Testosterone in males is produced primarily by specialized cells in the testes called Leydig cells, which account for roughly 95% of the hormone in circulation. The remaining small fraction, about 1%, comes from the adrenal glands that sit on top of the kidneys. But production isn’t a simple on/off switch. It’s controlled by a chain of signals that starts in the brain and loops back on itself to keep levels in a tight range.
The Brain Starts the Process
Testosterone production begins not in the testes but in a small region at the base of the brain called the hypothalamus. The hypothalamus releases a signaling hormone called GnRH in pulses throughout the day. These pulses travel a short distance to the pituitary gland, a pea-sized structure just below it, where they trigger the release of two hormones: luteinizing hormone (LH) and follicle-stimulating hormone (FSH).
LH is the direct trigger for testosterone production. It travels through the bloodstream to the testes, where it binds to receptors on the surface of Leydig cells. FSH, meanwhile, targets a different cell type in the testes called Sertoli cells, which play a supporting role in sperm development. Together, LH and FSH coordinate the two main jobs of the testes: making testosterone and producing sperm.
How Leydig Cells Build Testosterone
Leydig cells sit in the spaces between the sperm-producing tubes of the testes. When LH locks onto their surface receptors, it kicks off a chain of chemical reactions inside the cell that all begin with one raw material: cholesterol.
First, a transport protein shuttles cholesterol into the cell’s mitochondria, the structures that normally generate energy. There, an enzyme clips off part of the cholesterol molecule to create a precursor called pregnenolone. From pregnenolone, a series of additional enzymes in the mitochondria and other cell compartments modify the molecule step by step until it becomes testosterone. The whole conversion path, from cholesterol to finished hormone, happens entirely within the Leydig cell. This process was first demonstrated in 1969 by researchers who confirmed that cholesterol-to-testosterone conversion occurs specifically in these interstitial cells.
The rate-limiting step, the part that controls how fast the whole process runs, is the initial movement of cholesterol into the mitochondria. A protein called StAR (steroidogenic acute regulatory protein) acts like a gatekeeper, accelerating cholesterol transport when LH signals are strong and slowing it when they’re not. Other regulatory proteins cap the maximum amount of testosterone any single cell can produce, preventing overproduction at the cellular level.
The Feedback Loop That Maintains Balance
Your body doesn’t just produce testosterone and hope for the best. It continuously monitors circulating levels and adjusts production accordingly through a negative feedback loop. When testosterone in the blood rises high enough, it signals back to the hypothalamus to reduce GnRH pulses. This in turn lowers LH release from the pituitary, which slows Leydig cell output.
The feedback isn’t entirely direct. Testosterone doesn’t act on the GnRH-releasing neurons themselves. Instead, it targets nearby neurons called kisspeptin neurons in the hypothalamus, which then dial down GnRH signaling. Some testosterone is also converted to estradiol, which feeds back on the pituitary gland to reduce LH secretion specifically. The result is a self-correcting system: when levels drop too low, the brain ramps up signaling, and when they climb too high, it pulls back.
The Adrenal Glands’ Small Contribution
The adrenal glands, located above each kidney, contribute roughly 1% of circulating testosterone in males. Their main androgen output is actually a weaker precursor called DHEA, which can be converted into testosterone in other tissues. While this contribution is negligible in men with normally functioning testes, it becomes more significant in females, where adrenal androgens account for 30 to 50% of total testosterone.
Testosterone Levels Fluctuate Throughout the Day
Testosterone production follows a circadian rhythm, peaking in the early morning and declining as the day goes on. The highest levels typically occur between 5 and 8 AM, while the lowest fall between 6 and 11 PM. In younger men, this daily swing is substantial: men under 30 show a drop of about 79 ng/dL from morning to afternoon. One study of 266 healthy men found average morning levels of 502 ng/dL compared to 404 ng/dL in the evening.
This pattern flattens with age. In men around 30 to 40, morning testosterone runs 30 to 35% higher than afternoon levels. By age 70, the difference shrinks to about 10%. This is why clinical guidelines recommend blood draws before 10 AM when testing testosterone, since afternoon samples can give a misleadingly low reading.
Normal Testosterone Ranges by Age
In healthy, nonobese men aged 19 to 39, total testosterone ranges from about 264 to 916 ng/dL, with a midpoint around 531 ng/dL. That range shifts with age. The median stays relatively stable through the decades, hovering near 477 ng/dL from the 40s through the 70s. But the lower end of normal drops: the 5th percentile falls from 304 ng/dL in the 19-to-39 group to 252 ng/dL in the 70s and 218 ng/dL after 80.
In practical terms, this means that while many older men maintain testosterone levels similar to younger men, the floor gradually lowers, and a greater percentage of men dip below the thresholds associated with symptoms like fatigue, reduced muscle mass, and low sex drive.
Nutrients the Body Needs to Produce Testosterone
Because the production process depends on enzymes and chemical reactions, certain micronutrients serve as essential building blocks. Zinc plays a direct role in regulating testosterone production within the Leydig cells. Vitamin D functions as both a nutrient and a prohormone, and low levels have been associated with lower testosterone. Magnesium supports cellular energy production and protein synthesis, both of which factor into steroidogenesis.
Cholesterol itself is the most fundamental raw material. Since every testosterone molecule is literally built from a cholesterol molecule, extremely low-fat diets have been studied for their potential effects on hormone levels. The body manufactures most of its own cholesterol, so dietary intake isn’t usually the bottleneck, but the availability of cholesterol inside Leydig cells directly determines how much testosterone can be made at any given moment.