The left testicle does the same two jobs as the right: it produces sperm and releases testosterone. There is no division of labor between the two sides. Both testicles contribute roughly equally to hormone levels and fertility, and either one can sustain normal function on its own if the other is lost. What makes the left testicle distinctive isn’t its function but its anatomy, which creates a few unique vulnerabilities worth knowing about.
Sperm and Testosterone Production
Each testicle contains tightly coiled tubes where sperm cells develop over a roughly 74-day cycle. Surrounding those tubes are specialized cells that produce testosterone, the hormone responsible for muscle development, bone density, sex drive, and the deepening of the voice during puberty. The left and right testicles share this work without specializing. Neither side produces a different type of sperm or a different hormone.
If one testicle is removed (a procedure sometimes needed for testicular cancer), the remaining one compensates. In a study of men who had a single testicle removed, average sperm concentration actually increased from 26 million to 39 million per milliliter during the first year after surgery, and the majority of initially low-count patients returned to normal levels. Twenty-eight of the men in that study went on to father children. So while having two testicles provides a backup, either one alone can handle full-time duty.
How the Left Side Differs Anatomically
The left testicle hangs slightly lower than the right in most men. This is normal and likely helps prevent the two from pressing against each other. But there’s a more important structural difference beneath the surface: the veins that drain blood from each testicle take different routes back to the body.
The right testicular vein empties directly into the large vein running up the center of the abdomen (the inferior vena cava), a short and efficient path. The left testicular vein, by contrast, drains into the left kidney vein first, entering it at a steep angle. This longer, less direct route makes it harder for blood to flow upward against gravity, and it’s the main reason the left side is prone to certain problems the right side largely avoids. The two venous systems don’t even connect to each other in the scrotum or pelvis. The only crossover occurs higher in the abdomen, and only in about half of men.
Why Varicoceles Favor the Left Side
A varicocele is a swelling of the veins inside the scrotum, similar to a varicose vein in the leg. It can feel like a soft bag of worms above the testicle and sometimes causes a dull ache, especially after standing for long periods. About 80% of varicoceles occur on the left side alone, with another 11% being bilateral and only about 8% isolated to the right. That lopsided pattern is a direct result of the left vein’s longer drainage path and its angle of entry into the kidney vein, which creates more back-pressure.
Most varicoceles are harmless. They become a concern mainly when they affect fertility, since pooling warm blood can raise the temperature around sperm-producing tissue. If you notice enlarged, ropy veins on the left side of your scrotum, it’s worth mentioning at your next checkup, but it’s rarely an emergency.
Testicular Torsion Risk
Torsion happens when a testicle twists on its spermatic cord, cutting off its own blood supply. It causes sudden, severe pain and requires urgent treatment within hours to save the testicle. A large Japanese national study found that left-sided torsion occurs about twice as often as right-sided torsion, at a 2:1 ratio. The likely reason: the left spermatic cord tends to be slightly longer, giving the left testicle more room to rotate.
Size Differences Between Left and Right
Ultrasound studies of healthy males from infancy through adolescence show no statistically significant difference in volume between the left and right testicles. In one study of over 1,600 subjects, the left averaged 2.22 mL and the right 2.26 mL across all ages, a gap too small to be meaningful. That said, in most individual age groups the left side measured very slightly smaller than the right, a pattern consistent enough to notice but not large enough to matter clinically. In adults, a healthy testicle typically reaches around 12 to 20 mL.
A noticeable size difference between your two testicles, one that you can see or feel easily, is a different story. That can signal a varicocele, fluid buildup, or in rare cases a growth, and it’s worth having evaluated.
What a Healthy Testicle Feels Like
When you examine yourself, a normal testicle feels smooth, oval, and firm but not hard. Behind and slightly above each testicle, you’ll feel a softer, comma-shaped structure called the epididymis. This is the tube that stores and transports sperm, and it’s supposed to be there. It can sometimes be mistaken for a lump by someone doing a self-exam for the first time.
What you’re actually looking for is something on or within the testicle itself: a hard, painless lump, noticeable swelling, or a change in how the testicle feels compared to last month. Testicular cancer typically presents as a painless lump or area of hardness. Fluid-filled cysts (spermatoceles) tend to sit above the testicle near the epididymis and are almost always benign. Monthly self-checks take about a minute and are easiest after a warm shower, when the scrotal skin is relaxed.