There are many causes of hair loss in men and women, including disease, nutritional deficiency, hormone imbalance, and stress. However, by far the most common cause is what is called androgenic alopecia. Alopecia is simply the medical term for hair loss. Androgenetic refers to the fact that both a genetic predisposition to balding, and the influence of androgens, or male hormones, play a part in this type of hair loss.

In fact, there is a third factor, which is the passage of time, or aging. In other words, in order for androgenetic alopecia to occur, there must be:
1) a genetic propensity for balding
2) the presence of androgens, or male hormones
3) enough aging time to allow the first two factors to exert their influence on the hair follicles

Genetics

Genetics is not always simple, and such is the case with balding. Just the presence or absence of balding in one's parents or grandparents, on either the mother's or father's side, is not necessarily predictive of one's likelihood of balding. Certainly, if a man's father is completely bald, and this man begins to rapidly lose hair in his early twenties, it's a safe bet that he will develop extensive balding at some point. In short, it's very hard to accurately predict who will go bald and how rapidly.

This inherent uncertainly about the progression of balding is of utmost importance in planning surgical hair restoration, as we will see in later sections. We must always plan for a "worst case scenario" in order to give patients the best possible results in the long term, as well as in the short term. Anything less is irresponsible.

Androgenic Hormones

All normal men and women produce "male" hormones. The most common of these are testosterone, androsteinedione, and dihydrotestosterone (DHT). Androgens are produced by the testicles and adrenals in men, and by the ovaries and adrenal glands in women. These hormones are quite important in both sexes, but occur in different concentrations, being much more predominant in males than in females. This, in part, is responsible for the typical differences between the genders. It is the exposure of the hair follicles to DHT, in a genetically susceptible person, over a period of time, which leads to androgenetic alopecia, or male and female pattern baldness. How does this exposure to DHT occur?

In certain cells of the hair follicle, and in the sebaceous glands, there are high levels of an enzyme called 5-alpha-reductase. What this enzyme does is to convert testosterone, which is delivered to these areas by the blood, into DHT. This is important not only in understanding the mechanisms of balding, but also one medical treatment now available: Propecia (finasteride). What Propecia does is inhibit, or limit the activity of, this 5-alpha-reductase enzyme. Therefore, there is less conversion of testosterone to DHT, and lower levels of DHT are found in the follicle. In later sections, we will discuss this and other medical treatments in much greater detail.

Aging

There is no set age at which balding occurs. It is a process, and this is a simple, but oft-ignored fact. Like any process, it can be rapid or slow, it can begin toward the end of life or in the late teens, and it can progress in a predictably inexorable fashion, or it can stop and start, seemingly stabilize, and then begin again. Once we understand and accept this as a dynamic process, then we can better plan for the present and for the future in terms of how we treat it. This quest for understanding, which you have begun just by opening this book, will do more than all the despairing thoughts, hand- wringing, and self-pity, toward allowing a clear-eyed, rational, long term approach to the problem of hair loss.

So we now have looked at these three interdependent factors that play into the common types of balding. Again, they are: hormones, genetics, and Father Time. So what exactly does happen to the hair? Let's take a look.

Assuming we have a genetically predisposed person, then as the follicles are continuously exposed to DHT, an interesting phenomenon occurs. Remember the anagen phase, or active growth phase of the hair? This phase becomes gradually briefer and briefer, and eventually the hair becomes finer and shorter, and less deeply colored. We call this "miniaturization" of hairs. This is also the point at which hair loss tends to first be noticed. It's not that there are fewer hairs on the head, but that their caliber (cross-sectional area), color and length are so diminished that they no longer provide "coverage" for the scalp beneath. Light penetrates through to the shiny scalp, and this is perceived by the observer as "thinning" or balding.

Also, the ratio between hairs in the anagen phase and those in the telogen, or resting phase, is increased. This simply means that, at any given time, an increased number of hairs are in the telogen phase. These extra numbers of telogen hairs will be found in the susceptible zone for common balding, which is the front, top, and crown of the head. The so-called "permanent" zone, the familiar horseshoe-shaped wreath of hair around the back and sides, is unaffected by these changes. The telogen hairs are easily dislodged during washing, drying, or combing, and this is the second sign of balding: in addition to the apparent thinning seen with miniaturization, we begin to see larger numbers of hairs on the comb, the towel, the pillowcase, or in the bathroom drain. This can be quite traumatic, especially for the younger man or for women. In the next section, we will discuss the natural history of balding, that is, the way it first presents or appears, the different ways it progresses, and how it affects the different regions of the head.

For the sake of completeness, let's briefly mention some of the other patterns of hair loss, if only to distinguish them from androgenetic alopecia (male and female pattern baldness). There is alopecia areata, where discrete patches of scalp go bald; triangular alopecia, which tend to occur in a triangular pattern in the temporal area; alopecia universalis, in which the entire body may be affected; and various "toxic" alopecias, including those following a severe illness, sometimes with high fever, or following pregnancy. Toxic alopecias may also occur with low thyroid and/or pituitary gland function, or following chemotherapy. The cicatricial (scarring) alopecias occur following tissue destruction and inflammation.

Also seen are the so-called diffuse alopecias (patterned and unpatterned), in which there is widespread thinning that may affect the "permanent" zone as well as the areas vulnerable to balding. In any or all of these less common types of balding above, it may be necessary to have a complete physical and laboratory workup, possibly including scalp biopsy. So again, the common types of balding are directly related to the presence of male hormones in a genetically predisposed person over time. This can occur in both men and women. The process involves progressive miniaturization of the terminal hairs, and diminished length of the active hair growth cycle. Now, let's take a look at how this microscopic, cellular process is manifested on the head; we can call this the natural history of balding.