Alopecia has dozens of possible causes, ranging from genetics and hormones to stress, medications, and even the way you style your hair. The type of hair loss you experience, where it appears on your scalp, and how quickly it develops all point toward different underlying triggers. Understanding these causes is the first step toward figuring out which one applies to you.
Genetics and Hormones: The Most Common Cause
Pattern hair loss, known clinically as androgenetic alopecia, is by far the most widespread form. It affects both men and women, though the pattern differs. Men typically see a receding hairline and thinning at the crown, while women notice a gradual widening of their part line.
The driving force is a hormone called DHT (dihydrotestosterone), which is converted from testosterone by an enzyme in your hair follicle cells. DHT binds to receptors in the follicle and triggers a process called miniaturization: the follicle gradually shrinks with each growth cycle, producing thinner, shorter, lighter hairs until it eventually stops producing visible hair altogether. People with pattern hair loss tend to have more of these receptors in the frontal and crown areas of their scalp, which is why those zones thin first while the sides and back remain full.
DHT also disrupts the signaling pathways that tell follicle stem cells to regenerate. It ramps up proteins that push hair cells toward programmed death while suppressing the signals that keep them renewing. This is why the process is progressive: without intervention, the miniaturization continues over years or decades.
Hormonal Shifts in Women
Women face additional hormonal triggers beyond the standard androgenetic pattern. Polycystic ovary syndrome (PCOS), which raises androgen levels, is a well-known contributor to hair thinning. Menopause creates a similar situation from the opposite direction: as estrogen levels drop, the balance shifts in favor of androgens, even if androgen levels themselves haven’t changed. Research suggests that it’s the ratio of estrogens to androgens, rather than the absolute level of either hormone, that drives female pattern hair loss.
Pregnancy and childbirth also trigger hair loss, though through a different mechanism. The hormonal surge during pregnancy keeps more hairs in their growth phase than usual. After delivery, those hairs all shift into the resting phase at once, leading to noticeable shedding a few months postpartum. This is temporary and resolves on its own.
Autoimmune Attack: Alopecia Areata
Alopecia areata produces distinctive round, smooth patches of hair loss and affects roughly 2% of people at some point in their lives. It’s an autoimmune condition where your immune system mistakenly identifies hair follicles as a threat.
The key players are a specific type of immune cell: cytotoxic T cells carrying a surface marker called NKG2D. These cells are both necessary and sufficient to cause the disease, meaning they alone can trigger it without help from other immune cells. They infiltrate the skin around hair follicles and release inflammatory signals, particularly interferon-gamma, which creates a hostile environment that shuts down hair growth. The follicles also begin producing a protein called IL-15 on their surface, which attracts even more of these destructive T cells, creating a self-reinforcing cycle of inflammation.
The good news is that alopecia areata doesn’t destroy the follicle permanently. The stem cells survive beneath the surface, which is why hair can regrow even after years of loss. Some people experience a single episode that resolves, while others have recurring patches or, in rarer cases, lose all scalp hair or all body hair.
Stress-Related Shedding
Telogen effluvium is the medical term for widespread hair shedding triggered by a physical or emotional shock. Unlike pattern hair loss, which targets specific areas, telogen effluvium causes diffuse thinning all over the scalp. The hallmark feature is a delay: hair loss typically begins two to three months after the triggering event, which often makes it hard to connect the two.
Common triggers include:
- High fever or severe infections
- Major surgery or physical trauma
- Childbirth
- Significant psychological stress
- Crash diets, particularly those low in protein
- Thyroid disorders, both overactive and underactive
- Stopping birth control pills
What happens biologically is that the stressor pushes a large number of hair follicles out of their active growth phase and into the resting phase simultaneously. About three months later, those resting hairs fall out all at once. The acute form resolves within six months as follicles cycle back into growth. Chronic telogen effluvium, lasting longer than six months, is less common and usually tied to an ongoing issue like untreated thyroid disease or persistent nutritional deficiency.
Nutritional Deficiencies
Your hair follicles are among the fastest-dividing cells in your body, which makes them sensitive to nutritional shortfalls. Three deficiencies show up most consistently in research on hair loss.
Iron is the most studied. Low ferritin (your body’s iron storage protein) is linked to diffuse shedding, and studies of women with chronic hair loss have found a meaningful subset with ferritin levels at or below 20 micrograms per liter. The connection is strong enough that checking ferritin is a standard part of any hair loss workup.
Zinc deficiency is particularly associated with alopecia areata and telogen effluvium. Patients with these conditions are more likely to have serum zinc levels below 70 micrograms per deciliter compared to healthy controls. Zinc plays a role in cell division and immune regulation, both of which matter for hair growth.
Vitamin D deficiency has also been linked to multiple types of hair loss, though the exact mechanism is still being clarified. Many dermatologists now include vitamin D alongside ferritin in their initial blood panels for hair loss patients.
Medications That Cause Hair Loss
Drug-induced hair loss works through two main pathways. The more dramatic one, seen with chemotherapy drugs, involves a direct shutdown of rapidly dividing cells in the hair follicle, causing hair to fall out during its active growth phase. This type is usually temporary and reverses after treatment ends.
The subtler form mimics telogen effluvium: the medication pushes follicles into their resting phase prematurely, leading to diffuse shedding weeks to months later. Several common drug classes can do this, including blood pressure medications (beta-blockers and calcium channel blockers), antidepressants, retinoids used for acne or skin conditions, and NSAIDs like ibuprofen.
Blood thinners deserve special mention because their association with hair loss is underappreciated. Warfarin has generated over 700 reports of alopecia in global safety databases, while newer blood thinners like rivaroxaban have accumulated over 600 reports. Older heparin-based blood thinners may cause hair loss in as many as half of users. If you’ve started a blood thinner and notice increased shedding a few months later, the medication is a likely culprit.
Hairstyling and Traction Alopecia
Traction alopecia results from sustained mechanical pulling on hair follicles. It’s caused by tight hairstyles: braids, cornrows, tight ponytails, weaves, extensions, dreadlocks, and even tightly wrapped scarves. Chemical relaxers and frequent heat styling compound the damage by weakening the hair shaft, making follicles more vulnerable to the tension.
In the early stages, you might notice small bumps around the hairline or temples, tenderness when your hair is pulled back, or broken hairs along the areas of greatest tension. At this point, the damage is reversible if you change your styling habits. But with continued traction over months or years, the follicle undergoes a predictable progression: inflammation gives way to miniaturization, the tissue around the follicle develops scar tissue, and eventually the stem cells that regenerate hair are permanently destroyed. Once scarring sets in, the hair loss is irreversible. The follicles are replaced by fibrous tracts that can no longer produce hair.
The margins of the hairline and the temples are the most commonly affected areas because they bear the most tension from pulled-back styles.
Smoking and Environmental Damage
Smoking accelerates hair loss through multiple overlapping mechanisms. Nicotine constricts the tiny blood vessels that feed hair follicles, reducing their oxygen and nutrient supply. Cigarette smoke generates free radicals that damage the DNA inside follicle cells, both in the nucleus and in the mitochondria that power the cell. These free radicals also damage cell membranes, triggering premature cell death.
Beyond the direct toxic effects, smoking creates chronic low-grade inflammation around hair follicles. Over time, this inflammation leads to the same kind of scarring tissue buildup seen in traction alopecia. Nicotine can also overstimulate certain receptors on skin cells, eventually burning them out and activating cell death pathways. On the hormonal front, smoking appears to shift the balance toward androgens by interfering with estrogen metabolism, essentially amplifying the same hormonal mechanism behind pattern hair loss. One study found that male smokers had nearly twice the odds of baldness compared to nonsmokers.
Scarring Alopecia
Scarring (cicatricial) alopecia is a group of conditions where inflammation destroys the hair follicle and replaces it with scar tissue. Unlike most other forms of hair loss, this destruction is permanent. Causes include certain autoimmune skin conditions like lichen planopilaris and discoid lupus, severe bacterial or fungal infections of the scalp, and burns or other physical trauma.
The distinguishing feature is that the scalp in affected areas often looks shiny and smooth, with no visible follicle openings. A scalp biopsy, taken from the active edge of a spreading patch, is usually needed to identify the specific type and guide treatment. The goal with scarring alopecia is to stop the inflammation before it destroys more follicles, since regrowth in already-scarred areas isn’t possible.
How Doctors Figure Out the Cause
Diagnosing the specific type of alopecia typically starts with a close look at the pattern and location of hair loss. A hair pull test, where a doctor gently tugs on about 50 to 60 hairs, can indicate active shedding if more than five or six hairs come out easily. Trichoscopy (a magnified examination of the scalp) helps distinguish miniaturized hairs from broken ones and can reveal inflammation or scarring not visible to the naked eye.
Blood work usually covers thyroid function, iron and ferritin levels, vitamin D, zinc, and hormonal markers. In women, the workup often includes screening for PCOS and evaluating menstrual history. When the diagnosis is still unclear, or when scarring alopecia is suspected, a small scalp biopsy can provide a definitive answer by showing exactly what’s happening at the follicle level.