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Causes of Hair Loss       |      Assessing Hair Loss       |      Hair Loss in Women

Causes of Hair Loss

Throughout history man has searched for the cause of hair loss. It is only in recent years, with greater knowledge of genetics and the chemistry of sexual hormones, that we have begun to understand the causes. One thing that we do know for certain: no matter what we eat, what our lifestyles may be, or what kind of vitamins we take, we never grow more hair follicles than we were born with.

The most common type of baldness is called Male Pattern Baldness or more scientifically, Androgenetic Alopecia. In Androgenetic Alopecia, hair follicles that are producing healthy, terminal hairs begin to produce thinner, shorter, more brittle hairs with weaker shafts (this process is called Miniaturization and the hairs involved are called Miniaturized hairs). Eventually, these follicles produce only fine, almost invisible, short, vellus hairs, or they may die out altogether.

Androgenetic hair loss is caused by the effects of male hormones on genetically susceptible hair follicles. It is related to three interdependent factors:

Common baldness cannot occur without the presence of specific inherited genes. These genes can be passed on by either parent. A gene is a single bit of chemically encoded hereditary instruction that is located on a chromosome and actually represents a tiny segment of DNA. Chromosomes occur in pairs (humans have 23 pairs), and every individual gets one set of chromosomes from each parent. The genetics of androgenetic alopecia is complicated and hair loss is now felt to involve more than one gene. When several genes govern a trait, it is called polygenic.

Genes that are located on the X or Y-chromosomes are call sex-linked. Genes on the other 22 pairs of chromosomes are called autosomal. It is felt that the genes governing common baldness are autosomal. This means that the baldness trait can be inherited from the mother's side of the family or the father's side. The commonly held notion that baldness comes only from the mother's side of the family is incorrect, although for reasons not fully understood, the predisposition inherited from an affected mother is of slightly greater importance than that inherited from an affected father.

The term, "dominant" means that only one gene of a pair is needed for the trait to show up in the individual. A "recessive" gene means that both genes need to be present in order for the trait to be expressed. The genes involved in androgenetic alopecia are felt to be dominant.

Just because a person has the genes for baldness, doesn't mean the trait will manifest itself. The ability of a gene to affect one's characteristics, i.e. be visible in a particular individual, is called "expresivity". Gene expression is related to a number of factors, the major ones being hormones and age, although stress and other factors can play a role. To put it simply, a man whose father and uncles are severely bald may have minimal hair loss himself, either because the baldness genes have not been passed on or because the expression of the baldness gene that he did inherit is limited.

It is of interest that, to date, none of the genes for male pattern baldness have been identified. This suggests that any kind of genetic engineering to prevent common baldness is still many years away.

Hormones are biochemical substances that are made in various glands throughout the body. These glands secrete their products directly into the bloodstream so that the chemical they make is spread throughout the body. These chemicals are very powerful; only minute amounts of them have profound effects upon the body.

The major male sex hormone is called testosterone. Testosterone and other related hormones that have masculinizing effects are made primarily in the testicles, therefore, the hormonal levels that are seen in adults do not reached significant levels until the testicles develop and enlarge during puberty. In fact, these same hormones are the cause of many of the changes that occur in puberty; change in the voice, growth of the beard and mustache, development of an adult aroma in the sweat, change in the muscular development, and change in the basic body shape.

These same hormones that cause acne and beard growth can also signal the beginning of baldness. The presence of androgens; testosterone, and its related hormone DHT, cause some follicles to regress and die. In women, hormones with androgenic activity cause hair loss as well. In addition to the testicles, the adrenal glands located above each of our kidneys, produce androgenic hormones, and this would be similar in both sexes. In females, the ovaries are an additional source of hormones that can affect hair.

The specific relationship between testosterone and hormonally induced hair loss was discovered by a psychiatrist early in this century. At that time, castration was commonly performed on patients with certain types of mental illness. The rationale behind this procedure was that it was believed to be a treatment for mental illness at a time when there were no other forms of real treatment. Castration seemed to have a calming effect upon many patients and castration reduced the sex drive of patients who had no outlet for their desires. The doctor noted that the identical twin brother of one patient was profoundly bald while the mentally ill twin had a full head of hair. The doctor decided to determine the effect of treating his patient with testosterone, which had recently become available in a purified form. He injected his patient, the hairy twin, with testosterone to see what would happen. Within weeks, the hairy twin began to lose all but his wreath of permanent hair, just like his normal twin. The doctor, then, stopped giving the testosterone to see whether the process would be reversed, but the balding process continued and his patient never regained his full head of hair. It was apparent to him that eliminating testosterone will slow, or stop, further hair loss once it has begun, but it will not revive any dead follicles.

The hormone felt to be directly involved in androgenetic alopecia is actually dihydrotestosterone (DHT) rather than testosterone. DHT is formed by the action of the enzyme 5-a reductase on testosterone. DHT acts by binding to special receptor sites on the cells of the hair follicles to cause the specific changes associated with balding. Among other effects, DHT decreases the length of the anagen (growing) cycle, and increases the telogen (resting) phase, so that with each new cycle the hair shaft becomes progressively smaller.

It is interesting that testosterone effects axillary and pubic hair, whereas DHT effects beard growth, patterned baldness and the appearance of hair in the nose and ears (something that older men experience). Scalp hair growth, however, is not androgen dependent, only scalp hair loss depends on androgens.

In men, 5-a reductase activity is higher in the balding area. This is the enzyme that is inhibited by the hair loss medication finasteride (click on Propecia for more information). Women have ½ the amount of 5-a reductase overall as compared to men, but have higher levels of the enzyme aromatase, especially in their frontal hairline. Aromatase decreases the formation of DHT, and its presence in women may help to explain why the presentation of hair loss in women is different than in males.

One should keep in mind that the presence of the necessary genes and hormones are not, in themselves, sufficient to cause baldness. The reaction also requires time of exposure of susceptible hair follicles to the hormone for hair loss to begin. The time required for this to start varies from one individual to another and is related to a person's genetic expression and to the levels of testosterone in his bloodstream.

There is another time factor that is poorly understood. Hair loss does not occur all at once nor in a steady, straight-line progression. Hair loss is cyclical. People who are losing their hair experience alternating periods of slow and rapid hair loss and even stability. Many of the factors that cause the rate of loss to speed up or slow down are unknown.


The presence of the necessary genes and hormones are not alone sufficient to cause baldness. Even after a person has reached puberty, susceptible hair follicles must continually be exposed to the hormone over a period of time for hair loss to occur. The age at which these effects finally manifest themselves varies from one individual to another and is related to a person's genetic composition and to the levels of testosterone in the bloodstream.

Hair loss does not occur all at once, nor in a steady, straight-line progression. People who are losing their hair experience alternating periods of slow and rapid hair loss and even stability. Many of the factors that cause the rate of loss to speed up or slow down are unknown, but we do know that with age, a person's total hair volume will decrease.

Even when there is no predisposition to genetic balding, as a patient ages, some hairs randomly begin to miniaturize (shrink in length and width) in each follicular unit. As a result, each group will contain both of full terminal hairs and miniaturized hairs (similar to the very fine hairs that occur on the rest of the body and are clinically insignificant) making the area look less full. Eventually, the miniaturized hairs are lost, and the actual follicular units are reduced in number. In all adult patients, the entire scalp undergoes this aging process so that even the donor zone is not truly permanent, but will gradually thin, to some degree, over time. Fortunately, in most people, the donor zone retains enough permanent hair that hair transplantation is a viable procedure even for a patient well into his 70's.