I used to believe that more testosterone meant better fertility. It seemed straightforward: the hormone that builds muscle, drives libido, and puts hair on your chest should also make your sperm count soar. Then I dug into the actual physiology-and the published studies-and realized I had it backward. The relationship between testosterone and fertility is one of the most misunderstood topics in men's health. Here’s what the research actually shows.
Testosterone has two completely different jobs
Testosterone doesn't just float around doing one thing. It works in two separate compartments, and the split matters.
In your bloodstream, testosterone travels to muscles, bones, and brain. A standard blood test measures this circulating level. That's the number that influences muscle mass, energy, and sex drive. It also feeds back to your pituitary gland: when blood T is high enough, your brain dials down luteinizing hormone (LH), the chemical signal that tells your testicles to make more testosterone. This feedback loop acts like a thermostat.
Inside your testicles, testosterone plays a second role that a blood test never sees. Leydig cells produce local concentrations of T that are roughly 100 times higher than what you find in your veins. This intratesticular testosterone is what fuels the Sertoli cells, the nursemaids that shepherd sperm through development. Without that concentrated local dose, sperm production stalls-even if your blood testosterone looks perfect.
The two systems talk to each other, but they are not the same thing. A man can have a blood T number squarely in the reference range and still have low intratesticular testosterone, or a sperm problem that has nothing to do with testosterone at all.
How taking testosterone destroys sperm production
Here's where the assumption that "more T equals more fertility" crashes into a wall. When a man introduces testosterone from outside his body-anabolic steroids, prohormones, any exogenous androgen-the thermostat in his brain reads the surge and shuts off LH. With no LH signal, the Leydig cells stop producing their own T. Intratesticular testosterone collapses. Sperm production drops, often to zero.
This isn't a rare side effect. It's a predictable, well-documented response. Bodybuilding forums are full of men who built exactly the physique they wanted and then couldn't conceive. A review of the literature shows that most men on chronic anabolic-androgenic steroids become severely oligospermic or azoospermic-few sperm, or none at all. Recovery after stopping can take months. For some men, it's never complete. The body's own T factory doesn't always restart on command.
What makes this confusing is that these men usually feel fine. Their blood testosterone levels are sky-high. Libido is intact. But under a microscope, their semen is empty. The fertility problem hides behind the number everyone obsesses over.
This is not an argument against medically supervised hormone treatment. Endocrine disorders exist, and a specialist knows how to protect fertility when needed. But if you're tinkering with your own androgens based on gym wisdom, you're gambling with a system you don't fully understand.
Beyond testosterone: what really drives sperm health
Even for a man who never touches an exogenous hormone, focusing on testosterone alone misses what actually governs sperm quality. Three factors surface again and again in the research, and none of them appear on a standard hormone panel.
The temperature your testicles actually sit at
Sperm production runs best at 2 to 4 degrees Celsius below core body temperature. That's why the testicles hang outside the abdominal cavity. Sustained heat-from a fever, a hot bath, or repeated sauna sessions-temporarily lowers sperm count and can warp sperm shape. A 2013 study by Garolla and colleagues, published in Human Reproduction, found that regular sauna use reduced sperm count and motility. The effect reversed within a few months after the men stopped the heat exposure. The mechanism is direct: the germ cells that produce sperm are exquisitely temperature-sensitive. More heat, fewer healthy sperm.
Clothing matters here, too. Tight underwear and long periods of sitting push the testicles closer to the body and raise scrotal temperature. A 2005 study in Human Reproduction showed that tighter-fitting underwear led to higher scrotal skin temperatures compared with loose boxers. Later work, including a 2018 cohort study by Minguez-Alarcon and colleagues, linked boxer-style underwear to higher sperm concentration and total count. A few degrees, held for hours a day, add up over a sperm production cycle that lasts about 74 days.
If you're a sauna user, none of this means you have to stop. The body recovers. But if fertility is top of mind, you'd want to time your sessions carefully, keep the groin cooler during the heat, and make sure you aren't trapping heat against your skin the rest of the day.
Chemicals that mess with the hormonal orchestra
The last two decades of data point to a quiet decline in sperm health. A 2023 meta-analysis by Levine and colleagues in Human Reproduction Update found that sperm counts in Western countries dropped by more than 50 percent over the past 40 years. The causes are likely multiple, but a prime suspect is the class of chemicals called endocrine disruptors: bisphenols, phthalates, certain pesticides, and plasticizers that leach from food packaging, personal care products, and synthetic fabrics. These compounds don't just mimic estrogen; they can directly impair Leydig cell function, interfere with testosterone signaling inside the testis, and increase oxidative damage to sperm DNA.
The microplastics story adds another dimension. Since 2023, multiple studies have detected microplastic particles in human testicular tissue and semen-in some cohorts, every sample examined. The association with lower sperm parameters is real, though no research has definitively proven that the plastic particles themselves cause the damage. What's plausible is that the particles act as carriers for endocrine-disrupting chemicals. And the route of exposure may be more direct than most men think.
The skin on the scrotum is exceptionally permeable. A classic 1971 study by Maibach and colleagues showed that scrotal skin absorbs compounds far more readily than almost any other body site. When that area is warm, damp, and covered-during a sauna or sixteen hours in synthetic underwear-whatever chemistry is in the fabric has a better chance of getting in. Laboratory work has confirmed that synthetic fibers can release microplastics and chemical residues into sweat. Polyester, for example, is made using an antimony catalyst that can migrate into artificial sweat under test conditions. And heat turns the absorption dial up: research published in Toxicology in Vitro in 2020 found that raising skin temperature from 25°C to 39°C more than doubled the amount absorbed.
This isn't alarmist. It's a prompt to think about what you wear against the most absorbent skin you own, especially when heat and sweat are part of your daily life.
The stuff you can fix starting today
Sleep duration and quality are linked to both testosterone levels and sperm parameters in multiple observational studies. Strength training, especially heavy compound lifts like squats and deadlifts, provokes short-term testosterone spikes that support androgen sensitivity-though the effect on actual sperm production is modest unless you've been sedentary. Body composition matters, too: excess visceral fat converts testosterone to estradiol through the aromatase enzyme, lowering circulating T and tilting the hormonal balance away from what sperm production prefers.
Nutrients like zinc (oysters, pumpkin seeds, red meat) and selenium (Brazil nuts, fish) are non-negotiable for sperm development. None of these require a blood test or a prescription. They're the boring, effective foundation that often gets ignored while men chase sexier interventions.
What to actually do about it
If fertility is on your radar-now or a few years down the road-the playbook isn't about boosting a number. It's about protecting the environment your sperm need to develop normally.
- Respect the temperature gradient. Wear breathable, looser underwear that lets air circulate rather than trapping heat. If you use a sauna regularly and are actively trying to conceive, consider fewer sessions, shorter stays, or keep a cold pack on your groin during the heat. The research says the effect is temporary, so timing is everything.
- Audit your chemical exposures. Cut out plastic food containers in the microwave, switch to fragrance-free personal care products, and filter your drinking water if microplastics or PFAS are a concern in your area. When you buy underwear, choose fabrics that don't shed persistent plastic microfibers. Natural cellulosic fibers like TENCEL or modal manage moisture well without turning into plastic dust on your skin.
- Lock in the fundamentals. Seven to eight hours of sleep. Compound lifts two to four times a week. Enough zinc and selenium from food. Keep your body fat in a range that doesn't turn your testosterone into estrogen.
None of these steps require a hormone panel. They work independently of what your total testosterone number reads.
The fertility world has gotten loud about egg freezing and the female biological clock. The male side has stayed quieter, even as the sperm data bends downward. Men who understand the actual mechanics-the feedback loops, the temperature sensitivity, the chemical insults-have an advantage that doesn't come from a pill or a protocol. It comes from knowing that fertility isn't a testosterone problem. It's an everything problem. And the research says you can do a lot with that knowledge.
Frequently asked questions
can sauna use lower my sperm count
Yes, temporarily. Research shows that regular sauna exposure can reduce sperm count and motility (Garolla et al., 2013). The effect reverses once you stop the heat exposure, usually within a few months. Keeping the groin cool during sauna sessions with a cold pack or limiting frequency can help while still enjoying the sauna.
does wearing tight underwear affect male fertility
Studies indicate that tighter underwear raises scrotal temperature (Jung et al., 2005) and is associated with lower sperm concentration and total count (Minguez-Alarcon et al., 2018). Looser, breathable underwear helps keep the testicles closer to their optimal temperature for sperm production. This doesn't guarantee fertility outcomes, but it removes one well-documented source of heat stress.
how do microplastics end up in the testicles
Microplastic particles have been detected in human testicular tissue and semen, but the exact source isn't clear. They may enter through ingestion, inhalation, or dermal absorption, especially since scrotal skin is highly permeable. Synthetic clothing that sheds microfibers could contribute to local exposure, though no study has directly traced testicular microplastics back to underwear.
can taking testosterone boosters cause infertility
Exogenous testosterone-like anabolic steroids or certain prohormones-shuts down your body's own testosterone production by suppressing LH. This collapses intratesticular testosterone, which is essential for sperm development. Many men on these substances become nearly or completely azoospermic. The effect may be reversible after stopping, but recovery isn't guaranteed and can take months.
This content is for educational purposes only and is not medical advice. Oakman products are designed for physical comfort and cooling; they make no claims about fertility, sperm quality, or hormone levels. Consult a healthcare professional for personalized advice.