Causes of Hair Loss: All the Reasons at a Glance

How much hair loss is normal, and when does it become a problem?

Losing 50 to 100 hairs per day is considered normal, according to the American Academy of Dermatology. Only a clearly and persistently higher loss, visibly thinning hair, or bald patches indicate hair loss that needs treatment. A fixed figure such as “abnormal above 150 hairs” does not exist in the literature.

What causes hair loss in detail is closely tied to the hair growth cycle. About 85 to 90 percent of scalp hairs are in the growth phase (anagen, 2 to 6 years), roughly 1 percent in the transition phase (catagen), and 10 to 15 percent in the resting phase (telogen, about 2 to 3 months). At the end of the telogen phase, the hair falls out. More on this in the detailed article on the hair growth cycle.

Three markers offer a rough self-check: the pull test (if an unusual number of hairs come loose when you gently tug on a strand, that is a sign), visibly more hair in the drain, brush, and on your pillow, and a widening part. These signs are no substitute for a medical evaluation, but they give you a reference point.

The main types of hair loss at a glance

What causes hair loss can usually be assigned to one of three major forms: androgenetic (hereditary), diffuse (telogen effluvium), and patchy (autoimmune) hair loss. On top of these come rarer special forms such as scarring and mechanical hair loss. The pattern, triggers, and outlook differ markedly, as the following overview shows.

Reasons for hair loss in women and men

The reasons for hair loss differ between women and men primarily in pattern and frequency. Men lose hair overwhelmingly through androgenetic causes, often early (about 25 percent show first signs before age 21), with a receding hairline and crown thinning per the Norwood scale. The course is highly predictable and follows the typical male pattern.

In women the picture is more varied: diffuse causes such as iron deficiency, thyroid issues, and postpartum shedding are more common, alongside hormonal triggers (menopause, PCOS) and androgenetic alopecia in the Ludwig pattern, with thinning at the part rather than a receding hairline. Because several causes often coincide, a thorough evaluation is worthwhile. More detail in the article on hair loss in women.

Androgenetic hair loss (hereditary): the most common cause (L64)

Hereditary androgenetic hair loss is the most common cause of hair loss of all. According to epidemiological data, about 50 percent of men of Caucasian descent are affected by age 50 and up to 80 percent by age 70 (PMC11867384). The German public health portal gesund.bund.de cites a range of up to 70 percent of men and 40 percent of women.

The cause is not a disease in the strict sense but an inherited sensitivity of the hair follicles to dihydrotestosterone (DHT). It is not a high testosterone level but the receptor sensitivity that shortens the growth phase and makes the follicles shrink (miniaturization). About 25 percent of men show first signs before age 21 (PMC11867384). More on this in the article on androgenetic alopecia.

The pattern is characteristic: men lose hair per the Hamilton-Norwood scale at the temples and crown, while women follow the Ludwig scale with diffuse thinning at the part and a usually preserved hairline. Details on the female course in the article on hair loss in women.

As the evidence-based first line, the European guideline recommends topical minoxidil (for women and men) as well as oral finasteride 1 mg/day for men. Finasteride is prescription-only and can have sexual side effects, which is why a medical evaluation is necessary. Androgenetic hair loss progresses without treatment, but it can be slowed and treated. Timing is decisive: medication can curb the loss and preserve existing hair, but it cannot reactivate follicles that have already fully regressed. How density can be restored in areas that are already bald is addressed in the section on reversibility further below.

Diffuse hair loss (telogen effluvium) and its triggers (L65.0)

Diffuse hair loss (telogen effluvium) is spread evenly across the whole head and is set off by a trigger that typically dates back 3 to 4 months. Too many follicles shift into the resting phase at the same time. According to the review by Malkud (2015), this form is usually self-limiting and subsides within about 6 months once the cause is resolved.

Because months pass between the trigger and the visible shedding, the cause is often overlooked. The most common reasons for hair loss of this kind range from nutrient deficiencies to hormones and stress to medication. The following subsections sort them out.

Iron and nutrient deficiency

Iron deficiency is one of the most common, often overlooked causes of diffuse hair loss, especially in women. The decisive value is the storage marker ferritin: by guideline consensus, a deficiency is defined below 30 ng/ml, and some labs set the threshold below 15 to 20 ng/ml. When iron is lacking, the actively dividing hair matrix slows down and follicles are pushed prematurely into the telogen phase.

A higher “hair target value” of 40 to 70 ng/ml is discussed by some experts, traces back to the review by Trost et al. (2006), and is not backed by controlled studies. The authors themselves noted insufficient evidence for routine iron supplementation in non-anemic deficiency. The German Nutrition Society recommends 16 mg of iron per day for menstruating women and 11 mg for men. More on this under iron deficiency and hair loss as well as vitamins for hair loss.

Thyroid (overactive and underactive)

Both an underactive and an overactive thyroid can cause diffuse hair loss, because the hormones T3 and T4 help regulate the hair cycle. According to studies, about 25 percent of people with an underactive thyroid and up to 15 percent with an overactive thyroid report hair loss. The TSH value (reference range roughly 0.4 to 4.0 mU/l, lab-dependent) is the first screening test.

This hair loss is typically diffuse and reversible over several months once the values normalize. More detail in the hub on hair loss from the thyroid and in the detailed article on hair loss from hypothyroidism.

Hormonal shifts: childbirth, the pill, menopause, PCOS

Hormonal shifts are among the most important reasons for hair loss in women. After childbirth, postpartum shedding usually sets in 2 to 4 months later, because the drop in estrogen sends up to 30 percent of the follicles into the resting phase in sync (normally 10 to 15 percent). According to review articles, more than 95 percent of women recover within 6 to 12 months.

During menopause, the falling estrogen level unmasks the follicles’ genetic DHT sensitivity. This is often androgenetic alopecia becoming visible in the Ludwig pattern, not a pure effluvium. In younger women, PCOS (polycystic ovary syndrome, diagnosed by the Rotterdam criteria) is a central hormonal cause and should be evaluated by an internist. More on this interplay under hormones and hair.

Can stress cause hair loss?

Yes, severe physical or psychological stress can cause hair loss. Through activation of the stress axis (cortisol), follicles are pushed prematurely into the resting phase, producing a diffuse telogen effluvium, typically 2 to 3 months after the stressful event (JAAD Reviews, 2025). Once the stress eases, this loss is usually reversible.

An important distinction: stress does not trigger genetic hair loss, but it can accelerate an existing hereditary course. So anyone who suddenly sheds heavily under strain usually has a temporary effluvium, not newly developed hereditary hair loss.

Crash diets, protein and calorie deficiency

Crash diets and one-sided eating can cause hair loss, because the body lacks the building blocks for energy-hungry hair growth. After heavy weight loss, the diffuse shedding usually begins 6 to 12 weeks after the start of the diet. Reviews suggest that a protein intake below 0.8 g/kg of body weight per day might roughly double the risk of telogen effluvium.

Besides protein, zinc, essential fatty acids, and total calories also play a role in restrictive diets. Once you return to a balanced diet, the hair usually recovers.

Hair loss after COVID-19, surgery, childbirth, and fever

Acute physical stressors such as a COVID-19 infection, surgery, childbirth, or high fever frequently cause an acute telogen effluvium. After COVID-19 it often appears faster than usual, at a median of about 1.5 months after the infection (JAAD). According to one study, roughly 25 percent of those affected develop such hair loss in the first 2 to 3 months (PMC8407603).

The mechanism is always the same: physical stress pushes many follicles into the resting phase at once, and the loss only becomes visible with a delay. The peak usually falls around month 3 to 5, and full density typically returns within 6 to 12 months.

Medications

Various medications can cause hair loss, usually as telogen effluvium. The following table lists the most important drug classes. The YMYL principle is decisive: if you suspect a medication, never stop it on your own. Especially with blood thinners and cardiovascular drugs, consulting the prescribing physician is mandatory.

Sources for this overview include DermNet and pharmacovigilance-based adverse-event reports. The DOAC reports cited do not prove a causal link but are suspected cases. If a new medication lines up in time with the hair loss, the prescribing physician is the right person to ask.

Seasonal hair loss

Mild seasonal hair loss is real, but harmless. A cohort of 823 women showed the highest resting-phase proportion at the end of summer and beginning of fall (August to October), and the lowest in winter (PubMed 19407435). The effect is subtle and temporary and does not create visible gaps. It is clearly distinct from a true effluvium that needs treatment.

Patchy hair loss (alopecia areata): the autoimmune cause (L63)

Patchy hair loss (alopecia areata) is an autoimmune cause of hair loss in which the immune system attacks individual hair follicles and sharply defined, round bald patches form. According to the new German AWMF S3 guideline (registry 013-104, published 09/18/2025), about 170,000 people in Germany are affected, and the internationally pooled prevalence is around 2.1 percent.

So-called exclamation-mark hairs at the edge of a patch are characteristic. The condition can occur at any age and is frequently associated with other autoimmune diseases (such as Hashimoto’s or vitiligo) as well as with anxiety and depression. The course is unpredictable. Spontaneous remission is possible, but per the guideline occurs in under 10 percent of cases in the first 6 months. For dietary supplements, the guideline explicitly gives no recommendation.

One frequently described link with vitamin D is intriguing: in one study, 97 percent of alopecia areata patients had a vitamin D deficiency compared with 32.8 percent of controls. Whether the deficiency promotes the condition or is a consequence of it remains unclear. Patchy hair loss is treated dermatologically, for example with steroids or newer JAK inhibitors.

Scarring hair loss: the cause that counts as an emergency (L66)

Scarring hair loss (scarring alopecia, such as lichen planopilaris or frontal fibrosing alopecia) destroys the hair follicles permanently and is irreversible. That is why this cause of hair loss is the most urgent: the sooner the inflammation is stopped, the more follicles are preserved. The window of time matters, before the scarring is complete.

Warning signs are redness, itching, or burning together with shiny, scarred areas that have no visible follicle openings. Anyone who notices this combination should see a dermatologist immediately. The diagnosis is usually confirmed by a scalp biopsy, and trichoscopy provides early clues. Unlike diffuse effluvium, nothing here can wait.

Mechanical and other hair loss

Mechanical hair loss results from persistent pull. Traction alopecia typically affects the hairline and temple area and stems from tight braids, extensions, or a constant bun. In the early stage it is reversible, but with continued pull it can scar. In trichotillomania, people compulsively pull out their own hair, which needs to be addressed psychologically.

Another cause is tinea capitis, a fungal infection of the scalp that mainly affects children under 10. It shows scaly, sometimes inflamed bald patches with broken hairs, is contagious, and is treated with oral antifungals. Very rarely, an infection such as secondary syphilis can cause a “moth-eaten” alopecia, which should be evaluated by a doctor.

Is hair loss hereditary? And what role does lifestyle play?

Hereditary predisposition is the most common reason for hair loss. What is inherited, however, is not a single “baldness gene from the mother’s side” but the sensitivity of the hair follicles to DHT, and it is polygenic, passed down through both parental lines. The widespread belief that only the maternal grandfather is decisive is therefore disproven.

Lifestyle acts as an amplifying cofactor, not a sole cause. For smoking, a meta-analysis (Gupta et al., 2024) shows an odds ratio of 1.82 for androgenetic alopecia, and among heavy smokers (10 or more cigarettes/day) an almost threefold increased risk of severe forms. Heavy alcohol consumption can have a contributing effect through disrupted zinc and iron metabolism, while moderate consumption is not considered a relevant risk factor.

Myths: what does NOT cause hair loss?

Frequent hair washing, wearing hats, blow-drying, and most styling products do not cause permanent hair loss. These are widespread myths. The following table sets the most common misconceptions against the facts.

Identifying and diagnosing the cause correctly: when to see a doctor?

Which cause of hair loss you have can be recognized from three things: the pattern (where?), the course (how fast?), and blood values. The pattern is the fastest filter. The following guide is an aid, but it is no substitute for a medical diagnosis. Red flags such as scarring, itching, or sudden sharply defined patches always need to be evaluated immediately.

For diffuse hair loss, a simple time-window rule helps: if you are shedding more now, check for events about 3 months earlier (current month minus 3). Exactly this telogen lag of 3 to 4 months (Malkud 2015) often obscures the actual cause, such as an infection, a diet, or surgery.

Diagnostics include a medical history, the pull test, trichoscopy, and blood work. Which values make sense is shown in the following checklist. Details on lab diagnostics in the article on the blood test for hair loss.

Reversible or permanent? And: diffuse or genetic?

Whether hair loss is reversible depends on the cause: diffuse triggers (iron, thyroid, stress, hormones) are usually fixable, while hereditary and scarring hair loss progress without treatment. With diffuse effluvium, regrowth follows the hair cycle, which is why it takes a few months. The following timeline shows the typical course after the trigger has been resolved.

Diffuse hair loss across the whole head almost always has a fixable cause and is treated at the root, not surgically. Only genetic (androgenetic) hair loss with a typical pattern, meaning a receding hairline or thinning at the part, responds to hair growth medication or a hair transplant. The two forms often occur at the same time. That is why the question at the outset is always: which type do you have? A free hair analysis provides a visual classification and is no substitute for a medical blood test.

Frequently asked questions about the causes of hair loss

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