Yes, it is biologically possible for a person to have three kidneys, a condition known as a Supernumerary Kidney (SK). This is an extremely rare congenital anomaly where an individual is born with an additional, separate kidney.
Defining Supernumerary Kidney
A Supernumerary Kidney (SK) is defined as an extra kidney structure possessing its own capsule and collecting system. To be classified as a true SK, the third kidney must also have a separate blood supply, including its own artery and vein. This extra organ may be completely separate from the other kidney on the same side or connected to it by loose connective tissue.
The SK must be differentiated from other anomalies, such as a duplicated collecting system (duplex kidney) or a horseshoe kidney. A duplex kidney involves one kidney with two separate drainage systems within a single capsule. A horseshoe kidney is a fusion anomaly where the lower parts of the two normal kidneys join, resulting in a single, U-shaped organ.
The location of the extra kidney is variable, but it is typically found on one side of the body (ipsilateral anomaly). The supernumerary kidney is often positioned lower, or caudal, than the normal kidney on that side, frequently residing in the abdomen or pelvis. It may be drained by its own ureter connecting separately to the bladder, or it may share a bifid ureter that joins the normal kidney’s drainage system.
Developmental Origin and Prevalence
The presence of a third kidney originates during fetal development, specifically around the fifth to seventh week of gestation when the urogenital system forms. The cause is likely an aberration in the formation of the ureteric bud, an outgrowth from the mesonephric duct. Instead of one bud forming on each side, a second, separate ureteric bud develops prematurely or the nephrogenic cord divides abnormally.
This second bud interacts with the surrounding metanephric blastema tissue, inducing the formation of a separate, third renal structure. The accessory kidney develops through a replication of the normal process, resulting in an extra organ unit.
Supernumerary Kidney is an exceptionally rare condition, with fewer than 100 cases definitively documented in medical literature. While the true incidence is unknown, an estimated prevalence based on CT imaging studies is approximately 1 in 26,750 people. It is reported more often on the left side of the body and affects males and females with roughly equal frequency.
Clinical Detection and Functional Capacity
A Supernumerary Kidney is often asymptomatic and rarely causes issues. It is typically discovered incidentally during medical imaging procedures, such as ultrasounds or computed tomography (CT) scans, performed for unrelated health concerns. Advanced imaging allows for correct diagnosis by visualizing the separate capsule, blood supply, and collecting system.
The functional capacity of the third kidney varies widely. Most often, the accessory kidney is smaller than the two normal kidneys and may function poorly (hypoplastic). This diminished function can result from its abnormal position or drainage issues. The third kidney is also prone to complications, including kidney stones (calculi), recurrent infections (pyelonephritis), and fluid buildup (hydronephrosis) due to obstruction.
Clinical management depends on whether the extra kidney is causing problems. If the SK is asymptomatic, functional, and complication-free, it is generally monitored with regular follow-up imaging. If the third kidney is non-functional or causes recurrent pain, persistent infections, or obstruction, surgical removal (nephrectomy) may be necessary to resolve the symptoms.