The Androgen Paradox
Same hormone grows your beard and kills your hairline. DHT plays both sides.
Grounded in the research on Dihydrotestosterone (Wikipedia)
the paradox
The androgen paradox is one hormone doing two opposite things to your hair. Dihydrotestosterone (DHT), the most potent androgen in your body, thickens your beard, grows chest and pubic hair, and deepens your voice. The exact same molecule shrinks the follicles on your scalp until they stop producing visible hair. Masculinizing the face, demasculinizing the crown. Same signal, opposite outcome, same person.
the mechanism
Testosterone gets converted into DHT by the enzyme 5-alpha-reductase. DHT is roughly 3 to 5 times more potent than testosterone at the androgen receptor because it binds tighter and dissociates slower. The twist is tissue sensitivity. Beard and body follicles read DHT as a grow signal. Genetically susceptible scalp follicles read the same DHT as a shrink signal, miniaturizing over successive growth cycles until the hair is vellus fuzz. The hormone is identical. The receptor wiring of the follicle is not.