Hamilton and later Norwood have classified the patterns of male pattern baldness (see illustration below). Although the density of hair in a given pattern of loss tends to diminish with age, there is no way to predict what pattern of hair loss a young man with early male pattern baldness will eventually assume. In general, those who begin losing hair in the second decade are those in whom the hair loss will be the most severe. In some men, initial male-pattern hair loss may be delayed until the late third to fourth decade. It is generally recognized that men in their 20’s have a 20 percent incidence of male pattern baldness, in their 30’s a 30 percent incidence of male pattern baldness, in their 40’s a 40 percent incidence of male pattern baldness, etc. Using these numbers one can see that a male in his 90’s has a 90 percent chance of having some degree of male pattern baldness.

Hamilton first noted that androgens (testosterone, dihydrotestosterone) are necessary for the development of male pattern baldness. The amount of androgens present does not need to be greater than normal for male pattern baldness to occur. If androgens are present in normal amounts and the gene for hair loss is present, male pattern hair loss will occur. Axillary (under arm) and pubic hair are dependent on testosterone for growth. Beard growth and male pattern hair loss are dependent on dihydrotestosterone (DHT). Testosterone is converted to DHT by the enzyme, 5¤ -reductase. Finasteride (Propecia®) acts by blocking this enzyme and decreasing the amount of DHT. Receptors exist on cells that bind androgens. These receptors have the greatest affinity for DHT followed by testosterone, estrogen, and progesterone. After binding to the receptor, DHT goes into the cell and interacts with the nucleus of the cell altering the production of protein by the DNA in the nucleus of the cell. Ultimately growth of the hair follicle ceases.

The hair growth cycle is affected in that the percentage of hairs in the growth phase (anagen) and the duration of the growth phase diminish resulting in shorter hairs. More hairs are in the resting state (telogen) and these hairs are much more subject to loss with the daily trauma of combing and washing. The hair shafts in male pattern baldness become progressively miniaturized, smaller in diameter and length, with time. In men with male pattern baldness all the hairs in an affected area may eventually (but not necessarily) become involved in the process and may with time cover the region with fine (vellus) hair. Pigment (color) production is also terminated with miniaturization so the fine hair becomes lighter in color. The lighter color, miniaturized hairs cause the area to first appear thin. Involved areas in men can completely lose all follicles over time. Male pattern baldness is an inherited condition and the gene can be inherited from either the mother or father’s side. There is a common myth that inheritance is only from the mother’s side. This is not true.

In summary, male pattern hair loss (Androgenetic Alopecia) is an inherited condition manifested when androgens are present in normal amounts. The gene can be inherited from the mother or father’s side. The onset, rate, and severity of hair loss are unpredictable. The severity increases with age and if the condition is present it will be progressive and relentless.

Hair loss in men is likely to occur primarily between late teen-age years and age 40-50, in a generally recognizable “male-pattern” baldness known as androgenetic alopecia. Men with male-pattern hair loss may have an expectation of hair loss if they have male relatives who lost hair in a recognizably male pattern.

Norwood-Hamilton Scale of Male Pattern Baldness

Using pairs of identical twins, the study assessed the efficacy of dutasteride, a particular enzyme inhibitor, as a means of reducing hair loss and encouraging new growth, finding it to be an effective means of combating baldness.

In the study, one of each pair of twins was given 0.5 mg/day of the medication, the other receiving a placebo, with the results monitored over the course of a year. After both 6 and 12-month intervals, hair loss was found to not only have been halted in twins using dutasteride, but hair growth was actively improved. The rate of growth increased between the 6 and 12-month evaluations, while hair loss continued steadily in the placebo group.

MPHL is thought to be genetically controlled. As identical twins share the same genetic code, the study provides particularly convincing evidence of the efficacy of dutasteride.

There are many causes of baldness, including serious illness, reaction to certain medication and stress, but by far the most common is Male Pattern Baldness (MPB) or Andogenic Alopecia.

MPB is caused by a combination of two factors – a genetic predisposition and Dihydrotestosterone (DHT).

DHT is a byproduct of testosterone, and is therefore present in every male. Scientists have shown that DHT shrinks hair follicles, and that if DHT is suppressed, the follicles will continue to thrive.

In 1949 a landmark study was performed that explained the causes of MPB. A group of castrated convicts, males who were therefore not producing testosterone, were divided into two groups. The first group were those who had a family history of MPB, and the second group were those with no family history of balding. Both groups were injected with testosterone; the group with a family history of MPB began losing their hair within weeks, while the group with no family history lost no hair, but did begin to develop a mature hairline. When the injections were stopped, the progression of thinning stopped.

Looking past the inhumane and surprising extremes of this experiment, it was discovered that MPB was an inherited trait that is triggered with the presence of testosterone. Since that experiment, we have learned that it is not testosterone but the testosterone byproduct DHT that causes hair loss.

Hair follicles that are sensitive to DHT must be exposed to the hormone for a prolonged period of time for the affected follicle to complete the miniaturization process. Today, with proper intervention, this process can be slowed or even stopped if caught early enough.

What MPB sufferers inherit are hair follicles from their mothers or fathers with a genetic sensitivity to DHT. Hair follicles sensitive to DHT begin to miniaturize, shortening the lifespan of each hair follicle affected. Eventually, these follicles stop producing cosmetically acceptable hair.

MPB is characterized by a receding hairline and thinning in the temple, crown or vertex area. These areas are the most sensitive to DHT. For some men, DHT even thins the hair on the back of the head, although this area is usually spared, leaving a permanent donor area that can be used for modern hair transplants.

There are still many factors about the inheritance of baldness that we don’t understand. I often see a male with a strong family history of MPB that says he has an older brother with little or no balding. As well, some young adult males bald rapidly up until the age of 35, whereupon the balding pattern stops. In contrast, males can also experience little or no thinning up until their early 40s, and then lose all or most of their hair in the next 20 years, balding almost completely by age 60. Unfortunately, we cannot predict the exact course each individual’s balding will take when he is in the early balding stages. But generally speaking, the earlier you start losing hair, the more hair you are going to lose.