What is testosterone?
Testosterone belongs to a class of hormones called androgens. The body makes it from cholesterol, which is why it is a steroid rather than a peptide or protein. In men, most of it comes from Leydig cells in the testes. Women make much smaller amounts in the ovaries and adrenal glands, where it also serves as a building block for estrogen. Levels rise sharply at puberty, and in men they tend to decline gradually with age.
Medical testosterone is the same molecule the body makes. Many injectable forms attach it to an ester, such as cypionate or enanthate, which slows its release, and the body then removes the ester. It comes in several forms, including injections, gels, patches and implanted pellets, and it is prescribed mainly for men whose bodies do not make enough, a condition called hypogonadism.
How does testosterone work?
Testosterone works by binding to androgen receptors inside cells, which switches on genes involved in building muscle protein, making red blood cells, maintaining bone, body hair and sex drive. Some of it is converted into a stronger androgen, DHT, and some into estradiol, a form of estrogen that matters for bone and other functions in men too. The brain controls production through a feedback loop: the pituitary releases luteinizing hormone (LH), which tells the testes to make testosterone, and rising testosterone tells the brain to ease off.
What has testosterone research looked at?
- Low testosterone in older men
- A set of coordinated trials in older men with low testosterone found modest improvements in sexual function, mood, bone density and anemia, but little or no clear effect on energy, walking or memory.
- Heart safety
- A large trial in men with low testosterone and higher heart risk found no increase in major heart events compared with placebo. It did report more atrial fibrillation, blood clots in the lungs and acute kidney injury.
- Muscle and strength
- A well-known placebo-controlled study found that testosterone above normal levels increased muscle size and strength in healthy young men, with larger gains when combined with strength training.
- Body composition
- In men with low levels, trials have generally found that therapy increases lean mass and modestly reduces fat mass.
- Male contraception
- Testosterone suppresses sperm production strongly enough that it has been studied as a male contraceptive.
- Women
- Testosterone has been studied in postmenopausal women with low sexual desire, where trials reported modest improvements.
Testosterone is far better studied than most compounds in this library. The strongest evidence concerns men with clinically low levels. Benefits for men with normal levels, and the long-term effects of levels kept above the normal range, are much less clear.
How testosterone therapy affects natural production and fertility
When testosterone comes from outside the body, the brain senses the higher level and cuts back on LH and follicle-stimulating hormone (FSH). With less LH, the testes make less of their own testosterone, and with less FSH and less testosterone inside the testes, sperm production drops, often sharply. The testes may shrink. Recovery after stopping usually happens but can take months and is not guaranteed. This is why hormones such as HCG and HMG come up alongside testosterone in fertility discussions.
Testosterone side effects and cautions
Common effects of testosterone therapy include a rise in red blood cell count, acne or oily skin, fluid retention, breast tenderness or growth from conversion to estrogen, smaller testes and lower sperm counts. It can worsen untreated sleep apnea and urinary symptoms from an enlarged prostate. Because of these effects, testosterone therapy is normally monitored with blood tests. Anyone with health questions about it should talk to a qualified clinician.
- Red blood cells
- Testosterone raises hematocrit, and very high levels thicken the blood, which may raise the risk of clots.
- Prostate and breast cancer
- Prostate cancer and male breast cancer are standard reasons to avoid testosterone therapy, since these cancers can be driven by hormones.
- Fertility
- It lowers sperm production and can cause infertility, which matters for anyone hoping to have children.
- Heart and blood vessels
- A large trial was reassuring on major heart events in men with low levels but reported more atrial fibrillation and blood clots in the lungs, and a smaller trial found a greater increase in coronary plaque.
- Women and pregnancy
- In women, higher levels can cause acne, unwanted hair growth and a deeper voice, and testosterone can harm a developing baby during pregnancy. Testosterone gels can pass to women and children through skin contact.
Further reading
This page summarizes the research in plain words. To read the studies yourself:
- Published studies on Testosterone PubMed, US National Library of Medicine
- Registered human trials of Testosterone ClinicalTrials.gov
