testosterone
Testosterone is the main hormone that makes a male body develop and behave in a masculine way. Think of it as the master signal that, starting before birth and surging again at puberty, tells the body to grow a beard, deepen the voice, build muscle, and produce sperm. It is a steroid hormone, which means it is built from cholesterol and is fat-soluble, so it slips easily through cell membranes to act inside the cell.
Most of a man's testosterone is made by the Leydig cells of the testes, with a small fraction coming from the adrenal glands. Its production is controlled by the brain: the hypothalamus releases GnRH, which prompts the pituitary to release luteinizing hormone (LH), which in turn drives the Leydig cells. Testosterone then feeds back to the brain to keep its own level in check. In the blood most testosterone travels bound to carrier proteins such as sex hormone-binding globulin; only the small unbound (free) fraction is biologically active.
Inside many tissues testosterone is converted to more specialized hormones before it works: the enzyme 5-alpha-reductase turns it into the stronger androgen DHT (important for the prostate, skin, and external genitalia), while aromatase turns it into estradiol (important for bone, brain, and fat). A common misconception is that testosterone alone explains everything male; in reality many of its classic effects depend on these downstream conversions, and testosterone is also present and necessary, in much smaller amounts, in women.
A 30-year-old man with low libido and fatigue has a morning total testosterone of 180 ng/dL (reference roughly 300–1000), confirming hypogonadism and prompting a search for whether the problem lies in the testes or the pituitary.
Morning sampling matters because testosterone follows a daily rhythm, peaking in the early hours.
When clinicians measure testosterone they often report both total and free (or bioavailable) levels, because changes in binding proteins can make a total level look misleadingly high or low.