Androgenetic Alopecia (AGA), commonly known as pattern baldness, is the most frequent cause of hair loss, affecting a majority of men and a significant percentage of women. The condition is characterized by a genetically determined, progressive thinning of the hair on the scalp. The striking difference in presentation—men often develop severe bald spots while women typically experience diffuse thinning—stems from a complex interplay of sex hormones, enzyme activity, and genetic sensitivity. This biological interaction dictates why men frequently lose hair on the crown and temples, while women generally maintain their frontal hairline.
The Hormonal Mechanism: How DHT Attacks Hair Follicles
The primary biological driver of pattern hair loss involves a potent male sex hormone called Dihydrotestosterone (DHT). DHT is synthesized from the more abundant androgen, Testosterone, through the action of the enzyme 5-alpha reductase (5αR). This enzyme is present in the oil glands and hair follicles of the scalp. DHT is estimated to be several times more potent than Testosterone at binding to the hair follicle’s androgen receptors.
In individuals genetically predisposed to AGA, the hair follicles on certain areas of the scalp possess a heightened sensitivity to DHT. When DHT binds to these receptors, it triggers a process known as follicular miniaturization. This process progressively shrinks the hair follicle, causing it to produce shorter, finer, and lighter-colored hair strands with each successive growth cycle. The vigorous growth phase (anagen phase) is significantly shortened, while the resting phase is prolonged.
Eventually, the affected hair follicles shrink to the point where they can no longer produce visible, terminal hairs, resulting in a bald or nearly bald area. The concentration of the 5-alpha reductase enzyme and the number of androgen receptors are highest in the areas typically affected by male pattern baldness. This localized concentration explains the aggressive nature of hair loss in genetically susceptible men.
Distinct Patterns of Male and Female Hair Loss
The visible manifestation of AGA differs significantly between the sexes due to the concentration and distribution of hormone receptors across the scalp. Male pattern hair loss is characterized by a predictable progression often measured using the Hamilton-Norwood scale. This pattern typically begins with a receding hairline at the temples and thinning at the crown, forming a characteristic “M” shape. These two areas of loss eventually merge, often leaving only a horseshoe-shaped band of hair around the back and sides of the head.
Female pattern hair loss presents as a more diffuse thinning over the top and crown of the scalp, which is assessed using the Ludwig scale. Women rarely experience the complete baldness or receding frontal hairline seen in men. Instead, the hair part line gradually widens, and the overall volume decreases across the central scalp. This difference is largely attributed to the varying density of androgen receptors across the female scalp compared to the male scalp.
The female pattern preserves the frontal hairline, which is a key distinguishing feature from male pattern baldness. This preservation suggests that the hair follicles at the front of the female scalp are far less sensitive to the effects of DHT compared to the follicles on the male frontal scalp. The thinning tends to be uniform and less concentrated, which is why women experience reduced hair density rather than sharply defined bald areas.
The Role of Genetics and Hormonal Protection in Women
Genetic predisposition is a foundational element for pattern hair loss in both men and women, with the condition being polygenic, meaning multiple genes contribute to susceptibility. A significant factor is the Androgen Receptor (AR) gene, located on the X chromosome, which is inherited from the mother. Variations in this gene influence the sensitivity of hair follicles to androgens like DHT.
Women typically experience a milder form of AGA because their hormonal environment offers a degree of protection. Women naturally have significantly lower circulating levels of Testosterone and, consequently, less DHT than men. More importantly, women have much higher levels of Estrogen, a hormone that appears to counteract the effects of androgens on the hair follicle. Estrogen may inhibit the activity of the 5-alpha reductase enzyme, reducing the amount of DHT produced locally in the scalp.
This protective hormonal balance begins to shift after menopause, which coincides with a dramatic drop in Estrogen levels. The loss of Estrogen’s counteracting effect leaves the hair follicles more vulnerable to the existing levels of androgens, which is why female pattern hair loss often becomes more noticeable in postmenopausal years. The combination of polygenic inheritance and the powerful counter-regulatory effect of Estrogen explains why women tend to experience a less aggressive, diffuse form of hair loss compared to men.