Low sperm count has many possible causes, ranging from treatable medical conditions to everyday habits you might not suspect. A count is considered low when semen contains fewer than 39 million sperm per ejaculate, based on reference values from the World Health Organization. Some causes are reversible within a few months, while others require medical intervention.
Varicoceles: The Most Common Medical Cause
A varicocele is a swelling of the veins that drain the testicle, similar to a varicose vein in the leg. It’s the single most common identifiable cause of low sperm count. Several theories explain why varicoceles harm sperm quality, including oxygen deprivation, increased pressure, and the backflow of toxic metabolites. The most consistently supported explanation is heat: impaired blood flow raises the temperature inside the testicle. In animal studies, creating a varicocele reliably produces poor sperm function alongside elevated testicular temperature.
Varicoceles are found on the left side more often than the right, and many men have them without symptoms. They’re typically discovered during a fertility evaluation. The good news is that surgical correction often improves sperm numbers over the following months.
Hormonal Disruptions
Sperm production depends on a chain of hormonal signals that starts in the brain. The hypothalamus releases a signaling hormone that tells the pituitary gland to produce two key hormones: one that triggers testosterone production in the testicles and another that directly stimulates sperm development. If either the hypothalamus or the pituitary gland malfunctions, this hormonal chain breaks down, and sperm production stalls or stops entirely.
This type of hormonal failure can be something a person is born with. One well-known congenital form, Kallmann syndrome, pairs absent puberty with a missing sense of smell. In other cases, the signaling neurons in the hypothalamus simply fail to develop or function properly, leading to the same result without affecting smell.
Acquired hormonal disruptions are more common in adults. Causes include pituitary tumors, head injuries, brain radiation, excessive alcohol use, chronic exhausting exercise, and certain medications. Restoring normal sperm production in these cases requires both key pituitary hormones. Studies show that replacing only one of them restores sperm output to roughly 50% of normal, while combining both brings it back to full capacity.
Testosterone Therapy and Medications
This is one of the most misunderstood causes. Men who take supplemental testosterone to boost energy, muscle mass, or libido are often unaware that it suppresses their own sperm production. When testosterone enters the bloodstream from an outside source, the brain detects the elevated level and shuts down its signals to the testicles. Without those signals, sperm production drops dramatically or stops altogether.
The numbers are striking. Testosterone therapy drives sperm count to zero in 64% to 75% of men with normal counts within six months. One large study found that 93% to 98% of men achieved near-complete suppression after six months on monthly injections. This effect is so reliable that testosterone has been studied as a male contraceptive. The suppression is usually reversible after stopping therapy, but recovery takes time because a full cycle of sperm production requires about 65 days from start to finish, and the hormonal signals need to normalize first.
Other medications that can lower sperm count include certain antidepressants, anti-seizure drugs, long-term opioids, and anabolic steroids used for bodybuilding (which work through the same mechanism as testosterone therapy).
Genetic Conditions
Klinefelter syndrome is the most common chromosomal abnormality linked to male infertility. Men with this condition carry an extra X chromosome, giving them a 47,XXY pattern instead of the typical 46,XY. Most men with Klinefelter syndrome produce no sperm at all. A smaller subset, roughly 10% to 20%, have a mosaic form where only some cells carry the extra chromosome. These men may produce reduced amounts of sperm rather than none.
Interestingly, sperm has been identified in about 70% of ejaculated semen samples from adolescents with Klinefelter syndrome between ages 12 and 20, suggesting that production declines over time. For men who produce no sperm in their ejaculate, surgical extraction directly from the testicle finds sperm in approximately half of cases.
Other genetic causes include Y chromosome microdeletions, where small missing segments of the Y chromosome impair or eliminate sperm production, and cystic fibrosis gene mutations, which can block the tubes that carry sperm out of the body.
Heat Exposure
The testicles sit outside the body for a reason: sperm production requires a temperature 2°C to 4°C below core body temperature. Even modest warming can cause problems. Research suggests that a scrotal temperature increase of just 1°C above baseline may be enough to inhibit sperm production, and increases between 1°C and 2.9°C are consistently associated with sustained negative effects on fertility.
Everyday heat sources matter more than most people realize. Laptop computers placed on the lap raise scrotal temperature above the 1°C safety margin within 11 to 28 minutes, depending on leg position. Using a lap pad doesn’t fix the problem. Sitting with legs apart and a pad underneath still pushed scrotal temperature beyond the safe threshold. Hot tubs, saunas, heated car seats, and prolonged sitting in tight clothing all contribute similarly. The damage from heat exposure is reversible, but because sperm take 65 days to fully mature, improvements won’t show up on a semen analysis for roughly two to three months after eliminating the heat source.
Environmental Chemicals
Certain industrial chemicals interfere with the hormonal machinery inside the testicles. Phthalates, found in plastics, food packaging, personal care products, and vinyl flooring, are among the most studied. Inside the body, phthalate metabolites disrupt the Leydig cells responsible for producing testosterone. They do this by interfering with cholesterol transport into these cells, a critical step in making testosterone. At high enough concentrations, phthalates also generate oxidative stress that directly damages the cells’ ability to synthesize hormones.
These aren’t just laboratory findings in rodents. Phthalate exposure in animal models produces a cluster of reproductive problems, including undescended testicles, genital abnormalities, and reduced semen quality, that researchers group under the umbrella of testicular dysgenesis syndrome. The concern is that chronic low-level exposure in humans may contribute to the same pattern over time.
Body Weight
Carrying excess weight measurably reduces sperm count. In a study of over 2,400 men, median sperm concentration was 50 million per milliliter in men at a normal weight but dropped to 45.6 million in obese men, a statistically significant decline. The relationship held even after accounting for other variables.
The mechanism is partly hormonal. Fat tissue converts testosterone into estrogen through an enzyme called aromatase. Higher estrogen levels feed back to the brain and suppress the hormonal signals that drive sperm production, creating the same type of disruption seen in other hormonal causes. Excess weight also raises scrotal temperature simply through insulating fat deposits around the groin. Losing weight can reverse these effects, though again, you’ll need at least two to three months after reaching a healthier weight before a semen analysis reflects the change.
A Broader Decline
Individual causes aside, sperm counts appear to be falling globally. A 2022 meta-analysis covering decades of data found that sperm concentration among unselected men declined by about 1.16% per year after 1972. More concerning, the rate of decline has accelerated: since 2000, it has roughly doubled to 2.64% per year. The reasons behind this trend aren’t fully settled, but researchers point to rising obesity rates, increasing exposure to endocrine-disrupting chemicals, and lifestyle shifts as likely contributors. For any individual man with a low count, this backdrop means the causes listed above may be compounding each other in ways previous generations didn’t face.