Male pattern hair loss, or androgenetic alopecia, is the most common type of hair loss among males.

Hormonal factors appear to play a role, and especially a male sex hormone known as dihydrotestosterone (DHT).

Hair loss affects around half of all men over the age of 50 years, and around 50 million men in the United States (U.S.).

DHT has also been linked to hair loss in women, but this article will focus on male pattern baldness.

Fast facts on dihydrotestosterone

Here are some key points about dihydrotestosterone (DHT). More detail is in the main article.

  • DHT is an androgen and helps give males their male characteristics.
  • DHT is thought to cause hair follicles to miniaturize, and this contributes to male pattern hair loss.
  • By the age of 50 years, over half of the men in the U.S. will probably experience hair loss mediated by DHT.
  • Treatments that block DHT may help prevent hair loss.

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Male pattern baldness affects around half of men aged over 50 years in the U.S.

DHT has many roles. Apart from hair production, it is linked to benign prostatic hyperplasia, or enlarged prostate, and prostate cancer too.

DHT is a sex steroid, meaning it is produced in the gonads. It is also an androgen hormone.

Androgens are responsible for the biological characteristics of males, including a deeper voice, body hair, and increased muscle mass. During fetal development, DHT plays a vital role in the development of the penis and prostate gland.

In men, the enzyme 5-alpha-reductase (5-AR) converts testosterone into DHT in the testes and the prostate. Up to 10 percent of testosterone is normally converted into DHT.

DHT is more powerful than testosterone. It attaches to the same sites as testosterone, but more easily. Once there, it remains bound for longer.

Male pattern hair loss is the most common type of hair loss in men. Hair at the temples and on the crown slowly thin and eventually disappear.

The exact reason why this happens is unknown, but genetic, hormonal, and environmental factors are all thought to play a role. DHT is believed to be a major factor.

Three phases of hair growth

To understand male pattern hair loss, we need to understand hair growth.

Hair growth is split into three phases: anagen, catagen, and telogen:

Anagen is the growth phase. Hairs remain in this phase for 2 to 6 years. The longer it lasts, the longer the hair grows. Normally, around 80 to 85 percent percent of the hairs on the head are in this phase.

Catagen lasts only 2 weeks. It allows the hair follicle to renew itself.

Telogen is the resting phase. The follicle lies dormant for 1 to 4 months. Normally between 12 and 20 percent of hairs are in this phase.

After this, anagen begins again. The existing hair is pushed out of the pore by the new growth and naturally sheds.

Hair loss

Male pattern hair loss happens when the follicles slowly become miniaturized, the anagen phase is reduced, and the telogen phase becomes longer.

The shortened growing phase means the hair cannot grow as long as before.

Over time, the anagen phase becomes so short that the new hairs do not even peek through the surface of the skin. Telogen hair growth is less well-anchored to the scalp, making it easier to fall out.

As the follicles become smaller, the shaft of the hair becomes thinner with each cycle of growth. Eventually, hairs are reduced to vellus hairs, the type of soft, light hairs that cover an infant and mostly disappear during puberty in response to androgens.

Users of anabolic steroid drugs, including body builders, have higher levels of DHT. However, they often experience hair loss.

The hair on the head grows without the presence of DHT, but armpit hair, pubic hair, and beard hair cannot grow without androgens.

Individuals who have been castrated or who have 5-AR deficiency do not experience male pattern baldness, but they will also have very little hair elsewhere on the body.

For reasons that are not well understood, DHT is essential for most hair growth, but it is detrimental to head hair growth.

DHT is thought to attach to androgen receptors on hair follicles. Through an unknown mechanism, it then appears to trigger the receptors to begin miniaturizing.

In 1998, researchers found that both plucked follicles and skin from a balding scalp contain higher levels of androgen receptors than those from a non-balding scalp.

Some scientists believe that some people have a genetically transmitted susceptibility to otherwise normal levels of circulating androgens, particularly DHT. This combination of hormonal and genetic factors could explain why some people are more likely than others to lose their hair.

Why does DHT affect people in different ways?

DHT affects people in varying ways. This may be due to:

  • an increase in DHT receptors at the follicle
  • a greater local DHT production
  • higher androgen receptor sensitivity
  • more DHT produced elsewhere in the body and arriving through circulation
  • more circulating testosterone that acts as a precursor for DHT

It is known that DHT binds to follicle receptors five times more avidly than testosterone, but the amount of DHT in the scalp is tiny compared with the levels in the prostate.

How levels are controlled and why they change are not yet understood.

5-alpha-reductase (5-AR) is the enzyme that converts testosterone into the much more potent androgen, DHT.

If 5-AR levels increase, more testosterone will be converted into DHT, and greater hair loss will result.

There are two versions of 5-AR: type 1 and 2 enzymes.

  • Type 1 is predominantly found in sebaceous glands that produce the skin's natural lubricant, sebum.
  • Type 2 mostly sits within the genitourinary tract and hair follicles.

Type 2 is considered more important in the process of hair loss.

Male pattern hair loss can have a negative effect on the self-esteem of a man. To help relieve this, some treatments have already been developed.

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Work is in progress to develop a full and effective treatment for male pattern hair loss.

Finasteride, or Propecia, was approved for safety and efficacy in 1997, by the U.S. Food and Drug Administration (FDA).

It is a selective inhibitor of type 2 5-AR. It is thought to act on the 5-AR enzyme that concentrates in the hair follicles to inhibit production of DHT.

Studies of its efficacy have yielded apparently impressive results, but some people have questioned how effective it is.

Research has shown that it can stop baldness from progressing, and that, in some cases, hair will start appearing again. However, the number of hairs that were successfully grown in a square inch of the scalp over 5 years was 227, while the average number of hairs in a square inch is roughly 2,200.

Finasteride can be taken orally, at a dose of 1 milligram (mg) every day. Injections are also possible. If treatment stops, hair loss will continue.

Adverse effects include a loss of libido, a reduced ability to develop and maintain an erection, and a decrease in ejaculate.

Another theory proposed to explain male pattern hair loss is that, with age, the follicles themselves come under increasing pressure from the scalp.

In younger people, the follicles are buffered by the surrounding fat tissue under the skin. Youthful skin is also better at staying hydrated. As the skin becomes dehydrated, the scalp compresses the follicles, causing them to become smaller.

Testosterone also contributes to a reduction in the fat tissue, so higher levels of testosterone may further reduce the scalp's ability to buffer the hair follicles.

As follicles try to maintain their status, suggest some scientists, additional enzyme activity occurs in the site. More testosterone is converted to DHT, leads to further erosion and more hair loss.

Further investigation of DHT and male pattern hair loss may one day enable scientists to finally crack the code of male pattern baldness. For now, it is a waiting game.