Type "does polyester lower testosterone" into any search bar and the answer comes back a confident yes. The question is aimed at the wrong target. The bare fiber isn't where the hormone research lands. Two other things travel with synthetic gear: the plasticizer chemistry it carries, and the plastic particles the fabric sheds.
The short answer
The polyester fiber itself does not appear to lower your testosterone. The one experiment that put men in polyester and measured their hormones found serum reproductive hormones showed no significant change.16
Almost every page arguing the opposite traces back to two experiments run by the Egyptian physician Ahmed Shafik in the early 1990s, and both were sperm studies rather than hormone studies. A study published in Contraception (1992) fitted 14 men with a polyester scrotal sling for 12 months. Its companion, published in Urological Research (1993), kept 24 dogs, half in polyester underpants and half in cotton, for 24 months, with a further seven unclothed dogs as controls. Both tracked semen quality, testicular biopsy, and serum hormones.1617
Sperm counts fell in both and recovered in most subjects once the garment came off. Blood testosterone did not move: hormone measurements showed no statistically significant change in the dogs, and none in the men.1716
Those limits deserve saying out loud. Fourteen men and 12 polyester-wearing dogs is a small evidence base, one investigator ran both, the dog work is animal work, and no independent replication appears in the literature. The most-cited support for the claim is thin, old, and does not contain the claim.
The hormone worry people are reaching for belongs to two things synthetic activewear brings along. Phthalates, the plasticizers packaged into synthetic gear, are associated with lower testosterone in men in population studies. The microplastic particles the fabric sheds lower testosterone in animal studies and now turn up in human testis tissue. No single study has put a man in synthetic gear and proven it lowered his hormones, and that is worth being honest about. It is also not the bar the researchers who study these chemicals use. The Endocrine Society's scientific statement on endocrine-disrupting chemicals, published in Endocrine Reviews (2009), argues for invoking the precautionary principle precisely because animal, clinical, and population evidence converge long before any single study can prove causation in one specific wearer.5
What phthalates are, and why they're in your gear
Phthalates are a family of plasticizers. They keep plastic soft and pliable, and in clothing they bind the prints, the rubberized coatings, and the stretch in synthetic waistbands. Because so much athletic gear is built from synthetic fiber and finished with printed logos and stretch panels, phthalates are a routine part of the material rather than a fluke.
They are not a rare contaminant either. When researchers at NIST screened consumer polyester products, they detected diethyl phthalate in roughly 29 percent of the samples, the most common phthalate they found.6 A review in Reviews of Environmental Contamination and Toxicology (2022) measured phthalate concentrations from about 3 to 33 micrograms per gram of fabric, and found the levels ran higher in garments with heavy prints and coatings.7 Presence is the settled part.
What phthalates do to testosterone
In large population studies, higher phthalate exposure has been associated with lower testosterone in men.12 Those are observational findings, so they show a link rather than proof of cause, and the signal tends to be strongest in particular subgroups rather than uniform across every man.
The clearest human evidence comes from the high end of exposure. Factory workers handling di-n-butyl and di-2-ethylhexyl phthalate, with urinary markers running close to 200 times general-population levels, showed decreased free testosterone.3 No garment delivers a factory dose, so that study marks the ceiling rather than the everyday case. A systematic review of phthalates and male reproductive outcomes lands in the honest middle: the human evidence is real but mixed, stronger for some endpoints than others.4
Do microplastics from clothing lower testosterone?
Microplastics lower testosterone consistently in animal studies, and in humans the particles are now documented in testis tissue without any study yet showing that a garment moves a wearer's hormones.
Synthetic fabric is itself plastic, and it comes apart as you use it. Polyester breaks off tiny fibers steadily, in the wash and during wear. A single synthetic wash load can release millions of microfibers,10 and a typical polyester load sheds on the order of hundreds of thousands.11 Those are per-wash-load figures rather than per-garment counts, but they mark how freely the material sheds.
This stopped being only an environmental story once the particles started turning up in people. Researchers have detected microplastics in human blood,12 and a study by Hu, Garcia, Nihart and colleagues published in Toxicological Sciences (2024) found them in every human testis sample it tested.13
What the particles do to testosterone is, for now, an animal finding, and there it runs consistent. A 2024 meta-analysis in the Journal of Hazardous Materials that pooled 39 mammalian studies concluded that microplastics and nanoplastics reduce testosterone and impair the blood-testis barrier.14 Controlled mouse experiments point the same way, with chronic polystyrene microplastics lowering testosterone by stressing the cells that produce it.15 That is strong animal evidence sitting next to confirmed human presence, and it is not a study showing your shorts lower your testosterone.
Does any of it actually reach you?
Some of it does reach you: skin absorbs a measurable fraction of the phthalate chemistry, and the shed particles arrive mainly through the air rather than through the skin.
For the chemistry, a controlled study of 16 volunteers found they absorbed measurable amounts through the skin, though route for route that dermal uptake was generally smaller than what they took in by breathing.8 Skin is a partial barrier, not an open door. For the shed particles, the best-documented way in is not the skin at all but the air, since synthetic textiles are a leading source of the microplastics people breathe indoors. I walk through those routes into the body in a separate piece.
What a hard session adds is exactly what raises the transfer: heat opens skin permeability, sweat dissolves the compounds, and friction works fibers loose from fabric pressed against you for an hour or two. You don't need a precise per-garment number to make the call. When a material is known to carry endocrine-active chemistry and to shed a particle now found in human tissue, and a natural fiber does neither, the low-cost move is to remove the source, whether the absorbed dose turns out large or small.
First drop. 500 units. Late 2026.
Access before the public.
Is polyester underwear worse than a polyester T-shirt?
Skin-contact garments in the groin are the higher-exposure case, and the difference is site, contact time, and heat rather than a different fiber.
Start with the site. Skin doesn't absorb at one uniform rate across the body. A study published in the Journal of Investigative Dermatology (1967) applied radiolabeled cortisol to a series of body regions in male volunteers, and its authors, Feldmann and Maibach, found the scrotum absorbed more than any other site they tested.9 That was cortisol in a dosed topical application, not a phthalate migrating out of a waistband, so read it as a reason the site matters rather than as a number that transfers to clothing.
Then contact time and occlusion. A T-shirt hangs loose, moves air, and spends most of the day dry. A liner, a compression short, or a brief sits pressed against skin for a full session under heat, sweat, and friction, which are the conditions that raise dermal transfer of the phthalate chemistry in the first place.8 Heavier prints and rubberized coatings carry the higher phthalate loads,7 and a printed or coated waistband is one place that chemistry sits.
Heat is the third factor, and it is the best established of the three. A trapped, non-breathable layer in that spot works against a system built to stay cool, which is the sperm mechanism covered in the next section.
Notice that this is also where the Shafik experiments actually sit. They were scrotal-contact studies, which makes them the relevant evidence for underwear rather than for outerwear, and even there the sperm counts moved while testosterone did not.1617 What is established is regional permeability, the heat effect, and the phthalate load. What isn't established is a hormone dose from any garment. No study has compared polyester underwear against cotton underwear for testosterone in men, so treat the site argument as plausible dose rather than a measured one.
Does polyester affect sperm?
Through heat, yes, and the effect reverses once the heat source is removed. That mechanism doesn't rest on the Shafik studies at all. Sperm production runs best a couple of degrees below core temperature, and a tight, non-breathable synthetic layer traps warmth against skin that is built to stay cool.18
In the dog study, heat was not Shafik's explanation. Testicular temperature showed insignificant change while the pants were worn, the paper says outright that the cause of the effect is unknown, and Shafik proposed that electrostatic potentials generated by the polyester fabric played a role.17 The human study reads differently. There he attributed the effect to two mechanisms, an electrostatic field across the intrascrotal structures and disordered thermoregulation, and reported a significant fall in the difference between rectal and testicular temperature.16 So the temperature signal sits in the human paper rather than the dog one, which lines up with the heat evidence above. Read either explanation as the author's hypothesis about his own result rather than a settled mechanism.
For a lot of guys, reduced fertility from a hot synthetic layer is reason enough on its own. Either way it sits apart from the testosterone question: what the fabric does physically is a sperm story, while the plastic it sheds and the chemistry it carries are what the hormone research keeps flagging.
How polyester, organic cotton, and merino wool compare
| Polyester | Organic cotton | Merino wool | |
|---|---|---|---|
| What it is | PET plastic, spun from petroleum into filament | A plant cellulose fiber | An animal protein fiber |
| Plasticizer chemistry | Diethyl phthalate detected in roughly 29 percent of consumer polyester products screened by NIST,6 at about 3 to 33 micrograms per gram of fabric, higher with prints and coatings7 | None inherent to the fiber. Printed graphics, coatings, and stretch panels can still add it | None inherent to the fiber. Same caveat for prints and coatings |
| Microplastic shedding | Sheds plastic microfibers in wear and washing; a synthetic wash load releases hundreds of thousands to millions of fibers | Sheds fiber, and the fiber is cellulose rather than plastic | Sheds fiber, and the fiber is protein rather than plastic |
| What the evidence shows | The fiber did not move serum testosterone in the one human experiment that measured it.16 The concern attaches to the phthalates it carries and the microplastics it sheds | Ordinary cotton was the comparison fabric in the Shafik dog experiment, and those animals showed no testicular change17 | No comparable testosterone or sperm study exists for the fiber |
Read the last row carefully, because it is the honest state of the field. None of these fibers has a study showing it raises or lowers a wearer's testosterone. What separates them is what they carry and what they shed.
What should you train in?
The layer against your skin for hours a week is the variable you fully choose, and for a combat athlete it is the one worth choosing well. A natural fiber shell breathes instead of trapping heat, which answers the sperm-and-heat mechanism. It sheds no microplastics, so there is no particle load to breathe or absorb, and it carries no phthalate plasticizer and no per- and polyfluoroalkyl substances (PFAS) finish, which answers the chemistry. One material change addresses all three.
That is the short Fight Form set out to build. The Fight Short and Training Short use an organic cotton and TENCEL lyocell shell, a merino wool brief liner in the warmest spot, and a natural rubber waistband, with no polyester, spandex, or PFAS. It's performance fabric chosen for what it keeps off your skin as much as for how it moves, and the flexible fit is the point of the cut rather than the point of the material.
If you are working out which cut and inseam suit your sport before you get to fabric, we break the choice down by discipline in a separate guide. The materials breakdown lists every fiber by name, our wider look at whether polyester is bad for you covers the broader fabric question, and endocrine disruptors in men's activewear goes deeper on the hormone chemistry.
The bottom line
Does polyester lower testosterone? The fiber worn on skin did not in the one study that measured it, where testosterone held steady. The hormonal concern people are sensing is real, and it belongs to what synthetic activewear carries and sheds: the phthalate chemistry associated with lower testosterone in men, and the microplastic particles that lower it in animal studies and now sit in human tissue. No study has proven either one moves a specific wearer's hormones, and that's the honest limit of the evidence. It's also not a reason to shrug. The professional body that studies these chemicals doesn't wait for that proof before recommending precaution,5 and neither should you have to. The fiber's own separate effect is on sperm, through trapped heat, and it reverses. For a combat athlete spending hours a week in hot, printed, synthetic gear, the sensible move is not to wait for the final study but to remove the avoidable exposures from the layer against your skin.