The primary male sex hormone is testosterone, the most abundant androgen in the body. Testosterone belongs to a broader group of hormones called androgens, which drive the development of male physical traits, fuel reproductive function, and influence everything from muscle mass to mood. While testosterone gets most of the attention, other androgens like dihydrotestosterone (DHT) and DHEA also play important supporting roles.
How Testosterone Is Produced
Testosterone production starts with a signaling chain that runs from the brain to the testes. A region of the brain called the hypothalamus releases a hormone in pulses, which tells the pituitary gland (a pea-sized gland at the base of the brain) to release two messenger hormones: luteinizing hormone (LH) and follicle-stimulating hormone (FSH). LH travels through the bloodstream and binds to specialized cells in the testes called Leydig cells, triggering them to produce testosterone.
This entire system runs on a feedback loop. When testosterone levels in the blood rise high enough, the brain detects this and dials back its signals, reducing LH output. When levels drop, the brain ramps signaling back up. This constant adjustment keeps testosterone within a functional range throughout the day, with levels typically peaking in the morning and dipping in the evening.
The adrenal glands, which sit on top of the kidneys, also contribute to the androgen pool by producing most of the body’s DHEA and DHEA sulfate. These are weaker androgens that serve as building blocks the body can convert into testosterone or estrogen. However, the testes are responsible for the vast majority of circulating testosterone in men.
What Testosterone Does in the Body
Testosterone’s influence is remarkably broad. Its effects can be grouped into three main categories: development, reproduction, and ongoing body maintenance.
During fetal development, testosterone is essential for forming male genitals. Without adequate levels in the womb, a genetically male baby may be born with incompletely developed or ambiguous genitals. At puberty, rising testosterone drives the deepening of the voice, growth of facial and body hair, enlargement of the penis and testes, and the adolescent growth spurt in muscle mass.
In adult men, testosterone is directly involved in sperm production. It works on support cells within the testes called Sertoli cells, which nurture sperm through their maturation process. Testosterone is required for at least four critical steps: maintaining a protective barrier around developing sperm, supporting the cell division that creates sperm, keeping sperm attached to support cells as they mature, and releasing finished sperm. FSH, the other pituitary messenger hormone, also supports Sertoli cell function, making both hormones necessary for fertility.
Beyond reproduction, testosterone is osteogenic to bone (it promotes new bone formation by acting directly on bone-building cells), myogenic to muscle (it boosts protein synthesis by activating muscle stem cells and blocking signals that break muscle down), and catabolic to fat (it promotes the breakdown of stored fat for energy). Clinical trials have shown that six months of testosterone therapy increases hip bone density and total lean body mass in men with low levels.
The Role of DHT
Dihydrotestosterone, or DHT, is a more potent androgen that the body creates by converting testosterone. Rather than circulating through the bloodstream the way testosterone does, DHT primarily acts locally in the tissues that produce it, particularly the prostate and skin. This makes it more of an on-site hormone than a body-wide messenger.
DHT is critical during fetal development for the formation of male external genitals and the prostate. In adults, the prostate continues to produce DHT from circulating testosterone, and elevated DHT activity in the prostate is associated with benign prostate enlargement. The skin also converts testosterone to DHT using a different enzyme than the prostate uses, and DHT in the scalp has long been linked to male-pattern hair loss, though the relationship is more complex than once thought. Medications that block DHT production are used to treat both prostate enlargement and hair loss.
Normal Testosterone Levels by Age
Testosterone is measured in nanograms per deciliter (ng/dL) through a blood test. According to Cleveland Clinic reference ranges, levels change dramatically across the lifespan:
- Before puberty (under 10): Less than 25 ng/dL
- During puberty (11 to 15): Up to 830 ng/dL
- Late adolescence (16 to 17): 102 to 1,010 ng/dL
- Adults (18 and older): 193 to 824 ng/dL
The wide adult range reflects natural variation between individuals. A level of 250 ng/dL and a level of 750 ng/dL are both technically “normal,” but symptoms of deficiency can appear anywhere along that spectrum depending on what your body is accustomed to.
How Testosterone Declines With Age
Testosterone levels peak in late adolescence and early adulthood, then begin a slow, steady decline. Unlike the sharp hormonal drop women experience during menopause, the decline in men averages just over 1% per year, typically starting after age 30. By the time a man reaches his 70s, his testosterone may be 30 to 50% lower than it was at its peak. This gradual nature means many men don’t notice the change until cumulative effects become apparent.
Signs of Low Testosterone
When testosterone drops below the level a man’s body needs, the earliest symptoms are often reduced sex drive, lower energy, and depressed mood. These can be subtle and easy to attribute to stress, poor sleep, or aging itself.
As levels decline further, the effects become more visible. Muscle mass decreases, body fat increases (particularly around the midsection), and bone density drops, raising fracture risk. Some men experience hot flashes and difficulty concentrating, symptoms that closely mirror what women experience during menopause.
When low testosterone occurs before or during puberty, the consequences are more dramatic. Puberty may be delayed or incomplete, with limited facial and body hair growth, a voice that doesn’t deepen, and underdeveloped muscle mass. The arms and legs may grow disproportionately long relative to the torso, and breast tissue can develop, a condition called gynecomastia. In severe fetal deficiency, the genitals may not develop along typical male patterns at all.
Low testosterone in adults, called hypogonadism, can result from problems in the testes themselves (from injury, infection, or genetic conditions) or from disrupted signaling in the brain’s pituitary system. Obesity, certain medications, chronic illness, and heavy alcohol use can also suppress production. A simple blood test, ideally drawn in the morning when levels are highest, is used to confirm the diagnosis.