The testes are the reproductive glands that produce male hormones. These two oval-shaped organs, located in the scrotum, manufacture about 95% of the body’s testosterone and are the primary source of androgens in males. A small additional amount of weaker androgens comes from the adrenal glands, which sit on top of the kidneys, but their contribution to male hormone levels is minimal.
How the Testes Produce Testosterone
Inside the testes, specialized cells called Leydig cells are responsible for converting cholesterol into testosterone. These cells sit in the tissue between the sperm-producing tubes and make up the hormonal engine of the male reproductive system. When a signaling hormone from the pituitary gland (a pea-sized gland at the base of the brain) reaches the Leydig cells, it triggers a chain of chemical reactions that transform cholesterol into testosterone step by step.
This process begins before birth. During fetal development, Leydig cells produce the high levels of androgens needed to form male genitalia and shape brain development. After a quiet period in childhood, they ramp up again at puberty and continue producing testosterone throughout adult life.
The Hormones the Testes Make
Testosterone is the most well-known male hormone, but the testes contribute to several androgens that play distinct roles in the body.
- Testosterone: The primary male hormone. It drives muscle growth, bone density, red blood cell production, sex drive, and sperm development. In healthy men aged 19 to 39, normal blood levels range from about 264 to 916 ng/dL.
- Dihydrotestosterone (DHT): A more potent form of testosterone created when enzymes in tissues like the skin and prostate convert testosterone locally. DHT binds to hormone receptors roughly twice as strongly as testosterone. It is the main driver of facial hair, body hair, prostate growth, and male pattern baldness. Circulating DHT levels in the blood are only about 10% of testosterone levels, but in the tissues where it’s made, concentrations can be up to ten times higher.
- Androstenedione: A weaker androgen that serves mostly as a precursor, meaning the body uses it as a building block to make testosterone.
During embryonic development, DHT is essential for forming the external genitalia. It also helps guide the testes in their descent from the abdomen into the scrotum. Through puberty and adulthood, DHT takes the lead on traits like facial hair growth and prostate enlargement, while testosterone handles broader functions like muscle maintenance and libido.
The Role of the Adrenal Glands
The adrenal glands produce a group of weak androgens, primarily DHEA and androstenedione, from a layer called the zona reticularis. In adult men, however, these adrenal androgens account for less than 5% of total testosterone production. Their contribution to male characteristics is essentially negligible. Even when the adrenal glands overproduce these hormones, it typically causes no noticeable effects in men because the testes already dominate androgen output so heavily.
How the Brain Controls Hormone Production
Testosterone production doesn’t happen on autopilot. It’s tightly regulated by a feedback loop involving three levels: the hypothalamus in the brain, the pituitary gland just below it, and the testes themselves. The hypothalamus releases a signaling hormone that tells the pituitary to secrete two key messengers: luteinizing hormone (LH), which stimulates the Leydig cells to produce testosterone, and follicle-stimulating hormone (FSH), which supports sperm production.
When testosterone levels in the blood rise high enough, the hypothalamus and pituitary detect this and dial back their signals. When levels drop, they increase the signals. This negative feedback system keeps testosterone within a relatively stable range throughout the day, though levels are naturally highest in the early morning and lowest in the evening.
How Testosterone Changes With Age
Testosterone production peaks in early adulthood and begins a gradual decline. In men aged 40 to 70, total testosterone drops at a rate of about 0.4% per year. Free testosterone, the portion not bound to proteins in the blood and therefore available for the body to use, declines faster at roughly 1.3% per year. This is a slow process. Most men maintain levels well within the normal range for decades.
The lower boundary of normal does shift with age. For men in their 20s and 30s, the bottom of the reference range is around 264 ng/dL. By age 60 to 69, it drops to about 218 ng/dL. By age 80 and beyond, it falls to around 157 ng/dL. The American Urological Association considers a total testosterone level below 300 ng/dL a reasonable threshold for diagnosing low testosterone, provided a man also has symptoms like fatigue, reduced sex drive, or loss of muscle mass. Two separate early-morning blood tests are needed to confirm the diagnosis, since levels fluctuate throughout the day.
Signs the Testes May Not Be Producing Enough
When the testes underperform, the condition is called hypogonadism. It can stem from problems in the testes themselves (such as injury, infection, or genetic conditions) or from the brain failing to send the right signals. Common symptoms include persistent fatigue, low libido, difficulty concentrating, reduced muscle strength, increased body fat, mood changes, and erectile dysfunction. Because these symptoms overlap with many other conditions, blood testing is essential for a clear diagnosis.
In younger men, hypogonadism can also affect fertility, since the same hormonal signals that drive testosterone production are closely linked to sperm development. The Leydig cells and the sperm-producing cells in the testes depend on the same pituitary hormones, so disruptions in one process often affect the other.