balancing electrolytes and maintaining pH by removing waste materials, primarily urea, from the bloodstream. In some people, one kidney, usually the left, is a little larger than the other. Moderate enlargement of a kidney due to condition called hydronephrosis is not always a cause for concern.

Hydronephrosis is an enlargement of one or both kidneys due to accumulation of urine. This condition is not unusual in pregnancy because of pressure on the necks of the kidneys and on the ureters that connect them to the bladder, and because of the effects of the hormone progesterone. This hormone, which is particularly abundant during pregnancy, causes a growth in the "pelvis" of the kidney. Enlargement of the kidneys may begin in the second trimester and not resolve until six to twelve weeks after the baby is born

KIdney stones can cause urine to back up and the kidneys to enlarge, although that's really not the symptom one would notice first. Kidney stones cause intense flank pain and irritation during urination. When kidney stones actually block the flow of urine, they usually cause a "colicky," inconsistent pain, sometimes intense, sometimes letting up. Stones are relatively common in younger people, aged 20 to 49. If you have made it to the age of 50 without kidney stones, you probably won't develop them. Kidney stones are especially common in the South of the United States.

Prostate cancer in men can cause blockage to the passage of urine and kidney enlargement. Unlike hydronephrosis and kidney stones, prostate cancer usually causes enlargement of both kidneys, not just one. 

The conditions that cause shrinkage of the kidneys are usually more dangerous. Symptoms usually aren't noticed before the kidneys have already lost 50 percent of their function. Kidney shrinkage most often results from:

  • Congenital renal dysplasia is a birth defect caused by inadequate blood supply in the embryo. In this condition, the kidneys are 50 percent smaller than usual from birth. The condition usually affects both kidneys.
  • Chronic pyelonephritis, which is a long-term bacterial infection of the kidneys. This condition usually has three symptoms: Blood in the urine, fever, and flank pain. The infection can also cause nausea and vomiting. Flare-ups of this condition can be life-threatening. This condition may affect one of both kidneys.
  • End-stage kidney disease is most commonly a complication of poorly controlled diabetes. It causes weakness, bone problems, and intense itching. This condition affects both kidneys.

Almost anyone who suffers from one of these three conditions can experience nausea, vomiting, skin itching that doesn't respond to ointments, back pain, anemia, back pain, and blood in the urine. There can be frequent urination during the early stage of the disease followed by scant urination in the later stages of the disease. Treating the disease can stop it, but aside from a few experiments involving stem cell therapy, nothing restores the size of the kidney other than transplant.

The good news about kidney size is that kidney function doesn't usually depend strictly on size. Our kidneys have redundant capacity so that a greatly diminished kidney can continue to remove waste products and help to regulate calcium, electrolytes, pH, and blood pressure. One can lose a great deal of size without noticeable changes in health. However, by the time "size matters," medical management is usually very complicated and dialysis if often on the horizon.

There is at least one situation in which there is no worry whether an infant's kidneys are both the same size. That's when they are different sizes but both within the normal range, and there's no problem with kidney function.

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