The testes produce about 95% of a man’s testosterone. Specialized cells called Leydig cells, tucked in the spaces between the sperm-producing tubes of the testes, are the primary factory. The adrenal glands, which sit on top of the kidneys, contribute the remaining 5% or less. A healthy young man produces roughly 6 to 7 milligrams of testosterone per day, though that number depends on age, time of day, and overall health.
How Leydig Cells Make Testosterone
Leydig cells don’t build testosterone from scratch. They start with cholesterol, the same fatty molecule you hear about in blood tests. Through a series of enzymatic steps, cholesterol is converted into testosterone inside these cells. The process begins when a transport protein shuttles cholesterol into the cell’s mitochondria (its energy-producing compartment), where the first chemical transformation takes place. From there, additional enzymes modify the molecule step by step until it becomes testosterone, which is then released directly into the bloodstream.
This is why extremely low-fat diets can sometimes affect hormone levels. Your body needs a baseline supply of cholesterol to keep the production line running.
The Brain Controls the Whole Process
Testosterone production doesn’t just happen on autopilot at the testes. It’s directed by a signaling chain that starts in the brain, known as the hypothalamic-pituitary-gonadal (HPG) axis. Here’s how it works in three steps:
- Step 1: The hypothalamus signals. A small region at the base of the brain releases a hormone called GnRH in pulses throughout the day.
- Step 2: The pituitary responds. GnRH travels a short distance to the pituitary gland, which releases luteinizing hormone (LH) into the bloodstream.
- Step 3: Leydig cells activate. LH reaches the testes and binds to receptors on Leydig cells, triggering them to convert cholesterol into testosterone.
Without LH arriving from the pituitary, Leydig cells sit largely idle. This is why conditions affecting the brain or pituitary gland can cause low testosterone even when the testes themselves are perfectly healthy.
How the Body Keeps Levels in Check
The same signaling chain that starts production also shuts it down when levels get too high. When testosterone in the blood rises above a certain threshold, it feeds back to both the hypothalamus and the pituitary gland. At the hypothalamus, testosterone acts on specialized neurons that reduce the pulsing release of GnRH. At the pituitary, the gland becomes less responsive to whatever GnRH does arrive. The result: less LH reaches the testes, and production slows down.
When testosterone drops, the brakes come off, GnRH pulses pick up, and the cycle ramps back up. This negative feedback loop runs continuously, keeping testosterone within a relatively narrow range throughout the day. It’s also why men who take external testosterone (through gels, injections, or other forms) experience a drop in their natural production. The brain detects the incoming testosterone and dials down its own signaling, which can shrink the testes over time as Leydig cells become less active.
Testosterone Follows a Daily Rhythm
Testosterone levels aren’t constant. They follow a circadian pattern, peaking early in the morning (between about 5:30 and 8:00 a.m.) and hitting their lowest point roughly 12 hours later, in the early evening. This is why blood tests for testosterone are typically drawn in the morning, so the results capture your levels near their daily high. A late-afternoon blood draw can read significantly lower and may not accurately reflect your baseline.
Sleep plays a direct role in this rhythm. Most testosterone release happens during sleep, particularly during deeper sleep stages. Chronic sleep deprivation compresses that overnight production window and can measurably lower morning levels.
The Small Role of the Adrenal Glands
The adrenal glands produce a small amount of testosterone and larger amounts of weaker androgens (hormones in the same family). In men, this adrenal contribution is minor, less than 5% of total testosterone. It’s controlled by a separate signaling pathway involving ACTH from the pituitary, not LH. For most men, adrenal testosterone is a footnote. It becomes more clinically relevant in women, where the adrenal glands are a proportionally larger source of androgens.
Why Production Declines With Age
Starting around age 40, testosterone production drops by roughly 1 to 2% per year. This isn’t a sudden cliff like menopause. It’s a gradual slope, and many men don’t notice symptoms for years or even decades. The decline has multiple causes. Leydig cells become fewer and less responsive to LH over time. The hypothalamus and pituitary also become less efficient at sending strong signals. Body composition changes (particularly increased body fat) accelerate the decline because fat tissue converts testosterone into estrogen, which further suppresses the brain’s signaling chain.
By age 70 or 80, many men have testosterone levels that are 30 to 50% lower than what they had in their 20s. Whether that decline causes symptoms varies widely from person to person. Some men with modestly low levels feel fine; others with levels still technically in the normal range experience fatigue, reduced muscle mass, or lower libido. The relationship between a number on a lab test and how you actually feel is not always straightforward.