Testosterone is often a prohormone
Here is a surprise that trips up many learners: in several tissues, testosterone is not the final actor. It is converted into a more powerful or even a different hormone before it does its job. Think of testosterone less as a finished product and more as a versatile raw material. Two enzymes do the converting, and which one is present decides what testosterone *becomes* in that tissue.
5-alpha-reductase and aromatase
The first enzyme, 5-alpha-reductase, turns testosterone into dihydrotestosterone (DHT) — a *stronger* androgen that binds the receptor more tightly. DHT is the key player in the prostate, in male-pattern scalp hair, and in the genital skin during development. Drugs that block 5-alpha-reductase are used for an enlarged prostate and for hair loss, precisely because they lower DHT while leaving most testosterone intact.
The second enzyme, aromatase, does something more startling: it converts testosterone into estradiol — an estrogen. Yes, men make and need estrogen, just in smaller amounts. Estradiol from aromatase is essential for closing the growth plates in bone, for bone density, and for parts of the feedback to the brain. Aromatase lives in fat tissue especially, which is why body fat strongly shapes a man's estrogen levels — a fact that becomes central to the gynecomastia guide.
The receptor and the carrier
Both testosterone and DHT act through the same androgen receptor — an intracellular receptor that, once bound, moves into the nucleus and switches genes on and off. DHT simply grips this receptor harder, which is why it acts as the amplified version. If the androgen receptor is broken, the body can be drenched in testosterone and still not respond to it — a vivid reminder that a hormone is useless without a working receptor.
One more piece: in the blood, most testosterone is not floating free. It rides bound to sex hormone-binding globulin (SHBG). Only the small free fraction can actually enter cells and act. So a man's total testosterone can read normal while his *free* testosterone is low if SHBG is high — which is why good clinicians look at both numbers, not just the total.