In order to understand hair loss, it is vitally important to understand what normal hair growth is. An average person has 100,000 to 150,000 terminal hair follicles on his or her scalp. Hair follicles produce approximately 1 cm of new hair growth in a month. Hair follicles are on almost every part of the human body, and the follicles grow hair in cycles that continue to repeat until a person dies or the follicle is destroyed. The most common form of hair loss is androgenetic alopecia (male or female patterned hair loss), and the hair growth cycle progressively shortens.
4 Main Phases of the Hair Growth Cycle
There are four main phases of the hair growth cycle.
Anagen (Growth Phase)
For scalp hair, the actively growing anagen phase accounts for 85% of the hairs and lasts for approximately 2-7 years. The longer the anagen phase, the longer the new hair will grow before being shed. For eyebrow hair, the anagen phase only lasts approximately 1 month, and that is why eyebrow hair never grows that long before the hair is shed.
Catagen (Transition Phase)
The catagen phase is a transitional phase where the hair gets thinner and the growth slows down. This phase accounts for 2% of the scalp hair and lasts for approximately 3 weeks.
Telogen (Resting Phase)
The telogen phase is the resting dormant phase where the hair follicle no longer produces new hair. This phase accounts for approximately 12% of the scalp hairs and lasts for approximately 3 months.
Exogen (Shedding Phase)
The exogen phase is the very brief time period where the old hair shaft is pushed out of the hair follicle by a newly growing hair shaft that has entered the anagen phase again.
How the Cycle Is Disrupted in Hair Loss?
Hair loss occurs when the natural hair growth cycle is disrupted, causing shorter growth phases and increased shedding that lead to thinning hair. Below, we explore how disruptions in this cycle contribute to different types of hair loss.
Androgenetic Alopecia (AGA)
It is the most common form of hair loss in both men and women. In this condition, the anagen phase of the hair cycle gradually shortens over time. As the cycle becomes progressively disrupted, hair follicles begin to miniaturize, meaning they produce thinner, shorter, and less pigmented hairs. Eventually, these follicles stop producing visible hair altogether. This type of hair loss is chronic, and the affected follicles spend less time in the growing phase and more time in the resting (telogen) phase.
Telogen Effluvium
It is a condition in which a larger-than-normal number of hairs shift from the anagen phase to the telogen phase all at once. This is typically triggered by a physiological or emotional stressor, such as illness, surgery, childbirth, or sudden weight loss. The shedding usually becomes noticeable about two to three months after the triggering event. During this time, the normal balance of the hair cycle is disturbed, leading to widespread shedding as many hairs exit the scalp simultaneously during the exogen phase. Fortunately, telogen effluvium is often temporary and reversible once the underlying cause is resolved.
Alopecia Areata
It is an autoimmune condition in which the immune system mistakenly attacks healthy hair follicles, abruptly halting the anagen phase. As a result, hairs fall out suddenly, often in small, round patches. In more extensive cases, the shedding may involve large portions of the scalp or body. Unlike other forms of hair loss, the hairs may be shed without going through the typical transition into catagen or telogen. The follicle itself is not destroyed, and hair can regrow, although the condition may be recurrent and unpredictable.
Factors Affecting the Hair Growth Cycle
Below are some of the key factors that can affect the normal hair growth cycle:
Age
As we age, the hair growth cycle becomes less efficient. The anagen phase gradually shortens, which means new hair doesn’t grow as long or thick. Some follicles may even stop producing hair altogether. This age-related miniaturisation contributes to overall hair thinning and slower regrowth.
Genetics
Genetic factors determine how sensitive your hair follicles are to hormonal changes, especially DHT. In conditions like androgenetic alopecia (pattern hair loss), inherited sensitivity causes the anagen phase to shorten over time, resulting in thinner and weaker hairs.
Hormonal Changes
Hormones play a crucial role in regulating hair growth. Life events such as pregnancy, menopause, or thyroid disorders can disrupt hormonal balance, altering the duration and quality of growth phases. In particular, increased levels of DHT can accelerate follicle shrinkage in those predisposed to pattern hair loss.
Nutritional Deficiencies
Hair follicles require a constant supply of nutrients, such as iron, protein, biotin, zinc, and vitamins D and B12, to sustain normal growth. When the body is low on these nutrients, it prioritises essential functions over hair growth, pushing follicles into the resting (telogen) phase prematurely.
Environmental and Lifestyle Factors
External factors like excessive sun exposure, pollution, harsh hair treatments, or heat styling can weaken hair and damage follicles. Poor sleep, smoking, or a sedentary lifestyle may also reduce circulation and nutrient delivery to the scalp, further impacting the cycle
Medications and Hair Growth Cycle
Minoxidil and finasteride are the only two FDA-approved treatments that slow the progression of androgenetic alopecia. There are no other medications on the market that can legally and ethically claim that their product causes clinically significant hair growth. Unfortunately, there are plenty of “snake oil” potions that unethically claim to help hair loss, but do not, so beware.
Minoxidil
Minoxidil was the first medication FDA-approved in both men and women. The precise mechanism of action of hair growth is not known. Minoxidil is an indiscriminate hypertrichotic, which means it tries to get any hair follicle to produce a thicker and longer hair (i.e., lengthening the anagen phase). The results peak at 1 year, and then it slows the progressive miniaturization of the scalp hair. Unfortunately, discontinuation of minoxidil results in a loss of the hair gained during treatment (i.e., the patient reverts back to his or her genetic predisposition).
Finasteride
Finasteride (Propecia) is the most effective treatment for male pattern hair loss. Finasteride inhibits the conversion of testosterone into dihydrotestosterone (DHT) by blocking the enzyme alpha 5 reductase. Finasteride increases the number of scalp hairs in the anagen phase, making the follicles produce longer and thicker hair. The results peak at 3 years, and then it slows the progressive miniaturization of the scalp hair. If finasteride is discontinued, the patient will lose the benefit within 2 years.
How to Support Healthy Hair Growth?
There is a misconception that the balding scalp skin is not healthy. This scalp skin is just as healthy as all the other regions around the head (e.g., the skin on the sides/back of the scalp, the forehead, the beard region). But the different regions’ hair follicles have different genetics dictating if the follicle will produce a thick hair shaft, produce a miniaturized hair shaft, transform from producing a thick to a miniaturized hair shaft, or transform from producing a miniaturized to a thick hair shaft.
Finasteride and minoxidil are the only proven medical forms of therapy that can slow, stop, and/or somewhat reverse androgenetic alopecia. There is no shampoo or oral vitamin that can reverse or slow the miniaturization process of the susceptible hair. Shampoos and conditioners can cleanse and moisturize both the miniaturized hairs on the top of the scalp and the thick, good genetic hair around the sides and back of the scalp, but they do not change the course of androgenetic alopecia. Some shampoos and conditioners may help the hair, when styled, to appear thicker and fuller, but they do not actually change the diameter of the hair shaft.
Worried About Hair Loss? Let’s Talk.
Don’t wait and wonder, get the answers you need from a trusted expert. Dr. McAndrews is a board-certified hair restoration physician with over 20 years of experience helping patients understand and treat their hair loss. Whether you’re just starting to notice thinning or exploring advanced treatment options, we’re here to help.
Call us today at 877-HAIR-DOC or email us at appointments@hairgrowthdoctor.com to schedule your consultation.
