How Hair Loss Works

Androgenetic Alopecia (AGA), commonly referred to as male or female pattern baldness, is the most common form of hair loss. There are several underlying mechanisms of the hair loss cycle of AGA, with the biggest contributors being genetics and hormones. This condition impacts the hair on the top of the scalp, and usually presents as either receding or thinning on the front or crown.

Norwood scale showing the progression or stages of the hair loss cycle

Introduction to Hair Loss Cycle

Androgenetic Alopecia (AGA) is a common condition that is characterized by the gradual loss of hair on the top of the head. It typically follows a patterned distribution in men, with either receding or thinning hair on the front, temples, or crown. In women, it will often appear as diffuse hair thinning. While AGA is primarily a cosmetic concern, it can have significant impact on an individual’s self-esteem. To properly treat hair loss, it’s crucial to understand the molecular mechanisms behind it and how these can be overcome.

Genetic Predisposition

  • Identification of Genetic Markers: Numerous studies have identified specific genetic markers associated with AGA. A study published in the journal Nature Communications in 2017 identified over 200 genetic loci linked to male pattern baldness. These findings highlight the hereditary nature of AGA and emphasize the importance of genetic predisposition in its development.
  • Role of Androgen Receptor Gene: The androgen receptor gene is located on the X chromosome and plays a crucial role in AGA. A study published in the journal JAMA Dermatology in 2013 showed that variations in this gene can influence susceptibility to AGA, which impacts the hair growth cycle.

Hormonal Influences

  • The Role of Dihydrotestosterone (DHT): Dihydrotestosterone (DHT) is a potent androgen derived from testosterone and is a key player in AGA. DHT binds to androgen receptors in hair follicles, which leads to a process known as miniaturization within the hair growth cycle. A study published in the journal Dermatology in 2020 investigated the correlation between DHT levels and AGA severity, providing insights into the hormonal aspects of the condition.
  • Sensitivity of Hair Follicles: DHT sensitivity is not equal for all hair follicles. Some are genetically predisposed to be more sensitive, leading to miniaturization within the hair growth cycle. Research published in the Journal of Clinical Investigation in 2019 identified specific androgen receptor genes responsible for regulating hair growth. This helps to understand the molecular mechanisms underlying AGA within the hair growth cycle.

Follicular Miniaturization

  • Progressive Hair Thinning: One of the hallmark features of AGA is the gradual thinning of hair shafts within the hair growth cycle. Over time the affected hair follicles produce thinner, shorter hair. This is done through a process known as miniaturization, which disrupts the hair growth cycle.
  • Shrinking of Hair Follicles: Miniaturization within the hair growth cycle is accompanied by the shrinking of hair follicles. As hair follicles become smaller, they produce increasingly fine and weak hair. This eventually leads to the cessation of hair growth altogether.
  • Impact on Hair Growth Cycle: Miniaturized hair follicles have a shortened anagen (growth) phase and an extended telogen (resting) phase within the cycle. This imbalance in the hair growth cycle results in a progressive reduction in hair density and coverage.

Hair Growth Cycle

Understanding the hair growth cycle is crucial to comprehending hair loss in men and women. The cycle consists of three phases, broken out below:

  • Anagen Phase: This is the active growth phase when hair cells divide rapidly, and hair grows longer. The anagen phase can last for several years and is influenced by genetics. At any given time, approximately 90% of the hairs on your head are in the anagen phase.
  • Catagen Phase: This is a transitional phase during which hair growth slows down, and the hair follicle starts to shrink. The catagen phase tends to last for 10 days. Only about 5% of the hairs on your head are in the catagen phase.
  • Telogen Phase: This is the resting phase when the hair is no longer growing. During this phase, your hairs will no longer grow; however, they don’t usually fall out either. This is when new hairs start to form in the follicles released from the catagen phase. Many scientists have divided the telogen phase into two parts: the telogen and exogen phase.
  • Exogen Phase: The exogen phase is an extension of the telogen phase, where hair is shed from the scalp. This phase results in losing 50 – 100 hairs a day for normal hair. This stage typically lasts 2 – 5 months.
Stages of the hair growth cycle showing anogen, catagen, and telogen phases at Hairloss-Recovery

The growth of hair follicles at each phase for anagen, catagen, telogen, and exogen.


Androgenetic Alopecia (AGA) is a complex condition that is driven by genetics, hormones, and follicular miniaturization. All of these occur within the context of the hair growth cycle. Science has provided insight into the mechanisms underlying AGA, and the hereditary nature of the condition. It has shown the pivotal role that androgens, like DHT, have in the growth cycle. Being able to understand these intricate processes is crucial into the development of effective treatments aimed to stop or reverse hair loss. Future research holds the promise of further unravelling the molecular intricacies of hair loss.

Want to Learn More?

Click to view the latest scientific paper on Male Androgenetic Alopecia from the National Library of Medicine.

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