A person can indeed have three kidneys. The condition is called a supernumerary kidney, and it involves a fully separate extra organ with its own blood supply, drainage system, and surrounding capsule. Fewer than 100 cases have been formally documented in the medical literature, making it one of the rarest congenital anomalies of the urinary tract. Most people who have one never know it exists until imaging for an unrelated problem picks it up by accident.
What a Supernumerary Kidney Actually Is
A supernumerary kidney is not just a lump of extra tissue attached to a normal kidney. It is a distinct, encapsulated organ with its own artery feeding it from the aorta, its own vein draining into the vena cava, its own internal collecting system for urine, and its own ureter (or at least a connection to one).1PubMed Central. Right supernumerary kidney: A rare entity In one documented case, the extra kidney sat below the normal left kidney and received arterial blood from a separate branch of the aorta at a distinctly lower level than the native kidney’s artery, with its own vein emptying into the vena cava even further down.2Radiology Case Reports. Left supernumerary kidney: A rare case presentation The third kidney is typically smaller than the two normal ones, sometimes considerably so, and it usually sits on the same side as one of the native kidneys rather than floating off in some unexpected location.
One important distinction that comes up in diagnosis is the difference between a supernumerary kidney and a duplex kidney. A duplex kidney looks like one large kidney that has been internally divided into two separate collecting systems, sometimes draining through two ureters. But it shares a single capsule and a single blood supply. The supernumerary kidney, by contrast, is wrapped in its own capsule and fed by its own vessels. That independence is what makes it a genuinely separate organ rather than a kidney that split internally.1PubMed Central. Right supernumerary kidney: A rare entity The distinction matters clinically because the two conditions carry different risks and require different management approaches.
How a Third Kidney Forms Before Birth
During normal fetal development, each kidney grows from the interaction between two structures: a bud that sprouts from the developing urinary duct and a mass of tissue called the nephrogenic cord that the bud signals to start forming kidney tissue. Normally there is one bud per side, and one kidney per side results. A supernumerary kidney appears to form when something goes wrong with that process early on. The leading explanation is that the nephrogenic cord develops a double tail, stimulated either by a branching (bifid) bud or by two completely separate buds emerging on the same side.3PubMed. Supernumerary kidney Each bud induces its own mass of kidney tissue, and each mass develops its own capsule, blood supply, and collecting system. The result is two kidneys on one side and one on the other.
This is related, at a molecular level, to the same group of developmental pathways that produce other congenital kidney and urinary tract anomalies. Mutations in genes that control how and when the ureteric bud forms have been linked to duplex kidneys and other malformations in this family of conditions.4PubMed Central. Duplex kidney formation: developmental mechanisms and genetic predisposition Some of the genes implicated in these abnormalities are related to tiny hair-like cell structures called cilia, which appear to play a role in signaling during bud formation, though the exact connection is still being worked out.5Nature Communications. Genetic link between renal birth defects and congenital heart disease The same gene alterations can lead to a wide range of different structural outcomes in the urinary tract, which partly explains why supernumerary kidneys come in so many different configurations.6PubMed Central. Genetic Basis of Ureterocele
How Rare Is It, and Who Gets One
A supernumerary kidney is among the rarest congenital anomalies in medicine. Fewer than 100 cases have been reported in the published literature.7PubMed Central. A Case of Supernumerary Kidney One research group that reviewed CT scans systematically estimated the prevalence at roughly 1 in 26,750 people.8PubMed Central. Supernumerary kidneys: a clinical and radiological analysis of nine cases That estimate is probably closer to the truth than the case-report count suggests, because many people with a small, well-functioning extra kidney will go their entire lives without a scan that reveals it. The true number of people walking around with three kidneys is almost certainly higher than what the medical literature reflects.
Most reported cases have been in males and on the left side, and the extra kidney tends to sit below (caudal to) the normal kidney on that side.9PubMed Central. Supernumerary kidney laparoscopically treated Whether the male predominance is a true biological pattern or an artifact of who gets scanned is hard to say given how few cases exist. The first reported case dates back to 1656, described by a physician named Martius, and the condition has continued to generate interest and new case reports for more than 350 years since.10PubMed. Four miniature kidneys: supernumerary kidney and multiple organ system anomalies
The Surprising Range of Configurations
Not all supernumerary kidneys look alike, and the variety is striking. Some sit entirely separate from the native kidney on the same side, connected only by their shared position in the abdomen. Others are partially fused to the native kidney. In one reported case, a fused supernumerary kidney shared a common pelvis and a single ureter with the native kidney while maintaining its own separate arteries and veins.11Research and Reports in Urology. Fused Supernumerary Kidney with Single Pelvis and Ureter; Presenting with Stones: A Case Report and Literature Review In another, the third kidney was fused to the lower pole of the opposite native kidney, forming a horseshoe-shaped bridge of tissue across the midline.12PubMed Central. Supernumerary Kidney (Triple Kidney) With Horseshoe Malformation: A Case Report
The configuration can get even more complex. In one case, a patient had been thought to have a crossed fused ectopia, where one kidney migrates across the midline and fuses with the other. When a CT scan was performed, it turned out the kidney that had crossed the midline was not the patient’s native right kidney at all. It was a supernumerary kidney fused to the lower edge of the left kidney.13PubMed Central. Crossed Renal Ectopia with a Fused Supernumerary Kidney These kinds of diagnostic surprises underscore how difficult it can be to tell a supernumerary kidney apart from other congenital anomalies without high-quality imaging. And in one extreme case, a patient was found to have four miniature kidneys along with abnormalities in other organ systems.10PubMed. Four miniature kidneys: supernumerary kidney and multiple organ system anomalies
How People Find Out They Have a Third Kidney
The majority of supernumerary kidneys are discovered incidentally. Someone goes in for imaging because of abdominal pain, a urinary tract infection, or an entirely unrelated problem, and the radiologist spots an unexpected extra organ. The diagnostic toolbox includes ultrasound, CT urography, and MRI, with CT urography being the most definitive because it shows the anatomy of the blood supply, collecting system, and ureters in sharp detail.14PubMed Central. An incidental finding of a left-sided supernumerary kidney
Ultrasound sometimes picks up the anomaly first, but ultrasound alone is often not enough to tell whether you are looking at a supernumerary kidney, a duplex kidney, or a mass that is not a kidney at all. The reason CT urography is so useful is that it can trace the separate arteries, veins, and ureters that define a true supernumerary kidney, distinguishing it from lookalikes. Because the condition is so rare, the initial imaging report sometimes gets the diagnosis wrong, labeling it as a duplex kidney or crossed ectopia. It may take a specialist’s review of the images, or a second imaging modality, to get the anatomy right.
When a Third Kidney Causes Problems
Many supernumerary kidneys function quietly in the background for an entire lifetime. The extra kidney filters some amount of blood and produces some urine, and if its drainage is unobstructed, the person never develops symptoms. Problems arise when something goes wrong with that drainage or when the extra tissue becomes a site for infection or stone formation.
The most common complications reported in case studies include:
- Pyelonephritis: infection of the kidney, which may be the very symptom that leads to discovery of the extra organ.15African Journal of Urology. Late presentation of supernumerary kidney in a 35-year-old man: a case report
- Kidney stones: the extra collecting system can accumulate stones, sometimes large ones. In one case, a fused supernumerary kidney contained stones throughout both collecting systems, including one measuring roughly 4 cm.11Research and Reports in Urology. Fused Supernumerary Kidney with Single Pelvis and Ureter; Presenting with Stones: A Case Report and Literature Review
- Hydronephrosis: urine backs up and swells the kidney when the ureter is kinked or blocked.
- Abdominal mass: a swollen or enlarged extra kidney can be felt on physical exam, sometimes prompting worry about a tumor before the true nature is identified.16Radiology Case Reports. Case report of right supernumerary kidney in a 38 year old male
One case involved a 41-year-old man who came in with fever, abdominal pain, and painful urination. Imaging revealed not only a supernumerary kidney but one that was horseshoe-shaped and malrotated, with active pyelonephritis.12PubMed Central. Supernumerary Kidney (Triple Kidney) With Horseshoe Malformation: A Case Report In cases like these, the unusual anatomy of the extra kidney makes it more vulnerable to infection and obstruction than a normally positioned organ would be.
Cancer in a Supernumerary Kidney
Because the supernumerary kidney contains genuine renal tissue, it carries the same potential for malignancy as any other kidney. The numbers are tiny given how rare the condition itself is, but cancers have been documented. In a review of 51 reported supernumerary kidney cases, two involved carcinomas. A separate case report described the first confirmed urothelial carcinoma arising in a supernumerary kidney.17PubMed Central. Right supernumerary kidney with urothelial carcinoma: A case report Renal cell carcinoma, the more common kidney cancer type, has also been reported in a supernumerary kidney, with only three such cases documented in the literature at the time of the report.18Intisari Sains Medis. Renal cell carcinoma in a supernumerary left kidney: A very rare case report
There is no evidence that the extra kidney is more cancer-prone than a normal one. The small number of cancer cases likely reflects the small number of known supernumerary kidneys overall. Still, the possibility is worth keeping in mind. If you are one of the rare people who knows they have a third kidney, any surveillance imaging your doctor recommends should include the extra organ, not just the two native ones.
Treatment and When Surgery Is Needed
If a supernumerary kidney is functioning normally and causing no symptoms, the standard approach is to leave it alone. There is nothing inherently dangerous about having extra kidney tissue, and removing a healthy organ would carry more risk than monitoring it. For uncomplicated infections or non-obstructing stones, conservative treatment with antibiotics and pain management is the first-line approach.16Radiology Case Reports. Case report of right supernumerary kidney in a 38 year old male
Surgery comes into play when the extra kidney is chronically infected, producing recurrent stones, severely hydronephrotic, or harboring a mass suspicious for cancer. In one case, a woman with a small, poorly functioning supernumerary kidney and an abnormally draining ureter underwent laparoscopic removal of the extra kidney and its ureter. The surgery was successful, confirming that minimally invasive removal is feasible even for anatomically unusual cases. The patient did develop a urine leak (urinoma) and fistula afterward, but both resolved with conservative management rather than reoperation.9PubMed Central. Supernumerary kidney laparoscopically treated
One tricky surgical consideration is that the anatomy is never textbook. Surgeons cannot rely on standard anatomical landmarks when the blood supply, ureter path, and position of the extra kidney vary so much from case to case. Detailed preoperative imaging is essential, and even then, the operative field can hold surprises.
Why Duplex Kidneys Come Up So Often in the Same Conversation
Duplex kidneys are much more common than supernumerary kidneys and share some of the same embryological roots, which is why the two conditions are constantly discussed together. A duplex kidney has two separate internal collecting systems but remains a single organ under one capsule. It is one of the most frequent congenital urinary tract anomalies, and most people with one never develop symptoms.4PubMed Central. Duplex kidney formation: developmental mechanisms and genetic predisposition
When duplex kidneys do cause trouble, the problems follow a well-known pattern. The upper section’s ureter tends to insert into the bladder at an abnormally low and medial position, which can lead to obstruction. The lower section’s ureter tends to insert higher and more laterally, with a short tunnel through the bladder wall that makes it prone to urine refluxing backward toward the kidney.19PubMed Central. Complete renal duplication with lower pole obstructive ectopic ureter – An exception of the Weigert-Meyer rule This pattern, known as the Weigert-Meyer rule, holds true in most cases but not all. Exceptions exist where the lower section is the one that obstructs instead.20PubMed. Uncrossed complete ureteral duplication with dysplastic lower moiety: a violation of the Weigert-Meyer law
The practical overlap between duplex and supernumerary kidneys is that both involve extra renal tissue, both can produce extra ureters, and both can cause confusion on imaging. A duplex kidney with two ureters draining separately might initially look like a supernumerary kidney until the shared capsule and blood supply are identified. Going in the other direction, a fused supernumerary kidney that shares a ureter with its neighbor can mimic a duplex kidney until a closer look at the vascular anatomy reveals two separate organs.
Animals Get Them Too
Supernumerary kidneys are not unique to humans. They have been documented in other species, including cats. In one reported case, a necropsy of an eight-year-old mixed-breed cat revealed four kidneys: two on the left side and two on the right. Each kidney had its own cortex and medulla, but the kidneys on each side shared a partially fused pelvis that drained through a single ureter.21PubMed Central. Bilateral fusion of a supernumerary kidney in a cat The cat had died from kidney failure and chronic intestinal disease, though it is unclear how much the extra kidneys contributed to the renal problems versus other factors.
The fact that supernumerary kidneys show up across species reinforces the idea that the underlying developmental process is deeply conserved. The molecular signals that tell the ureteric bud where to sprout, how many times to branch, and when to stop are broadly similar across mammals. When those signals misfire, extra kidney tissue can form regardless of whether the animal has two legs or four. Veterinary cases are even rarer in the published record than human ones, partly because animals do not routinely undergo the kind of cross-sectional imaging that catches incidental findings in people.