What Does It Mean to Have Small Testicles?

Small testicles can signal anything from a normal anatomical variation to an underlying hormonal, genetic, or lifestyle-related condition that deserves medical attention. Testicular size is closely tied to two functions most people care about: testosterone production and sperm output. A testis that falls below the typical adult range doesn’t automatically mean something is wrong, but it often prompts questions about fertility, hormone levels, and long-term health that are worth answering with real data rather than anxiety.

What Counts as “Small”

Testicular volume in healthy adult men varies more than most people realize, and the number you get depends heavily on how the measurement is taken. In clinical settings, doctors either use a Prader orchidometer (a string of egg-shaped beads held against the scrotum for comparison) or ultrasound. Ultrasound is more precise, but even ultrasound results shift depending on which mathematical formula the technician applies. One study comparing three common formulas found that median volumes on the right side ranged from about 14 mL to 19 mL depending on the formula used, with the left testis consistently measuring a bit smaller than the right.¹ All three formulas in that study also underestimated true volume compared to direct water-displacement measurement.²

As a rough guide, most references place a normal adult testicular volume somewhere in the range of 12 to 25 mL per testis. Below about 12 mL is where clinicians start considering the testis “small,” though context matters: a 13 mL testis in a man with perfectly normal hormones and sperm counts is different from a 13 mL testis paired with low testosterone and absent sperm. It’s also worth knowing that your left testicle being slightly smaller than the right is the norm, not a red flag.¹

Why Size Matters for Testosterone

The bulk of each testicle is made up of seminiferous tubules, the tightly coiled tubes where sperm are produced. Nestled between those tubules are Leydig cells, which are the body’s primary testosterone factories.³ When testicular volume shrinks, it usually means there’s less functional tissue available for one or both of those jobs. A study comparing men with infertility to fertile controls found that testosterone levels in the infertile group were roughly half those of the controls.⁴

That said, size and hormone levels don’t always track neatly. Some men with smaller-than-average testes have testosterone levels well within the normal range. The relationship is statistical, not absolute. If you’re worried about testosterone, a blood test is the definitive check, not a tape measure.

The Fertility Connection

Because seminiferous tubules account for most of the testicular volume, smaller testes often produce fewer sperm. Research on infertile men found that those with no sperm in their ejaculate (azoospermia) had a mean testicular volume of about 10 mL, while men with low but present sperm counts averaged around 13 mL. Fertile controls, by contrast, averaged about 18 mL.⁵ The correlation between testicular volume and sperm count is one of the more robust findings in male reproductive medicine.

Testicular volume also correlates strongly with inhibin B, a hormone produced by the cells that support sperm development. In infertile men, both inhibin B levels and testicular volume were significantly lower than in fertile men, and the two were tightly linked.⁴ Still, volume alone doesn’t predict whether sperm can be retrieved surgically for assisted reproduction. In men with no ejaculated sperm, neither testicular volume nor standard hormone markers reliably predicted whether a surgical procedure would find usable sperm.⁶ The biology is more complex than “big equals fertile, small equals infertile.”

Genetic Conditions That Cause Small Testes

Klinefelter syndrome is the most common genetic reason for persistently small testicles. Men with this condition carry an extra X chromosome (47,XXY instead of the usual 46,XY). It affects roughly one in 500 to 600 live male births, though many cases go undiagnosed into adulthood. The hallmark features are small, firm testes, reduced testosterone, and infertility, but the effects extend well beyond reproduction: men with Klinefelter syndrome have higher rates of metabolic, cardiovascular, and autoimmune conditions.⁷ It remains the most frequent chromosomal cause of male hypogonadism and infertility.⁸

Congenital hypogonadotropic hypogonadism is a rarer genetic cause. In this condition, the brain doesn’t produce the signaling hormones (LH and FSH) that tell the testes to grow during puberty. Without that signal, the testes remain prepubertal in size. Unlike Klinefelter syndrome, the testes themselves are structurally capable of working; they just never received the instruction to start. Hormone replacement therapy can sometimes trigger testicular growth and even fertility in these men.

Anabolic Steroids and Testicular Shrinkage

If you’ve heard that steroid use “shrinks your balls,” the mechanism is straightforward. When you flood your body with external testosterone or other anabolic androgenic steroids, the brain detects the excess and shuts down its own signals to the testes. Without those signals, the testes stop producing testosterone and sperm, and the tissue atrophies. Power athletes who used high-dose testosterone and anabolic steroids for three months were still in a state of suppressed testicular function three weeks after stopping.⁹

The good news is that the effect is usually reversible. Men who have used anabolic steroids for less than a year typically recover normal hormonal function within about a year after stopping.¹⁰ Long-term or very heavy use is a different story: recovery may be slower or incomplete, and some men need medical intervention to restart the system. Testosterone replacement therapy prescribed for low testosterone has the same suppressive effect on testicular size and sperm production, which is why fertility preservation is an important conversation before starting it.

Varicocele and Undescended Testes

A varicocele is an enlargement of the veins within the scrotum, similar to a varicose vein in the leg. It almost always appears on the left side and can gradually reduce testicular volume, mainly through elevated scrotal temperature. In a study of adolescents with varicoceles, about a third already showed measurable shrinkage of the affected testis. Among those simply observed without surgery, nearly 40% developed new testicular shrinkage. Surgical repair (varicocelectomy) allowed catch-up growth, particularly in boys who had elevated testicular temperatures before the procedure.¹¹

Cryptorchidism, or undescended testicle, is a condition where one or both testes fail to descend into the scrotum during fetal development. If the testicle remains in the abdomen or inguinal canal, it’s exposed to higher temperatures than it was designed for, which damages the tissue over time. Even after surgical correction in childhood, the formerly undescended testicle tends to remain smaller than the normally descended one. A long-term follow-up study found significant volume differences between operated and non-operated testes, as well as between men who had unilateral versus bilateral undescended testes.¹² Interestingly, paternity rates didn’t differ based on testicular volume in that study, suggesting that one normally functioning testis can often compensate.

Cryptorchidism also raises the risk of testicular cancer later in life, and early surgical correction matters. Men who had orchiopexy (the corrective surgery) before age 13 had roughly double the testicular cancer risk compared to the general population, while those treated at 13 or older had more than five times the risk.¹³ The elevated risk doesn’t go away entirely with surgery, but early treatment substantially reduces it.

Infections and Injury

Mumps orchitis is the classic infectious cause of testicular shrinkage, though it’s become less common in countries with widespread vaccination. When the mumps virus infects the testicle, the resulting inflammation can permanently damage the tissue. In a small ultrasound study of men who developed testicular atrophy after mumps orchitis, the affected testes were on average about 45% smaller in volume than the unaffected side.¹⁴ Most cases were one-sided, which is fortunate for fertility since the remaining testis can often pick up the slack.

Bacterial infections (epididymo-orchitis from sexually transmitted infections or urinary tract infections) can also cause atrophy if severe or untreated. Testicular torsion, where the spermatic cord twists and cuts off blood supply, is a surgical emergency: if not corrected within a few hours, the testis may be permanently damaged or lost. Physical trauma severe enough to cause significant swelling can also lead to long-term volume loss.

Medical Treatments That Shrink the Testes

Radiation therapy for prostate cancer can cause testicular atrophy as a side effect, even though the testes aren’t the treatment target. The frequency of testicular atrophy in men who received prostate radiotherapy has been reported at roughly two and a half times higher than in men who didn’t receive it.¹⁵ Chemotherapy drugs can also damage the sperm-producing cells, sometimes permanently. For younger men facing cancer treatment, sperm banking before starting therapy is routinely recommended.

Aging and Gradual Shrinkage

Testicular volume doesn’t stay fixed throughout life. Starting in a man’s 30s or 40s, the testes begin a slow decline in volume. The mechanism involves a gradual loss of Leydig cells (the testosterone producers) combined with fibrotic changes to the testicular tissue, likely driven by chronic microvascular disease similar to what happens in other aging organs.¹⁶ This parallels the age-related decline in testosterone that most men experience. Obesity and metabolic conditions like high cholesterol can accelerate both Leydig cell aging and testosterone decline.³

The age-related decline is typically gradual enough that most men never notice a change in the mirror. It becomes clinically relevant mainly when testosterone drops low enough to cause symptoms like fatigue, reduced sex drive, or loss of muscle mass.

Environmental Chemicals and Testicular Health

A growing body of research points to endocrine-disrupting chemicals as a factor in male reproductive health, though the evidence in humans is less tidy than in animal studies. Bisphenol A (BPA) and phthalates, found in plastics, food packaging, and personal care products, have been shown in animal models to disrupt the hormonal signals that drive testicular development. Maternal exposure during pregnancy appears to have the most significant effects, increasing the risk of developmental abnormalities in the male reproductive tract.¹⁷ After birth, exposure is associated with changes in the hormonal axis that governs testicular function.

In human epidemiological studies, smaller adult testis size and lower sperm counts have been linked to shorter anogenital distance, a marker that researchers believe reflects prenatal androgen exposure.¹⁸ The picture is far from complete, though. Separating the effects of specific chemicals from the dozens of other environmental and genetic factors that influence testicular size remains a major challenge. The evidence is strong enough in animals to warrant concern but not strong enough in humans to pin specific effect sizes on specific chemicals for most men.

When to See a Doctor

Most men don’t regularly measure their testicular volume, so the question usually comes up in one of a few scenarios: trouble conceiving, symptoms of low testosterone, a noticeable change in size, or a clinical finding during an exam for something else. If any of those apply, the diagnostic workup is relatively straightforward. It typically includes a physical exam, blood tests for testosterone, FSH, LH, and possibly inhibin B, a semen analysis if fertility is a concern, and scrotal ultrasound if the doctor wants precise measurements or suspects a structural issue like a varicocele or mass.

Inhibin B has gained attention as a marker because it reflects what the sperm-supporting cells are actually doing. In studies of infertile men, inhibin B correlated more strongly with both testicular volume and sperm counts than FSH or LH did.⁴ However, no single blood test or measurement reliably predicts fertility outcomes in every individual. The clinical picture is always a combination of findings, not a single number.

Treatment Options

Treatment depends entirely on the underlying cause. For men whose small testes are due to hormonal deficiencies (like congenital hypogonadotropic hypogonadism), gonadotropin therapy can stimulate testicular growth and sometimes restore sperm production. For men with Klinefelter syndrome or other conditions where sperm are absent from the ejaculate, microsurgical sperm extraction (micro-TESE) can sometimes retrieve usable sperm directly from testicular tissue. In some cases, hormonal therapy before the procedure improves the odds of finding sperm or even leads to sperm appearing in the ejaculate, potentially avoiding surgery altogether.¹⁹

Even in severe cases, fertility isn’t always impossible. A case report described a successful live birth in a man with one of the most severe forms of testicular failure (Sertoli cell only syndrome), achieved through hormonal therapy that stimulated just enough sperm production for use in assisted reproduction.²⁰ These are edge cases, not the norm, but they illustrate how far reproductive medicine has come.

For varicoceles, surgical repair can reverse testicular shrinkage in many adolescents and may improve semen quality in adults. For steroid-induced atrophy, stopping the offending agents is the first step, with medical support if recovery stalls. And for men whose primary concern is low testosterone rather than fertility, testosterone replacement therapy is an option, though it will further reduce testicular size and sperm production rather than restore them.

Testicular Size and Parenting Behavior

One study that drew public attention examined the relationship between testicular volume and caregiving in fathers. Researchers found that smaller testes volume was associated with greater nurturing-related brain activity when fathers viewed photos of their own children. Activity in a brain region tied to reward and motivation predicted more involved caregiving and was negatively related to testes volume.²¹ The authors framed this as reflecting a biological trade-off between mating effort and parenting effort, a concept from evolutionary biology.

This is a single study with a small sample, and it is worth treating as an interesting observation rather than a firm rule about who makes a good father. Many factors influence parenting, and reducing it to anatomy would be both scientifically and personally misleading. Still, the finding is a reminder that testicular biology connects to more aspects of life than most people would expect.

Puberty and What’s Normal for Younger Males

During puberty, the testes are the first organ to enlarge, typically before pubic hair appears or the growth spurt begins. A study of adolescent boys aged 10 to 18 found that mean testicular volume rose from about 4.5 mL at the earliest stage of development to roughly 23 mL by the final stage.²² If a boy’s testicles have not begun to enlarge by age 14, doctors consider this delayed puberty and investigate possible hormonal or genetic causes.

Parents sometimes worry about testicular size in young boys, but prepubertal testes are supposed to be small. Volume under 4 mL before puberty is completely normal. The timing of testicular growth varies by several years across healthy boys, and comparing one adolescent to another can create unnecessary anxiety. What matters is that growth is progressing, not that it matches a specific calendar date.