DHT
DHT, or dihydrotestosterone, is the androgen that drives male pattern hair loss. An enzyme converts testosterone into it, and susceptible scalp follicles shrink in response.
Where you'll meet it
In every explanation of why finasteride works, and in the marketing for a great many things that do not.
What it does
The enzyme 5-alpha reductase converts testosterone into DHT. DHT binds to receptors in follicles that are genetically susceptible, and over repeated hair cycles those follicles miniaturise.
Crucially, susceptibility is local rather than systemic. Follicles at the crown and hairline respond; follicles in the donor area largely do not, and they keep that property when they are moved. That single fact is why transplants work.
What blocks it
Finasteride blocks one form of the enzyme and lowers scalp DHT substantially. Dutasteride blocks more forms and lowers it further. Both are prescription drugs and both need a proper conversation about side effects.
Nothing over the counter blocks DHT to a degree that has been shown to hold hair. Saw palmetto is the usual claim and the evidence is much weaker.
A common misunderstanding
Having more DHT is not the problem, and blood testosterone levels say almost nothing about whether you will go bald. Sensitivity of the follicle is what varies, and it is inherited from both sides of the family, not just the mother's father.
Related pages
Thinking about a transplant?
The Start Here guide explains why medical treatment comes first, how much donor hair you actually have, and what a transplant can and cannot move.
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