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What are the causes of hair loss?

Hair loss is a hair-related issue that can affect both men and women. It is a natural phenomenon in the hair renewal cycle. However, when hair loss results in progressive thinning, it could be a case of alopecia. Discover what the causes of hair loss are.

Hair loss, a natural phenomenon.

Hair loss is part ofthe hair cycle stage. It is adaily phenomenon but completely normal. Indeed, each hair has its own life cycle composed of three phases.

  • The anagen phase (2 - 6 years): This is the growth phase of the hair during which there is proliferation of the matrix cells that form the inner sheath of the root, the cortex, and the medulla of the hair shaft. The synthesis and pigmentation of the hair shaft only occur during this phase. Its duration determines the length of the hair, generally 1 cm per month.

  • The catagen phase (2 - 3 weeks): The first sign of the catagen phase is the cessation of melanin production, the pigment responsible for the color of our hair, in the hair bulb. The hair stops growing but remains attached to its hair follicle.

  • The telogen phase (2 - 3 months): After the catagen phase, the follicles rest in a dormant phase, the telogen phase. The hair shaft eventually detaches from its follicle, which is already starting to produce a new hair under the skin.

Each hair has an asynchronous life cycle. Some may be in a growth phase, while others are about to fall out. This is why we naturally lose between 50 and 150 hairs per day. Their life cycles are regulated by hormonal or nutritional variations. However, certain factors can cause an abnormal hair loss.

Heredity, a key factor in hair loss.

Today, we can identify numerous forms of hair loss that are hereditary. For example, theandrogenetic alopecia is due to genetic factors where the scalp follicles react abnormally to the action of testosterone. Indeed, testosterone is converted under the action of the 5-alpha reductase enzyme into dihydrotestosterone (DHT). This latter is known to shorten the lifespan of hair by accelerating the hair cycle and thus promoting hair loss. There are also other hereditary forms of hair loss.

Hormonal changes as the cause of hair loss.

One of the reasons that can cause hair loss is hormonal changes. This problem is thus quite common during pregnancy and after childbirth. Similarly, around the age of 50, women experience menopause. This stage is characterized by the cessation of reproductive hormones produced by the ovaries.

As a result, menopause can trigger a hormonal imbalance which is characterized by a deficiency in estrogen concentration and an increase in androgen levels. However, the latter leads to an acceleration of the hair cycle and the hair follicle renewal capacity will prematurely deplete. This upheaval will then cause hair loss, also known as "androgenic alopecia".

Thehyperandrogenism indicating an excess of androgen is often associated with hair loss. Men are most affected because androgens, the main hormones involved in hair loss, are male hormones. Hyperandrogenism can occur as a result of congenital adrenal hyperplasia, a genetic disorder that involves a mutation in some of the enzymes that metabolize steroid hormones, or a tumor originating from the adrenal gland. Women are also affected by hyperandrogenism, which can occur as a result of polycystic ovary syndrome or an ovarian tumor.

Alopecia caused by stress and anxiety.

Stress or anxiety can cause hair loss. An emotional shock can trigger a chain reaction in the body leading to hair loss. Indeed, during a period of stress, a locally released substance P can directly inhibit the growth of keratinocytes in hair follicles, or it can induce their apoptosis and inhibit their proliferation through the release of hair growth-inhibiting cytokines, such as TNF-α, IL-154, and IL-55. TNF-α is an inflammation mediator that triggers the transition of hair into the telogen phase, which is their shedding phase. Depression caused by stress can also prolong the disruption of hormones. Thus, people suffering from depression can lose a lot of hair over a long period.

Nutritional deficiencies promote hair loss.

Hair loss can also be attributed to dietary deficiencies. Indeed, diet has a direct impact on the health and appearance of the hair.

  • Iron Deficiency.

    Iron is a cofactor of the ribonucleotide reductase, an enzyme that limits the speed of DNA synthesis. The cells of the hair follicle matrix are among the cells that divide the fastest in the body and can be extremely sensitive, even to a minor decrease in iron availability, thus leading to a decrease in hair growth. Nevertheless, further studies need to be conducted to elucidate its mode of action and to affirm that an iron deficiency can be responsible for hair loss.

  • Zinc deficiency.

    Clinical studies have shown that among 312 patients diagnosed with alopecia areata (AA), male hair loss, female hair loss, or telogen effluvium (TE), their serum zinc levels had significantly decreased compared to a control group. Indeed, zinc plays a role in the regulation of hair growth, as it acts as a catagen inhibitor through its inhibitory action on apoptosis-related endonucleases. Therefore, a zinc deficiency can promote hair loss.

The change of seasons causing hair loss.

Hair loss is a daily occurrence, but you may have noticed that it is more pronounced during the arrival of fall.Indeed, during the summer, we observe a decrease in the production of melatonin, which is inhibited in the presence of light. Melatonin is involved in the hair cycle by prolonging the anagen phase.

Clinical studies have shown that in women suffering from androgenetic alopecia, a solution of melatonin applied to their scalp was able to increase hair growth by extending the anagen phase compared to control groups. Thus, in the summer, melatonin is inhibited and the hair enters the catagen phase and then the telogen phase. This process can take a few months, which explains why our hair only falls out at the beginning of autumn.


  • RANDALL V. A. & al. Seasonal changes in human hair growth. British Journal of Dermatology (1991).

  • ARCK P. C. & al. Indications for a brain-hair follicle axis: inhibition of keratinocyte proliferation and up-regulation of keratinocyte apoptosis in telogen hair follicles by stress and substance P. The FASEB Journal (2001).

  • HADSHIEW I. M. & al. Burden of hair loss: Stress and the underestimated psychosocial impact of telogen effluvium and androgenetic alopecia. Journal of Investigative Dermatology (2004).

  • OLSEN E. A. & al. Iron deficiency in female pattern hair loss, chronic telogen effluvium, and control groups. Journal of the American Academy of Dermatology (2010).

  • KIL M. S. & al. Analysis of serum zinc and copper concentrations in hair loss. Annals of Dermatology (2013).

  • LIN R. L. & al. Systemic causes of hair loss. Annals of Medicine (2016).


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