Testosterone is the primary sex hormone in males, responsible for everything from muscle and bone development to sperm production and sex drive. It’s produced mainly in the testes, with small amounts coming from the adrenal glands. While both men and women have testosterone, men produce far more of it, and it plays a central role in male development from before birth through old age.
What Testosterone Does in the Body
Testosterone drives many of the physical traits associated with male development. During puberty, it deepens the voice, triggers facial and body hair growth, and spurs the growth of muscle and bone. In adult men, it continues to maintain muscle size and strength, keeps bones dense and resistant to fracture, and fuels sperm production. It also influences sex drive, energy levels, and mood.
These effects aren’t just cosmetic. Men with very low testosterone are at higher risk for brittle bones and fractures, a condition more commonly associated with women but real for men too. Restoring testosterone levels in men with osteoporosis can meaningfully improve bone strength.
How the Body Produces Testosterone
Testosterone production starts in the brain. The hypothalamus releases a signaling hormone called GnRH, which tells the pituitary gland to release luteinizing hormone (LH). LH then travels through the bloodstream to specialized cells in the testes called Leydig cells, which use cholesterol as a raw material to synthesize testosterone. This chain of signals, from brain to pituitary to testes, is called the hypothalamic-pituitary-gonadal axis.
The system runs on a feedback loop. When testosterone levels rise high enough, the brain reduces its signaling, slowing production. When levels drop, the brain ramps signaling back up. This is why anything that disrupts the brain’s ability to sense or send those signals, whether obesity, certain medications, or pituitary problems, can throw testosterone levels off.
Normal Testosterone Levels by Age
Testosterone is measured through a blood test, reported in nanograms per deciliter (ng/dL). The American Urological Association uses 300 ng/dL as the general cutoff below which a man may be considered to have low testosterone. But that number is a simplification. Research published in The Journal of Urology found that healthy ranges vary meaningfully by age:
- Ages 20 to 24: 409 to 558 ng/dL
- Ages 25 to 29: 413 to 575 ng/dL
- Ages 30 to 34: 359 to 498 ng/dL
- Ages 35 to 39: 352 to 478 ng/dL
- Ages 40 to 44: 350 to 473 ng/dL
These ranges represent the middle third of the population for each age group. A man in his early twenties with a testosterone level of 350 ng/dL is technically above the 300 cutoff but would fall below the 33rd percentile for his age, sitting at a level more typical of men 15 to 20 years older.
After age 30, testosterone drops an average of about 1% per year. This gradual decline is normal and doesn’t necessarily cause symptoms. Some men maintain healthy levels well into their sixties and seventies, while others drop below the threshold much earlier.
Total vs. Free Testosterone
Not all the testosterone circulating in your blood is actively doing anything. Most of it is bound to a protein called sex hormone binding globulin (SHBG), which is produced by the liver. Testosterone that’s attached to SHBG can’t interact with your tissues, so it’s essentially inactive. A smaller portion floats freely in the bloodstream, unbound to any protein. This “free” testosterone is what actually enters cells and drives biological effects like muscle growth and reproductive function.
A standard blood test measures total testosterone, which includes both the bound and free forms. If total testosterone comes back normal but symptoms suggest otherwise, a doctor may check free testosterone or SHBG levels separately. High SHBG can trap more testosterone in its inactive form, leaving less available to do its job even when the total number looks fine.
Signs of Low Testosterone
The symptoms of low testosterone depend on when the deficiency occurs. In the most extreme cases, when testosterone is insufficient during fetal development, a genetically male baby can be born with ambiguous or incompletely developed genitals. During puberty, low testosterone can delay or stall development, sometimes causing disproportionate limb growth relative to the trunk, or breast tissue growth.
In adult men, the signs are subtler but can significantly affect quality of life. Early symptoms often include reduced sex drive, lower energy, and depression. Over time, men may notice loss of muscle mass, increased body fat, thinning body hair, and difficulty concentrating. Bones gradually weaken, raising fracture risk. Fertility can also be affected since testosterone is essential for sperm production.
Because these symptoms overlap with many other conditions, from thyroid problems to simple aging, diagnosis requires blood work. The American Urological Association recommends that low testosterone only be diagnosed after two separate blood draws, both taken in the early morning when levels are at their daily peak.
What Affects Testosterone Levels
Beyond the natural age-related decline, several lifestyle factors influence how much testosterone your body produces. Sleep is one of the most significant. A meta-analysis of 18 studies found that going 24 hours or more without sleep causes a measurable drop in testosterone. Interestingly, partial sleep deprivation, like getting five or six hours instead of eight, did not produce a statistically significant effect in the short term. The takeaway: occasional short nights are unlikely to tank your levels, but pulling all-nighters or consistently severe sleep loss can.
Body weight plays a major role too. Fat tissue, particularly abdominal fat, disrupts the hormonal signaling chain between the brain and testes. In extreme cases, obesity in youth can impair LH and FSH production enough to mimic a condition called hypogonadotropic hypogonadism, where the testes simply never receive the signal to produce adequate testosterone. Losing weight often partially reverses this effect.
Exercise, particularly resistance training, has a well-documented positive effect on testosterone. Chronic stress, heavy alcohol use, and certain medications can push levels downward. Diet matters in a foundational way: since testosterone is synthesized from cholesterol, extremely low-fat diets can theoretically limit the raw materials available for production, though this is rarely the primary cause of deficiency in practice.