The primary male sex hormone is called testosterone. It belongs to a broader group of hormones known as androgens, which drive the development of male physical traits and regulate everything from muscle mass to sperm production. While testosterone is the most well-known androgen, it works alongside several other hormones that each play distinct roles in male biology.
Testosterone and the Androgen Family
Testosterone is the most common and most potent androgen circulating in the male body. But it’s not the only one. The androgen family includes several other hormones, most of which serve as building blocks for producing testosterone or estrogen:
- Androstenedione: a precursor hormone that the body converts into testosterone
- DHEA and DHEA sulfate: precursor hormones produced mainly by the adrenal glands, which also feed into testosterone production
- Dihydrotestosterone (DHT): a more potent form of testosterone that the body creates by converting roughly 10% of its daily testosterone supply
DHT deserves special attention because it plays a critical role in male sexual development before birth, particularly the formation of external genitalia. An enzyme called 5-alpha reductase converts testosterone into DHT. When this enzyme is missing or deficient, male sex characteristics don’t develop normally in the womb, even if testosterone levels are adequate.
Where Testosterone Is Made
The vast majority of testosterone in males comes from specialized cells in the testes called Leydig cells. These cells sit in the tissue between the sperm-producing tubes and are the body’s primary testosterone factory. The adrenal glands, which sit on top of the kidneys, contribute a smaller amount of androgens like DHEA.
Testosterone production doesn’t happen on autopilot. It’s controlled by a hormonal relay system that starts in the brain. The hypothalamus releases a signaling hormone in pulses, which tells the pituitary gland to release two key hormones: luteinizing hormone (LH) and follicle-stimulating hormone (FSH). LH is the direct trigger for Leydig cells to produce testosterone, while FSH supports sperm development.
This system has a built-in thermostat. When testosterone levels rise high enough, they send a signal back to the hypothalamus and pituitary to slow down production. When levels drop, the brain ramps signaling back up. This feedback loop keeps testosterone within a functional range throughout the day, though levels naturally peak in the early morning and dip in the evening.
What Testosterone Does During Puberty
Testosterone is the driving force behind the physical changes boys experience during puberty. It triggers increases in height, the growth of body and pubic hair, deepening of the voice, and the enlargement of the penis, testes, and prostate gland. It also activates sex drive for the first time. These are often called secondary sex characteristics, and they’re almost entirely dependent on rising testosterone levels during adolescence.
Testosterone’s Role in Adult Men
After puberty, testosterone continues to serve several essential functions. It is required for sperm production and signals the body to make new red blood cells. It helps maintain bone density and muscle mass, and it sustains sex drive and a general sense of well-being.
Interestingly, a portion of testosterone’s benefits actually come from its conversion into estrogen. An enzyme called aromatase, found in Leydig cells, fat tissue, and other organs, converts some testosterone into estradiol (the primary form of estrogen). This process is essential for bone health in men. Research in Physiological Reviews has shown that when this conversion is blocked, men lose the bone-protective benefits of testosterone, and reduced aromatase activity is linked to decreased bone mass.
Testosterone, Mood, and the Brain
Low testosterone is commonly associated with cognitive and emotional symptoms, including depressive mood, irritability, poor concentration, reduced motivation, and memory complaints. These symptoms are real and recognized in clinical guidelines, but the relationship between testosterone and brain function is more complicated than it might seem.
Clinical trials have tested whether raising testosterone levels actually improves thinking skills. In one trial of nearly 500 men aged 65 and older who had both low testosterone and age-related memory problems, a full year of testosterone treatment produced no improvement in verbal memory, visual memory, executive function, or spatial ability compared to a placebo. A separate three-year trial in healthy older men with low testosterone found similar results. So while low testosterone correlates with cognitive symptoms, replacing it doesn’t reliably reverse them, at least in older men.
Signs of Low Testosterone
The American Urological Association defines low testosterone as a total level below 300 ng/dL, confirmed by two separate blood draws taken in the early morning. A diagnosis requires both low levels on lab work and the presence of symptoms. The number alone isn’t enough.
The symptoms recognized by clinical guidelines fall into three categories:
- Physical: reduced energy and endurance, fatigue, loss of body and facial hair, decreased lean muscle mass, increased body fat (particularly around the midsection), and diminished physical performance
- Cognitive and emotional: depressive symptoms, irritability, poor concentration, reduced motivation, and memory problems
- Sexual: reduced sex drive, erectile difficulties, and loss of morning erections
One important caveat: many of these symptoms overlap with chronic stress, poor sleep, depression, and general fatigue. That’s why clinical guidelines require lab confirmation rather than relying on symptoms alone. A man experiencing several of these changes may have low testosterone, or he may have an entirely different underlying issue producing the same effects.