Testosterone is a steroid hormone built from cholesterol. In men, the bulk of it is made by Leydig cells in the testes, with a small top-up from the adrenal glands. Production runs on a feedback loop: the hypothalamus signals the pituitary gland, the pituitary releases luteinising hormone (LH), and LH tells the testes to make testosterone. When levels rise, the brain eases off; when they fall, it pushes harder.
Once testosterone reaches the bloodstream, most of it is carried by proteins, mainly SHBG and albumin. Only a small slice circulates unattached as free testosterone. A standard blood test measures the total, which is why doctors sometimes need extra numbers to get the full picture.
Some testosterone is converted in the tissues into two other active hormones: DHT, a stronger androgen that acts on the prostate, skin and hair, and oestradiol, an oestrogen men genuinely need. So when people talk about testosterone doing something, part of that work is often being done by its by-products.
What does testosterone do in men?
Testosterone switches on male development at puberty and then keeps sexual function, sperm production, muscle and bone ticking over through adult life. It also feeds into energy and mood, though those links are looser than supplement adverts suggest.
Before birth and again at puberty, testosterone builds the male body: a deeper voice, facial and body hair, a larger penis and testes, and more muscle. In adulthood the job shifts to maintenance. Levels do not need to be sky-high for things to work; the body needs enough, and more than enough rarely adds much.
- Sex drive (libido)
- Erections, including spontaneous and morning erections
- Sperm production, working alongside FSH
- Muscle mass and strength
- Bone density
- Red blood cell production
What is a normal testosterone level?
In a large study that harmonised results across four cohorts in the US and Europe, healthy non-obese men aged 19 to 39 ranged from 264 to 916 ng/dL, roughly 9 to 32 nmol/L.1 Laboratories use slightly different methods, so always read your result against the range printed on your own report.
That same work found that a lot of the variation between studies came down to the assay rather than the men, which is a useful reminder that a single number is less precise than it looks.1 Testosterone also follows a daily rhythm, peaking in the morning, and it can dip for a while after illness or a run of poor sleep.
For that reason, the Endocrine Society recommends measuring fasting, morning total testosterone with a reliable assay, and repeating the test to confirm a low result before anyone makes a diagnosis.4 One borderline reading on its own is not a verdict.
Does testosterone drop with age?
Yes, gradually. In the Massachusetts Male Aging Study, total testosterone fell by about 1.6% a year within individual men followed for up to a decade, and the more active fractions fell faster, at about 2% to 3% a year.2
Part of the reason the active fractions fall faster is that SHBG, the protein that grips testosterone, climbed by about 1.6% a year across age groups in the same study.2 Health matters too: men who were free of chronic illness, obesity, regular prescription medication and heavy drinking had androgen levels roughly 10% to 15% higher.2 We cover the practical side in how testosterone changes after 40.
What lowers testosterone?
Short sleep, excess weight, chronic illness, some medicines and heavy drinking are the usual suspects. In one small study, a week of sleeping five hours a night cut daytime testosterone in young healthy men by 10% to 15%.3
None of these factors is exotic, which is good news: they are also the ones you can do something about. If sleep is the weak link, our piece on sleep and sex drive is a sensible starting point. If you have symptoms such as low desire, fewer morning erections or erectile problems alongside a low result, talk to a doctor rather than self-treating, because the cause is worth finding.
Common questions
What time of day should testosterone be tested?
In the morning and after fasting, which is what the Endocrine Society guideline recommends.4 A low result should be confirmed with a second morning test before it is treated as meaningful.
Can you raise testosterone naturally?
If your level is being dragged down by short sleep, excess weight or heavy drinking, fixing those can help, and they are worth doing anyway. Several supplement ingredients have also been studied for testosterone, including zinc, ashwagandha and tongkat ali; our guide to supplement ingredients and the research goes through what trials have found.
Does higher testosterone mean a higher sex drive?
Not in a straight line. Desire does depend on having enough testosterone, but stress, sleep, health and how things are going with a partner all matter, and a higher number within the normal range is no guarantee of more interest. There is more in does testosterone affect sex drive?
Read more on this
How testosterone changes after 40 (and what actually helps)
Does testosterone affect sex drive? Separating myth from mechanism
Sleep and sex drive: the overlooked connection
Sources
- Travison TG, et al. Harmonized reference ranges for circulating testosterone levels in men of four cohort studies in the United States and Europe. J Clin Endocrinol Metab. 2017;102(4):1161-1173. PubMed
- Feldman HA, et al. Age trends in the level of serum testosterone and other hormones in middle-aged men: longitudinal results from the Massachusetts Male Aging Study. J Clin Endocrinol Metab. 2002;87(2):589-598. PubMed
- Leproult R, Van Cauter E. Effect of 1 week of sleep restriction on testosterone levels in young healthy men. JAMA. 2011;305(21):2173-2174. PubMed
- Bhasin S, et al. Testosterone therapy in men with hypogonadism: an Endocrine Society clinical practice guideline. J Clin Endocrinol Metab. 2018;103(5):1715-1744. PubMed
Not medical advice. Written by Eric Stephens from the research cited above; not reviewed by a doctor. If something worries you, see a GP.