The short answer: dihydrotestosterone, or DHT, is a potent hormone derived from testosterone. Your body needs it, but in genetically sensitive follicles it drives the shrinking that causes male pattern hair loss. Understanding it explains why the main tablet treatment works the way it does.
What DHT is
DHT is an androgen, a male sex hormone. It is made when an enzyme called 5-alpha-reductase converts testosterone into DHT, which binds to hormone receptors more strongly than testosterone itself. So although there is less of it circulating than testosterone, it is more potent where it acts. Only a small proportion of your testosterone, roughly 5 to 10 per cent, is converted to DHT, but that fraction does a lot of work, and it is why lowering the conversion step rather than testosterone itself is the target for hair loss treatment.
Male pattern hair loss, the condition DHT drives, is extremely common: it affects around half of men to some degree by their fifties and is the single most frequent cause of hair loss in men. The NHS overview of hair loss describes it as a normal part of ageing for many men rather than a disease, which is worth remembering: the aim of treatment is to slow a natural, hormone-driven process in people who want to, not to fix something that is medically wrong.
There are two main forms of the 5-alpha-reductase enzyme, type I and type II, and they are found in different tissues. Type II predominates in hair follicles and the prostate, which is why a medicine that targets it can influence scalp hair. This detail is not trivia: it is precisely why finasteride, which mainly inhibits type II, has the effect on hair that it does.
What it does in the body
DHT is not a villain by design. Before birth and during puberty it drives the development of male sexual characteristics. In adulthood it continues to act on the prostate and on body and facial hair. This is worth stressing, because it explains why medicines that lower DHT are prescription-only and reviewed carefully: DHT does useful work, and you are adjusting a hormone with several roles.
The paradox of hair
Here is the twist that surprises people: DHT promotes hair growth on the face and body, yet drives hair loss on the scalp. The difference is in the follicles, not the hormone. Scalp follicles at the hairline and crown can be genetically sensitive to DHT, and in those follicles it triggers miniaturisation, the gradual shrinking that produces finer, weaker hairs over successive cycles. With each growth cycle the affected follicle produces a slightly shorter, thinner hair, until eventually it makes only fine, colourless vellus hair or stops producing a visible hair at all. This is why pattern loss is gradual and follows a predictable map, thinning at the temples and crown while the back and sides, whose follicles are not DHT-sensitive, usually hold firm.
This is why pattern loss is really about sensitivity rather than sheer quantity. Two men with similar DHT levels can have very different hair, because their follicles respond differently. The inherited part of male pattern baldness is largely this follicle sensitivity.
Why blocking DHT helps
Finasteride works by blocking the 5-alpha-reductase enzyme, so less testosterone is converted into DHT and scalp levels fall substantially. With less DHT, miniaturisation slows and some recently shrunken follicles can recover. We go into the detail in how finasteride works.
DHT and the prostate
Because DHT acts on the prostate, medicines that lower it also affect it, and related medicines are used for prostate enlargement. For hair loss the dose is much lower, but this connection is one reason a prescriber takes a full history: the same hormone touches more than your hairline.
This shared biology also explains a practical point that catches some men out at the pharmacy counter. Because DHT-lowering medicines affect the prostate, they can lower the PSA blood test used in prostate checks, so it matters that any doctor arranging that test knows you take one. The finasteride product information sets this out, alongside the reasons the medicine is not for women who are or may become pregnant, since lowering DHT can affect the development of a male foetus. None of this makes DHT dangerous to have; it simply reflects that you are adjusting a hormone with jobs beyond the scalp.
Can you lower DHT naturally?
People often ask whether diet or supplements can lower DHT enough to help hair. The honest answer is that nothing you eat comes close to the effect of a medicine that blocks the converting enzyme. Some compounds, such as saw palmetto, are marketed on a claimed DHT-lowering effect, but the evidence is weak and the effect small, as we discuss in natural remedies for hair loss. General good health supports your hair, but it does not meaningfully shift scalp DHT.
The honest limits
Lowering DHT slows an ongoing process rather than switching off the genetic tendency. It works while treatment continues and fades if it stops, and it does not undo loss from years ago. Knowing what DHT is makes those trade-offs easier to understand rather than mysterious. It also explains why treatment is a long-term commitment: the follicles remain genetically sensitive, so the moment DHT rises again, the same pressure returns.
