Male factors contribute to 30% to 50% of all infertility cases among couples trying to conceive, with about 20% of those cases caused by male factors alone. The causes range from physical problems like enlarged veins in the scrotum to hormonal imbalances, genetic conditions, lifestyle choices, and environmental exposures. Often, more than one factor is at play.
Varicoceles: The Most Common Physical Cause
A varicocele is an enlargement of the veins inside the scrotum, similar to a varicose vein in the leg. It’s the most frequently identified physical cause of male infertility. These swollen veins allow blood to pool and flow backward through the testicular vein system, which disrupts the body’s built-in cooling mechanism for the testicles.
Testicles need to stay a few degrees cooler than core body temperature for sperm production to work properly. When a varicocele raises scrotal temperature, the heat interferes with enzymes responsible for building DNA and proteins inside sperm cells. Research published in the Journal of Assisted Reproduction and Genetics found that men with varicoceles show decreased activity of key enzymes involved in DNA synthesis. The heat also disrupts how proteins are distributed within sperm, compromising their ability to fertilize an egg. A varicocele is typically diagnosed through a physical exam and confirmed with ultrasound, which has a sensitivity of 97% and specificity of 94%.
Hormonal Imbalances
Sperm production depends on a signaling chain that starts in the brain. The hypothalamus releases a hormone that tells the pituitary gland to produce two critical hormones: one that stimulates the cells supporting sperm development, and another that triggers testosterone production in the testicles. A disruption at any point in this chain can reduce or halt sperm production entirely.
When the brain doesn’t send enough of these signals, a condition called hypogonadism develops. It can be congenital (present from birth) or acquired later in life from tumors, head injuries, or certain medications. The result is low testosterone and poor sperm production. In some cases, the testicles themselves are the problem, failing to respond to normal hormonal signals due to damage from infection, injury, or radiation.
Testosterone Therapy and Anabolic Steroids
This is one of the most misunderstood causes of male infertility. Men prescribed testosterone replacement therapy, or those using anabolic steroids for muscle building, often don’t realize these treatments can shut down sperm production. When testosterone enters the body from an outside source, the brain reads it as a signal that no more is needed and stops sending the hormones that drive sperm production in the testicles.
The effect is dramatic. Testosterone levels inside the testicles plummet, and the tissue responsible for making sperm shrinks. One study found that injectable testosterone caused a complete absence of sperm in roughly 75% of men after just six months of use. Up to 40% of men on exogenous testosterone develop this condition. All anabolic steroids work through the same mechanism, so the impact on fertility is identical regardless of which specific steroid is used. The good news is that sperm production often recovers after stopping, though it can take months to over a year.
Genetic Conditions
Genetic factors account for roughly 15% to 20% of severe male infertility cases where sperm counts are extremely low or sperm are completely absent. The two most common genetic causes are Klinefelter syndrome and Y-chromosome microdeletions.
Klinefelter syndrome, where a man carries an extra X chromosome, is the single most common genetic cause. Y-chromosome microdeletions rank second. These involve missing segments on the Y chromosome in regions called AZF zones, which contain genes essential for sperm production. The type of deletion matters considerably. AZFc deletions are the most common, making up about 80% of Y-chromosome microdeletions, and they produce a range of outcomes from very low sperm counts to no sperm at all. When sperm are absent, there’s still about a 50% chance of retrieving viable sperm surgically. AZFa and AZFb deletions, though rarer, are more severe and almost always result in a complete absence of sperm with no realistic chance of surgical retrieval.
Heat Exposure and Occupational Hazards
Prolonged heat exposure is a well-documented threat to sperm quality, and it goes beyond varicoceles. A meta-analysis in the Iranian Journal of Public Health pooled data from studies on steel workers, ceramic factory employees, and regular sauna users. The overall findings showed that high ambient temperature significantly decreases semen volume, sperm concentration, total sperm count, motility, and the percentage of normally shaped sperm. The effect on normal sperm shape was the most severe of all parameters measured.
Steel workers exposed to heat stress for four to five hours daily showed reduced sperm quality across multiple measures. Sauna users who attended two sessions per week for three months at 80 to 90 degrees Celsius experienced disruptions in sperm DNA integrity and mitochondrial function. Even wearing a heating belt in a controlled study produced measurable changes in sperm concentration, shape, motility, and DNA integrity. That said, a few smaller studies found no significant differences, suggesting individual susceptibility varies. Practical sources of concern include prolonged laptop use on the lap, tight underwear, long hot baths, and occupations involving sustained heat.
Environmental Chemicals
A growing body of evidence links rising male infertility rates to environmental pollutants that interfere with the hormone system. The main culprits include phthalates (found in plastics, food packaging, and personal care products), bisphenol A or BPA (in plastic bottles and can linings), pesticides, and heavy metals like cadmium and lead.
These chemicals disrupt hormones in different ways. BPA mimics estrogen by binding to estrogen receptors. Phthalates suppress testosterone production by blocking the enzymes that make it. Certain pesticides act as direct blockers of testosterone’s receptor, preventing it from doing its job even when levels are normal. Exposure to any of these has been linked to reduced sperm count and motility in both human and animal studies. PCBs, an older class of industrial chemicals still persistent in the environment, interfere with thyroid signaling, adding another layer of hormonal disruption.
Oxidative Stress and Sperm DNA Damage
Oxidative stress occurs when harmful molecules called free radicals overwhelm the body’s ability to neutralize them. In the context of male fertility, these molecules directly attack sperm DNA, causing breaks in the genetic material, and they damage the fatty membranes that sperm need for movement and fertilization. The byproducts of this membrane damage are themselves toxic to DNA, creating a cycle of worsening harm.
Free radicals also trigger a self-destruct process in sperm cells, which further increases the production of more free radicals and accelerates DNA fragmentation. This mechanism is particularly relevant in cases of unexplained infertility, where standard semen analysis looks normal but a couple still can’t conceive. Some specialists now recommend testing for oxidative stress in these situations, as it may reveal damage that conventional tests miss.
Ejaculation Problems
Retrograde ejaculation, where semen travels backward into the bladder instead of out through the penis, is a relatively common mechanical cause of infertility. It happens when the muscle at the bladder neck fails to close properly during orgasm. You might notice very little or no fluid when you ejaculate, or your urine may appear cloudy afterward.
Diabetes is one of the leading causes, as nerve damage from high blood sugar can impair the muscle’s function. Multiple sclerosis, spinal cord injuries, and pelvic surgeries carry the same risk. Prostate surgery, particularly a procedure called TURP used for enlarged prostates, causes retrograde ejaculation in the majority of cases. Several classes of medication can also trigger it, including drugs for high blood pressure, depression, and benign prostate enlargement.
How Male Infertility Is Diagnosed
Evaluation typically begins after 12 months of trying to conceive without success, or sooner if the female partner is over 35 or there’s a known risk factor. The process starts with a detailed medical history and physical exam, followed by a semen analysis. If the first analysis is abnormal, it’s repeated four to six weeks later to confirm, since sperm production naturally fluctuates.
The World Health Organization’s 2021 reference values set the lower bounds for normal semen at 39 million total sperm per ejaculate, 42% total motility, and 4% normally shaped sperm. Falling below these thresholds doesn’t guarantee infertility, but it signals reduced fertility potential and the need for further investigation. Depending on results, next steps may include hormone blood tests to check for signaling problems, scrotal ultrasound to evaluate for varicoceles or structural abnormalities, and genetic testing if sperm counts are very low or absent.