The causes of hair loss

Occasional or chronic hair loss or androgenetic alopecia: anyone can be affected by the phenomenon. There can be many causes that are not always easy to identify. Here's an overview.



What does the hair growth cycle look like?

To fully understand the process of hair loss and its causes, it is important to keep the hair growth process in mind. Hair development is cyclical in nature, characterized by 3 phases: growth, regression and shedding:

  • Anagen phase: the hair growth phase. During this phase, the cells of the hair root multiply. It is during this phase that the hair, also called hair shaft, is produced by the hair follicle. This phase, during which the hair grows one centimeter per month, lasts on average from 2 to 6 years
  • Catagen phase: the transition phase during which the hair follicle becomes inactive. The cells stop multiplying and the hair stops growing. This phase lasts approximately 2 to 3 weeks.
  • Telogen phase: the shedding phase. Shedding is not immediate, it lasts 2 to 3 months. "Dead" hair eventually falls out, pushed out by the hair in the anagen phase of the next cycle, and so on. Under normal conditions, each hair follicle follows its own cycle, so that the phases of the hair cycle do not occur simultaneously throughout the scalp. On a healthy scalp with approximately 100,000 hairs, 86% of the hair follicles are in the anagen phase*. So, it is normal to lose 50 to 100 hairs per day, corresponding to normal hair renewal. Generally, this shedding is imperceptible, we do not necessarily notice it in our daily lives.

Why does hair fall out?

When the hair life cycle is disrupted or even stopped due to the influence of various factors, daily hair loss can increase and sometimes reach 300 hairs per day. At this point, it becomes noticeable and you start to find hair on the pillow case, on your clothes, on the floor...
Certain factors can accelerate the hair cycle. Cycles come one after the other much faster than normal and the follicle eventually becomes exhausted. Hair no longer has time to grow properly, it becomes thinner and thinner, then disappears permanently.

Hair loss in women: hormonal causes are common

It is not always easy to identify the precise cause of hair loss, especially when it is temporary. However, hormonal variations are among the first elements specialists look into. In fact, scientists have observed and recognized that hormones have a more or less significant degree of involvement in the different types of hair loss: acute telogen effluvium (reactional hair loss), chronic telogen effluvium (chronic hair loss), androgenetic alopecia and hair loss related to menopause.
The majority of people who consult a specialist for hair loss are women. In women, hair loss due to hormonal variations is quite common:

  • It is not unusual to observe a link between the contraceptive pill and hair loss. This is due to the composition and dosage of hormones, which can vary from one contraceptive pill to another. For example, progestogens alone have androgenic effects on hair that can cause diffuse reactional hair loss. While progestogens associated with estrogens can have a positive effect on hair quality. Another common cause of hair loss in women under 30 is polycystic ovary syndrome (PCOS: a hormonal disorder that causes the excessive production of androgenic hormones and that can be responsible for fertility problems). Hormonal imbalances related to this pathology are often responsible for alopecia.
  • Hormonal changes observed during the period following childbirth, or post-partum, often cause reactional hair loss. The reason: the sharp drop in estrogen hormone levels after childbirth. Specialists estimate that a third, or even half, of women are affected by hair loss after childbirth**.
  • Thyroid disorders are often mentioned as one of the causes of alopecia, specifically chronic telogen effluvium (the scientific term for chronic hair loss, i.e., fluctuating progressive hair loss that lasts for more than 6 months). This type of hair loss can stem from the same causes in men.
  • The mode of onset for androgenetic alopecia is also hormonal, involving the androgen receptors at the hair root. This is the primary cause of hair loss in men. This type of chronic localized hair loss (hair loss that is progressive and sets in over the years) generally occurs between the ages of 30 and 40. 70-80% of men are affected in their lifetime compared to 29-42% of women***. In many cases, alopecia is hereditary. And this type of alopecia has the same causes in women.
  • Hormonal changes following menopause can also be the cause of alopecia. Hair loss related to menopause can trigger or aggravate alopecia already present in some women.

The cause of alopecia is not always hormonal...

Hair loss in younger women can have other causes, not just the so-called hormonal ones.

  • Stress-related hair loss induces a particular physiological mechanism: in the scalp, cells subjected to acute or profound stress release neurotransmitters that trigger an acute inflammatory cascade. This inhibits the normal hair cycle: hair then enters the telogen phase prematurely, causing sudden and severe hair loss 2 to 3 months after the triggering factor. This is reactional hair loss or acute telogen effluvium.
  • A link between fatigue and hair loss does exist. Physical and psychological fatigue can be involved in several types of hair loss:

- either reactional, following temporary but intense fatigue.

- or chronic, for example, due to anemia caused by an iron deficiency. Closely related to fatigue, hair loss related to a nutritional deficiency can cause fluctuating hair loss over more than 6 months.

  • Some people experience hair loss at the change of season, mainly in autumn. This phenomenon could be linked to the impact of the sun on the production of hormones that positively influence the hair life cycle during the summer. This positive influence disappears with the arrival of the shorter, less sunny days of autumn, causing reactional hair loss. It is very temporary and lasts a maximum of 4 to 6 weeks.
  • Traction alopecia is hair loss caused mechanically. It is a localized form of alopecia that results in progressive loss of hair in the frontal area and at the temples. It is caused by hair styling practices that are traumatic to the hair root (buns, tight ponytails, braids, heavy braids, weaves, extensions, straightening). If these practices are not abandoned quickly, hair follicles can be destroyed irreparably causing the alopecia to become scarred and irreversible. A progressive recession of the hairline is observed. Traction alopecia is common in women, especially African women. It can affect up to 30% of them****.
  • Hair loss related to spot baldness is a type of alopecia that occurs in patches of varying size that can sometimes even affect the whole scalp. The cause of this type of hair loss is an autoimmune disease that is often triggered by intense stress, hormonal changes or viral infections. The hair follicles are suddenly considered foreign bodies by the immune system, which decides to remove them. The hair follicles are not completely destroyed, so the hair can grow back, but this can take several years.
  • Ringworm is another form of alopecia that occurs in patches. In this case, the cause of alopecia is infectious: a microscopic fungus that infects hair follicles and causes hair loss. It is most common in children and people with weakened immune systems, and can be highly contagious. It is treated with medication to kill the fungus (antifungal) and hair grows back normally once healed.
  • Syphilis, a sexually transmitted infection, can cause hair loss in patches or diffuse hair loss, or both. Syphilis can cause alopecia in its secondary form, usually affecting the back and top of the head. Hair loss may be accompanied by generalized redness on the body without itching. Antibiotic treatment can quickly treat this infection, which allows hair to grow back normally.
  • Scarring alopecia is a form of localized alopecia, defined by an area of smooth scarred skin on which no hair grows back. The hair follicles have been completely destroyed and replaced by fibrous tissue. It is caused by trauma, burns, radiotherapy to the scalp or inflammation of the scalp.
  • The term medication-related alopecia describes a type of hair loss that is a reaction to the use of certain treatments. The main families of drugs to watch out for: anticoagulants, antidepressants, anticonvulsants, antihypertensives, certain anti-inflammatory drugs, thyroid treatments, beta blockers, certain cholesterol-lowering treatments, lithium-containing drugs and retinoids.
    The administration of cancer chemotherapy or exposure to radiotherapy on the head and neck is also responsible for a certain type of severe hair loss called anagen effluvium.

COVID-19 is a new factor responsible for reactional hair loss (or acute telogen effluvium). The hair loss observed after COVID-19 is related to the inflammatory state caused by this viral infection. The episodes of high fever that accompany this hair loss have also been implicated. Therefore, a history of COVID-19 should be sought in individuals with reactional hair loss. In the unusual context of a pandemic, stress and anxiety can be factors that add to the consequences of the illness and worsen the hair loss.

When to worry about hair loss

Hair loss can affect anyone at any age. It can be quite temporary and last only a few months before the hair returns to its normal state.
When hair loss is long term (more than 6 months) and is accompanied by a progressive decrease in density or symptoms such as itching, pain or redness, it is best to consult a doctor. This will help determine the cause and treat it if possible.
* Source: Olsen, 1994; Shapiro, 1996
**Source: Grover and Khurana, 2013
*** Source: Blume-Peytavi et al., 2011; Wang et al., 2010)
**** Source: Billero et al. 2018

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