Why am I losing my hair?

Hair loss is something most people notice at some point, whether they see it in the shower, on their pillow, or in their hairbrush. This can be worrisome, but hair loss isn’t always abnormal. It’s helpful to distinguish between hair shedding—when more hairs fall out—hair thinning—a gradual reduction in density over time—and hair breakage—when the hair shaft becomes brittle. These can reflect different underlying patterns.

In many cases, what is perceived as excessive hair loss is part of the natural hair cycle. In other situations, hair loss may be due to actual thinning of the hair resulting from underlying biological changes. Understanding these differences is important and can help you determine when to take action and what steps to take.

How Hair Normally Grows

The average human scalp contains approximately 100,000 hair follicles, and each hair follicle produces hair in cycles. Hair on the scalp consists of two main types of hair fibers: terminal hair and vellus hair. Terminal hair is thick, pigmented hair that makes up most of the visible hair on the head, while vellus hair is finer, shorter, and less pigmented.

The balance between these hair types reflects normal hair follicle function and becomes clinically relevant in conditions that affect hair growth. Hair follicles do not produce hair continuously; instead, they follow a recurring cycle consisting of three main phases: [2,3]

• Anagen phase, the growth phase: Active hair growth that lasts 2 to 6 years. Typically, about 85 to 90 percent of hairs are in this phase at any given time. During this phase, hair grows an average of 0.35 mm per day, or about 1 centimeter per month. [1,3]
• Catagen phase, the transitional phase: A short phase lasting 2 to 3 weeks. During this period, the hair follicle begins to shrink. The lower part of the hair follicle undergoes a controlled breakdown, known as apoptosis, and the hair stops growing. Typically, less than 1 percent of hairs are in this phase at any given time. [1,3]
• Telogen phase, the resting phase: A resting period of about three months. During this time, the hair no longer grows, but it remains anchored in the hair follicle until it eventually falls out. About 10 to 15 percent of the hairs on the scalp are typically in this phase at any given time. At the end of this phase, the hair shaft detaches. This process is sometimes referred to as a separate fourth phase, the exogen phase. In practice, the new hair pushes the old resting hair out of the hair follicle when the new growth cycle begins. [1,3]

This asynchronous cycle causes hair loss to occur gradually rather than all at once. [1,3]

Why Hair Loss Often Feels Sudden

Hair loss is often perceived as something that happens suddenly, even though the underlying changes may have begun earlier. Many people don’t notice individual strands of hair falling out, but become aware of hair loss when they see more hair in the shower, on their brush, or when thinning becomes visible. This experience is largely influenced by the phases of the hair growth cycle.

When many hair follicles enter the resting phase due to a triggering factor, hair loss does not occur immediately. It usually becomes noticeable 2 to 3 months later, which corresponds to the end of the telogen phase, when the resting hair sheds. Hair in the telogen phase also becomes more noticeable when mechanical forces pull the hair out during brushing and showering. The increased hair shedding can last for 3 to 6 months. [4]

The prolonged period of increased hair loss, combined with the time it takes for new hair to grow to a cosmetically visible length, is a common source of frustration and can undermine confidence in the treatment and advice.

When normal hair shedding turns into actual hair loss, alopecia

Due to the natural hair cycle, it is considered normal to lose 50 to 100 hairs per day, although some people may lose up to 150 to 200 hairs due to physiological and seasonal variations. A temporary increase in hair shedding is sometimes observed in the fall and winter. This may be because more hairs enter the resting phase in late summer. [1,17,18]

This process usually goes unnoticed because new hair is constantly growing in to replace the hair that falls out.

Hair loss and thinning, however, become noticeable when the balance between hair loss and regrowth is disrupted. When hair falls out faster than it grows back, overall hair density gradually decreases, and thinning eventually becomes visible once a significant portion of the hair on the head has been lost. In such cases, where the hair cycle is disrupted or the function of the hair follicles is impaired, leading to thinning and baldness, the condition is known as alopecia, the medical term for hair loss. [6]

Thinning hair and hair loss do not manifest the same way in everyone. It is usually multifactorial, meaning that several contributing factors—such as genetics, hormonal profile, age, and gender—interact and influence both the pattern and progression of hair thinning. Understanding the pattern helps determine the cause.

In most cases of alopecia, the hair follicle is still present, but its function is impaired. The hair may fall out more easily or regrow thinner, but the hair follicle is not destroyed. These conditions are not classified as scarring alopecia and have the potential for regrowth. [6]

Less commonly, the hair follicle becomes damaged and is eventually destroyed as part of inflammation in the surrounding skin. In such cases, the hair follicle is replaced by scar tissue, and hair cannot regrow in the affected area. This is called scarring alopecia and requires early treatment to prevent irreversible hair loss. [12,13]

The Most Common Causes of Hair Loss

Androgenetic alopecia, pattern hair loss

The most common cause of hair thinning in both men and women is androgenetic alopecia, often referred to as male- or female-pattern hair loss. AGA is a genetically determined condition in which hair follicles gradually shrink over time and produce thinner and shorter hair. Up to 80 percent of people have a family history of AGA. [7–9]

The most important biological process is follicular miniaturization, in which thick hairs are gradually replaced by finer and shorter hairs. At the microscopic level, the hair on the scalp is organized into follicular units consisting of two to four terminal hairs, thicker than 60 µm in diameter, and one to two vellus hairs, thinner than 30 µm in diameter. In male- and female-pattern hair loss, the degree of miniaturization is specific to each individual hair within these units, leading to a gradual reduction in thickness, length, and number of hairs per follicular unit. [20,21] This process is accompanied by a shortened anagen phase and a prolonged latent phase between active growth cycles. The affected hair follicles therefore spend less time producing visible hair and more time in an inactive state. [19,20] Unlike temporary hair loss, this reflects a progressive change in hair follicle function.

Clinically, these changes in hair thickness manifest as increased variation in the diameter of the hair shafts. This can be assessed diagnostically and during follow-up through close examination of the scalp and hair follicles using magnification equipment, known as trichoscopy, which clearly reveals the difference between thick terminal hair and finer, miniaturized hair. [9]

AGA manifests itself differently in men and women.

• In men, it usually manifests as:
• Receding hairline
• Thinning on the crown of the head
• In women, it more often manifests as:
• Diffuse thinning across the center of the scalp
• Wider gaps, often with a pattern described as a Christmas tree [7,8]
• Involutional alopecia, or simply a later manifestation of male-pattern hair loss

In addition to classic pattern hair loss, a gradual reduction in hair density or hair diameter—or both—is often observed in older individuals. This age-related change, sometimes referred to as senescent or involutional alopecia, remains a subject of debate. It is unclear whether it is a distinct condition or merely a later manifestation of pattern hair loss. [19]

Telogen effluvium, increased hair loss

Telogen effluvium (TE) is one of the most common causes of sudden and diffuse hair loss. It occurs when more hair follicles than normal prematurely enter the telogen phase—that is, the resting phase—which leads to hair loss several weeks or months later. [4,7]

This type of hair loss is usually temporary and occurs in response to physiological stress or other factors, such as: [4,7]

• Illness or infection, such as COVID-19 [10]
• Major surgery
• Changes in Medication Use
• Childbirth, Postpartum Hair Loss
• Rapid weight loss or strict dieting
• Psychological stress

Unlike androgenetic alopecia, telogen effluvium does not involve progressive miniaturization of the hair follicles. The condition is usually reversible once the triggering factor is resolved, and the hair typically grows back to its normal thickness within six to nine months, although full cosmetic recovery may take even longer. [4,7] It is worth noting that chronic telogen effluvium—persistent hair loss lasting more than six months—can occur concurrently with female pattern hair loss, which sometimes makes it difficult to distinguish between the two conditions. [22]

Nutritional factors

Hair follicles are highly sensitive to nutritional status and metabolic health. A deficiency in certain nutrients can disrupt the normal hair growth cycle. Key factors include:

• Iron deficiency, ferritin deficiency
• Protein deficiency
• Zinc and Vitamin D Deficiency
• B vitamins, such as B12 and biotin

Correcting actual deficiencies can help reduce excessive hair loss. For most healthy people with a varied diet, nutritional deficiencies are not the primary cause of hair loss. Supplements beyond normal physiological levels do not appear to improve hair growth and may, in some cases, be counterproductive. High intake of certain nutrients, particularly selenium, has been linked to increased hair loss.

Although so-called nutricosmetic products are widely marketed, the clinical evidence supporting their efficacy remains inconclusive. [11]

Hormonal effects

Hormonal changes can significantly affect the hair growth cycle. For example, elevated estrogen levels tend to prolong the anagen phase—that is, the growth phase—while a sudden drop can cause many hair follicles to simultaneously enter the telogen phase. This is often seen after childbirth and usually peaks around four months postpartum. [16]

Hormonal changes during menopause can also affect hair density. As estrogen levels decline, the effects of androgens may become more pronounced in women who are predisposed to this condition. In addition, systemic conditions such as thyroid disease can lead to diffuse hair loss. [1,16]

Scalp and Skin Conditions

Inflammatory conditions of the scalp can affect the environment for hair growth and contribute to increased hair loss or thinning. Persistent inflammation can disrupt the normal function of the hair follicle and can also lead to hair breakage, irritation, and mechanical stress from scratching. In these situations, hair loss is usually secondary to inflammation and external stress on the scalp, rather than a primary disorder of the hair follicle itself, and it often occurs only in a subset of patients, particularly those with scalp involvement and more severe disease. [6]

Common examples include:

• Psoriasis of the scalp
• Seborrheic eczema
• Atopic eczema
• Fungal infections, tinea capitis

These conditions can cause symptoms such as flaking, itching, redness, or tenderness of the scalp. Treating the underlying skin condition can often help reduce hair loss associated with inflammation. [6]

Mechanical and external factors

Not all hair loss occurs in the hair follicle. External stressors can cause hair to break or detach from the scalp. [6]

Examples include:

• Traction alopecia, caused by tight hairstyles, braids, ponytails, or hair extensions
• Chemical damage, such as bleaching, perms, or excessive heat styling
• Physical hair breakage and brittle hair

These forms of hair loss may resemble thinning hair, but are often caused by breakage of the hair shaft rather than actual loss of hair follicles. Gentler hair care can help prevent further damage. [6]

Other Causes of Hair Loss

Alopecia areata

Alopecia areata is an autoimmune condition in which the immune system attacks the hair follicles, leading to patchy hair loss. It usually presents as sudden, round, or oval bald patches, often without scaling. In some cases, it can progress to affect the entire scalp (alopecia totalis) or the entire body (alopecia universalis). [3]

Scarring alopecia, cicatricial alopecia

Scarring alopecias are a group of inflammatory conditions that destroy the hair follicle. Unlike non-scarring alopecia, regrowth is not possible once the hair follicle is permanently damaged. This category includes rare conditions such as lichen planopilaris, frontal fibrosing alopecia, and central centrifugal cicatricial alopecia. [12,13] Among these, frontal fibrosing alopecia, which was first described in 1994, has become the most common form of cicatricial alopecia encountered clinically. [19] It primarily affects postmenopausal white women and presents as receding of the frontal and parietal hairline, perifollicular erythema, loss of follicular openings, and loss of eyebrows. These features may occur independently of one another and in varying sequences over time. [23]

These conditions are less common but clinically important because early diagnosis and treatment can prevent further progression, since the associated destruction of the hair follicle is irreversible. Symptoms may include scalp pain, burning, redness, pustules, flaking, or smooth, shiny areas with hair loss. [12,13]

Anagen effluvium and chemotherapy-related alopecia

Anagen effluvium is an abrupt, usually diffuse pattern of hair loss characterized by the rapid shedding of hair in the active growth phase, and is often noticed as sudden hair loss on the scalp. It is classically associated with cytotoxic chemotherapy and usually begins within days to a few weeks after treatment starts. It can also affect areas of the body other than the scalp. [14]

Trichotillomania and Compulsive Hair-Pulling Behavior

Trichotillomania is a self-inflicted form of alopecia that typically presents as irregular or incomplete patches of broken hairs of varying lengths, often with a stubby appearance, and may involve the scalp, eyebrows, or eyelashes. The course is often intermittent or fluctuating, and episodes may be linked to stress or habitual hair pulling. Patients are not always aware of or willing to disclose this behavior. [15]

When Should You Seek a Medical Evaluation?

Mild hair loss is often normal. However, an evaluation by a healthcare professional is recommended if the hair loss is accompanied by:

• Rapid or significant hair thinning
• Patchy areas of hair loss
• Symptoms on the scalp, such as pain, a burning sensation, redness, or flaking
• Signs of scarring, such as smooth, shiny skin where hair used to grow
• Hair loss that occurs along with systemic symptoms, such as fatigue, weight change, or menstrual irregularities

A dermatologist can help identify the underlying cause of hair loss through a structured evaluation. This typically includes a medical history, a visual examination of the scalp, and a close inspection using magnification, known as trichoscopy. [5,6] A targeted medical history should include age at onset, progression over time, menopausal status, signs and symptoms of possible hyperandrogenism—such as hirsutism, irregular menstrual cycles, infertility, or cystic acne—current medications or dietary supplements that may have androgenic properties, as well as hair care habits. In addition, blood tests are often useful, including assessment of androgen levels in those with signs of hyperandrogenism, as well as a complete blood count, iron and vitamin D levels, and thyroid function, to identify the key factors that may influence regrowth. [19]

Hair growth patterns can be assessed by examining the distribution of hair in different growth phases—known as a trichogram—as well as through simple clinical tests such as the hair pull test or the hair pluck test. [5,6]

Further investigations may include laboratory tests and, in selected cases, a small skin sample—a biopsy—for microscopic examination. [5,6]

Conclusion

Hair shedding is a normal part of the growth cycle. However, when the balance between hair shedding and regrowth is disrupted, visible hair thinning can occur. The causes range from genetics and hormones to nutritional status and scalp health. Understanding these biological processes provides a solid foundation for interpreting changes and determining when further medical evaluation is necessary.

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