Hair thinning and loss, particularly on the top of the head, is a common occurrence. This pattern results from complex biological interactions. Understanding these reasons can demystify a widespread concern.
The Primary Drivers: Genetics and Hormones
The primary cause of hair loss on the top of the head is androgenetic alopecia, often called male or female pattern baldness. This condition is largely influenced by an individual’s genetic makeup, determining susceptibility. Studies suggest genetics account for up to 80% of androgenetic alopecia cases. The inheritance pattern is polygenic, meaning multiple genes contribute to its development, and these can be inherited from either parent.
A key hormonal player in androgenetic alopecia is dihydrotestosterone, or DHT. DHT is an androgen, a type of sex hormone, synthesized from testosterone by an enzyme called 5-alpha reductase. While testosterone is produced in the testes in males and ovaries in females, approximately 10% of this testosterone is converted into DHT daily in various tissues, including hair follicles. DHT is a more potent androgen than testosterone, binding to androgen receptors with a significantly higher affinity.
How Hair Follicles Respond
When DHT binds to specific androgen receptors in genetically susceptible hair follicles, it initiates hair follicle miniaturization. This process causes hair follicles to shrink, producing progressively thinner, shorter, and finer hairs. Eventually, these miniaturized follicles may stop producing hair altogether, leading to visible thinning and bald spots.
DHT also disrupts the natural hair growth cycle, which consists of three main phases: anagen (growth), catagen (transition), and telogen (resting). In affected follicles, DHT shortens the anagen phase. Concurrently, it prolongs the telogen (resting) phase. This imbalance results in increased hair shedding and a reduced number of active hair follicles, further contributing to the appearance of hair loss.
Why the Top of the Head is Affected
The specific pattern of hair loss in androgenetic alopecia, primarily affecting the crown and frontal areas, stems from a differential sensitivity of hair follicles across the scalp. Hair follicles located on the top, front, and crown (vertex) of the head are genetically predisposed to be more sensitive to DHT. This heightened sensitivity means these follicles are more likely to undergo miniaturization and disruption of their growth cycle when exposed to DHT.
In contrast, hair follicles on the sides and back of the head possess a lower sensitivity to DHT. This difference explains why these areas retain hair, even in advanced stages of baldness, forming the “horseshoe” pattern. The localized effect of DHT on these vulnerable follicles results in the characteristic receding hairline and thinning at the crown that defines androgenetic alopecia.