Your testicles weigh about 40 grams together. Inside that small amount of tissue, two completely different factories run at once, every day. One makes your testosterone. The other makes roughly 1,000 sperm cells per heartbeat.
That, to me, is the real story of this organ. Not that it is fragile, but that it holds down two jobs at once, and those two can fail independently of each other.
It explains something men often find confusing. You can have a perfectly good testosterone value and still get a disappointing semen analysis. The reverse happens too.
What do your testicles actually do?
Your testicles have two jobs. They make testosterone, the hormone that helps steer your muscle mass, your bones, your libido and your mood. And they make sperm. Two different cell types do that work, they are driven by different hormones, and they can start underperforming separately.
That split is the heart of this piece.
The Leydig cells sit between the tubes and make your testosterone. The Sertoli cells sit inside the tubes and are the nursery for your sperm.
Together, your scrotum holds around 500 metres of coiled seminiferous tubules. That is the length of five football pitches, folded into something the size of a plum.
How do your testicles make testosterone?
Your brain gives the starting signal. The pituitary sends LH into your bloodstream, LH reaches the Leydig cells in your testicles, and they convert cholesterol into testosterone. As soon as your testosterone rises, that damps down LH production again. A thermostat, essentially.
That feedback loop is what makes LH so useful to measure.
Picture two men with exactly the same low testosterone value, say 9 nmol/l. In the first, LH is high. His brain is shouting, and the testicles are not answering. The problem sits downstream.
In the second, LH is low. The testicles are simply never given the order, and the problem sits upstream, in the pituitary.
Same testosterone value, completely different story. That is why a lone testosterone result says so little, and why LH belongs next to it as standard.
How do your testicles make sperm?
Making sperm takes time. A sperm cell needs roughly 72 days to mature from its first division, plus a few more weeks ripening in your epididymis. So what shows up in a semen analysis today is a snapshot of something that started three months ago.
That detail gets skipped almost every time, and it changes how you read a result.
If you ran a heavy fever ten weeks ago, it may still be visible in today's result. If you quit smoking last month, it will not show up yet.
The Sertoli cells do this work, driven by FSH. Those same Sertoli cells also make inhibin B, a substance you can measure in your blood. The more active the sperm production, the more inhibin B tends to circulate.
In 2021 the WHO published the sixth edition of its laboratory manual, including a lower limit of 16 million sperm per millilitre, 30 percent progressive motility and 4 percent normally shaped cells.
Note what those numbers are. They are fifth percentiles taken from men who fathered children, not a dividing line between fertile and infertile. Sitting under the limit does not mean it will not happen.
Why do your testicles hang outside your body?
Because sperm production wants to run cooler than the rest of you. Your scrotum keeps your testicles a few degrees below your core temperature, and it manages that actively: it pulls tight in the cold and hangs loose in the heat. At structurally higher temperatures, sperm production may run less well.
The mechanism is solidly established. The practical advice people derive from it, much less so.
A 2005 study sat 29 men down with a laptop on their lap and measured a rise in scrotal temperature of around 2.8 degrees (PMID 15591087). Except that study measured temperature, not sperm.
Online, that difference gets flattened into "laptops make you infertile". That is not what it says.
Something similar goes for tight underwear. Research among men at a fertility clinic found that boxer-wearers had a higher average sperm concentration (PMID 30102388), but that is a cross-section in a selected group, and the authors call the literature on it inconsistent themselves.
My reading: heat demonstrably does something to the process, and the effect is largely reversible. Whether your underwear makes the difference, nobody knows.
Which blood values say something about your testicles?
Four values tell you the most together, and they split neatly along those two factories. LH and testosterone cover the hormone side, FSH and inhibin B the sperm side. Only when you lay them side by side does it become visible whether the problem sits in the testicle itself or in the signal coming from your brain.
Below you can see which value reports on which factory.
| Value | Which factory | What a deviation may mean |
|---|---|---|
| Total testosterone | Leydig cells (hormone) | The output of the hormone factory, heavily dependent on the time of the draw |
| LH | Signal from the pituitary | High when a testicle is not answering, low when the brain signal never arrives |
| FSH | Signal to the Sertoli cells | Often rises when sperm production falls short |
| Inhibin B | Sertoli cells (sperm) | Tracks sperm production, usually in the same direction |
| SHBG | Transport in your blood | Binds testosterone, so your total value can look normal while your free value runs low |
Inhibin B and FSH are each other's mirror image. In a Danish study of 349 men, inhibin B tracked positively with sperm concentration while FSH moved the other way (PMID 9398713).
That is exactly what you would expect from a thermostat not getting enough feedback.
If you want those hormone values in view together, the Hormones Man panel looks at them. For the testosterone story on its own, there is the Testosterone panel.
What can go wrong with your testicles?
Roughly four things. Something can get inflamed, something can twist, something can grow, or something in the blood vessels can go wrong. Some of those hurt, others do not, and that difference says little about how serious it is.
That last part is the trap. Pain feels alarming and a painless lump feels harmless, while it is often the other way around.
Sudden, severe pain is the one true emergency. According to Thuisarts, sudden severe pain in your scrotum means calling your GP or the out-of-hours service straight away. Which pain carries which urgency is set out in our piece on testicle pain.
A painless lump always belongs with your GP, even when it feels like nothing. What such a lump can be, and what a blood test does and does not say about it, is covered in testicular cancer symptoms.
If something goes wrong in the veins above your testicle, that is called a varicocele. It may affect your sperm quality and is treated by a urologist. More on that in varicocele and your fertility.
And if you want the bigger picture of how all this connects to having children, start with our guide to male fertility.
If you feel something change in a testicle, in shape, weight or sensation, go and see your GP. With their hands and, if needed, an ultrasound, they can rule out in minutes what you cannot rule out at home.
References
- Jensen TK, et al. Inhibin B as a serum marker of spermatogenesis: correlation to differences in sperm concentration and follicle-stimulating hormone levels. J Clin Endocrinol Metab. 1997. PMID 9398713.
- Boitrelle F, et al. The Sixth Edition of the WHO Manual for Human Semen Analysis: A Critical Review and SWOT Analysis. Life (Basel). 2021. PMID 34947899.
- Sheynkin Y, et al. Increase in scrotal temperature in laptop computer users. Human Reproduction. 2005. PMID 15591087.
- Minguez-Alarcon L, et al. Type of underwear worn and markers of testicular function among men attending a fertility center. Human Reproduction. 2018. PMID 30102388.
- Durairajanayagam D, et al. Causes, effects and molecular mechanisms of testicular heat stress. Reproductive BioMedicine Online. 2015. PMID 25456164.
- Thuisarts. Pijn in je balzak. thuisarts.nl
Every blood test result at Caliber includes a professional assessment by a BIG-registered doctor. For treatment decisions, always discuss your results with your GP.
Author
