The testes are the gland that produces both sperm and testosterone. Each testis performs this dual role using two distinct sets of specialized cells, making it one of the few organs in the body that functions as both an endocrine gland (releasing hormones into the bloodstream) and an exocrine gland (producing cells that exit through a duct).
Two Jobs, Two Cell Types
The testes handle sperm and testosterone production in separate compartments. Testosterone is made by Leydig cells, which sit in the tissue between the sperm-producing structures. These cells convert cholesterol into testosterone inside their mitochondria, then release it directly into the bloodstream. This is the endocrine function.
Sperm production happens inside tightly coiled tubes called seminiferous tubules. Within those tubules, immature germ cells gradually develop into mature sperm cells with the help of support cells called Sertoli cells. Sertoli cells act like scaffolding and nurseries combined: they physically cradle developing sperm, feed them nutrients, and create the precise chemical environment they need to mature. This is the exocrine function, because the finished sperm cells leave the body through a series of ducts.
These two functions aren’t independent. Testosterone produced by the Leydig cells outside the tubules actually seeps into the seminiferous tubules, where it’s essential for sperm development. Without adequate local testosterone levels, sperm production stalls.
Why the Testes Sit Outside the Body
Sperm production is remarkably sensitive to heat. For optimal development, the testes need to stay 2 to 4°C cooler than core body temperature. That’s the reason they’re housed in the scrotum rather than inside the abdominal cavity like the ovaries. The scrotum acts as a temperature regulator, pulling the testes closer to the body in cold conditions and letting them hang lower when it’s warm. Prolonged exposure to elevated temperatures, whether from fevers, hot tubs, or tight clothing, can temporarily impair sperm production.
How Long Sperm Take to Develop
A single sperm cell takes roughly 64 days to develop from a stem cell into a mature, motile spermatozoon. This process runs in overlapping waves, so the testes aren’t producing one batch at a time. Instead, new groups of sperm are constantly starting development while older groups are finishing. The full cycle of the germinal lining takes about 16 days, and a sperm cell passes through approximately four of these cycles before it’s complete. This timeline matters if you’re trying to improve sperm quality through lifestyle changes: it takes about two to three months to see the effects, because you’re waiting for an entirely new generation of sperm to develop.
The Brain Controls the Whole Process
The testes don’t operate on autopilot. A hormonal feedback loop running between the brain and the testes governs how much testosterone and sperm are produced at any given time. A region of the brain called the hypothalamus releases a signaling hormone (GnRH) in pulses, which tells the pituitary gland at the base of the brain to release two hormones of its own: LH and FSH. LH travels through the blood to the Leydig cells and triggers testosterone production. FSH targets the Sertoli cells and supports sperm development.
The system is self-correcting. When testosterone levels rise high enough, testosterone feeds back to the hypothalamus and pituitary, dialing down the release of GnRH, LH, and FSH. The Sertoli cells also release a hormone called inhibin B that specifically puts the brakes on FSH secretion. This keeps testosterone and sperm production within a functional range without the system overshooting in either direction. It’s also why taking external testosterone (such as for hormone therapy or performance enhancement) can shut down the body’s own sperm production: the brain senses high testosterone and stops sending the signals that drive the seminiferous tubules.
Normal Testosterone Levels
For adult males aged 18 and older, a typical total testosterone range falls between 193 and 824 ng/dL, according to Cleveland Clinic reference values. That’s a wide range, and where you fall within it depends on age, time of day (testosterone peaks in the morning), body composition, and the specific lab running the test. Two different labs can return slightly different numbers using different assays, so your results should always be interpreted against the reference range printed on your specific lab report.
Testosterone production gradually declines with age, typically dropping about 1 to 2% per year after age 30. This is a normal part of aging and doesn’t necessarily mean something is wrong. Problems arise when levels fall low enough to cause symptoms like persistent fatigue, loss of muscle mass, reduced sex drive, or mood changes.
When the System Breaks Down
When the testes underperform in producing testosterone, sperm, or both, the condition is called hypogonadism. It comes in two forms depending on where the problem originates.
Primary hypogonadism means the testes themselves are damaged or dysfunctional. The brain is sending the right signals, but the gland can’t respond properly. Causes include genetic conditions, physical injury, infection, or certain medical treatments. In this form, LH and FSH levels are typically high because the brain keeps ramping up its signals trying to get a response.
Secondary (or central) hypogonadism means the problem is in the brain, specifically the hypothalamus or pituitary gland. The testes are capable of working, but they aren’t receiving the hormonal signals they need. This can result from pituitary tumors, head injuries, or certain medications. Symptoms sometimes include headaches, vision changes, or other hormonal deficiencies beyond just low testosterone, because the pituitary controls multiple hormone systems. When a pituitary issue is suspected, imaging of the brain is typically part of the workup.
The distinction matters for treatment. In secondary hypogonadism, restoring the brain’s signaling can sometimes restart both testosterone and sperm production. In primary hypogonadism, the testes may need to be bypassed entirely with hormone replacement, though this approach supports testosterone levels without restoring fertility.