Testosterone is the primary sex hormone in males, responsible for driving puberty, building muscle and bone, maintaining sex drive, and influencing mood throughout life. It’s produced mainly in the testes, with small amounts coming from the adrenal glands. In adult men, normal levels generally fall between 193 and 824 ng/dL, though these numbers can vary depending on the lab performing the test.
How Your Body Makes Testosterone
Testosterone production starts in the brain. The hypothalamus releases a signaling hormone in pulses, which tells the pituitary gland to produce two key hormones: luteinizing hormone (LH) and follicle-stimulating hormone (FSH). LH travels through the bloodstream to specialized cells in the testes called Leydig cells, where it triggers testosterone production. FSH, meanwhile, supports sperm production.
This system runs on a feedback loop. When testosterone levels rise high enough, the brain detects the increase and dials back its signaling, slowing production. When levels drop, the brain ramps signaling back up. This loop keeps testosterone within a relatively stable range day to day, though levels do fluctuate on a predictable schedule. Testosterone peaks around 8 a.m. and drops to its lowest point around 2 p.m., a decline of roughly 15% over the course of a morning. This is why blood tests for testosterone are typically drawn early in the morning.
Free vs. Total Testosterone
Not all testosterone circulating in your blood is actively doing work. Most of it, around 97 to 98%, is bound to proteins, primarily sex hormone-binding globulin (SHBG) and albumin. Only about 2 to 3% floats freely, and this “free testosterone” is the fraction that can enter cells and drive biological effects. When doctors evaluate your hormone levels, they may measure total testosterone, free testosterone, or both, depending on your symptoms and health history.
What Testosterone Does During Puberty
The surge of testosterone during adolescence is what transforms a boy’s body into an adult male’s. It triggers the growth of the penis and testes, the appearance of pubic hair, and eventually facial and underarm hair (typically about two years after pubic hair begins). It thickens the vocal cords, deepening the voice. It accelerates bone growth and increases muscle mass. These are collectively called secondary sex characteristics, the visible markers of male development beyond the reproductive organs themselves.
When testosterone is insufficient during puberty, development stalls or proceeds incompletely. Boys may experience delayed growth of body hair, minimal muscle development, growth of breast tissue (gynecomastia), and disproportionate limb growth relative to the trunk.
Roles in the Adult Body
After puberty, testosterone continues to play a central role in several systems.
Bone health. Testosterone drives cortical bone growth, the dense outer shell of your bones, and promotes the expansion of bone surfaces. It does this partly through direct action on bone-building cells called osteoblasts and partly by converting into estradiol (a form of estrogen) via an enzyme in bone tissue. This conversion is essential for trabecular bone, the spongy interior structure, and for slowing bone breakdown. In adult men, testosterone helps preserve the microarchitecture that gives bones their strength.
Muscle and body composition. Testosterone promotes muscle protein synthesis and helps maintain lean body mass. It also influences fat distribution, which is why low testosterone often leads to increased body fat, particularly around the midsection.
Sexual function. Testosterone sustains sex drive and plays a role in erectile function. A drop in libido is often one of the earliest noticeable symptoms when levels fall too low.
Mood and cognition. Testosterone has measurable effects on the brain. Studies in older men have found positive associations between testosterone levels and spatial reasoning, verbal memory, and visual memory. Short-term supplementation trials have shown improvements in spatial cognition and working memory, though these studies were small and ranged from one to twelve months in duration.
How Testosterone Changes With Age
In most men, testosterone begins a gradual decline starting around age 40, dropping an average of just over 1% per year. This is nothing like menopause. There’s no sudden cliff. Some men maintain high levels well into old age, while others experience a more noticeable decline. Over decades, though, even a 1% annual drop adds up. A man in his 70s may have substantially lower levels than he did at 30.
This gradual decrease can contribute to reduced energy, lower sex drive, loss of muscle mass, increased body fat, and decreased bone density. These changes often develop so slowly that they’re easy to attribute to aging in general rather than to a hormonal shift specifically.
Signs of Low Testosterone
Low testosterone, clinically called hypogonadism, produces different effects depending on when it occurs. In a developing fetus, severely low testosterone can affect genital development, resulting in genitals that are incompletely formed or ambiguous. In boys before puberty, it can delay or prevent the typical changes of adolescence.
In adult men, the symptoms tend to be subtler but wide-ranging. Reduced sex drive is often the first sign. Other common symptoms include fatigue, difficulty concentrating, depressed mood, loss of muscle mass, increased body fat, and reduced bone strength. Some men also notice decreased body hair over time. Because these symptoms overlap with many other conditions, including depression, thyroid disorders, and simple sleep deprivation, low testosterone is confirmed through blood testing rather than symptoms alone.
Notably, men who already have low testosterone don’t show the same daily fluctuation pattern as men with normal levels. In men with normal testosterone, levels drop predictably from morning to afternoon. In men with testosterone deficiency, levels stay relatively flat throughout the day. Clinical guidelines still recommend morning blood draws for diagnosis, since that’s when the difference between normal and low is most apparent.
What Affects Your Testosterone Levels
Beyond age, several factors influence how much testosterone your body produces and how much remains available. Body fat is one of the most significant: fat tissue contains an enzyme that converts testosterone into estrogen, so higher body fat generally means lower circulating testosterone. Sleep matters too, since much of the day’s testosterone is produced during sleep, and chronic sleep restriction reliably lowers levels.
Exercise has a complex relationship with the hormone system. Resistance training tends to acutely raise testosterone, while excessive endurance training can suppress it. Chronic stress elevates cortisol, which competes with and can suppress testosterone production. Alcohol, certain medications, and conditions affecting the pituitary gland or testes also play a role.
SHBG levels add another layer of complexity. Conditions like obesity and type 2 diabetes tend to lower SHBG, which can make total testosterone appear lower while free testosterone remains adequate. Conversely, aging and liver disease can raise SHBG, binding up more testosterone and reducing the active fraction even when total levels look normal. This is why doctors sometimes order both total and free testosterone when the clinical picture doesn’t match a single number.