Alopecia has no single cause. It can result from genetics, autoimmune attacks on hair follicles, hormonal shifts, nutritional gaps, physical stress on the scalp, medications, or a combination of these factors. The type of hair loss you experience depends on which underlying cause is at work, and identifying it is the first step toward the right treatment.
Hair loss falls into two broad categories: non-scarring and scarring. In non-scarring types, the hair follicle stays intact and regrowth is possible. In scarring types, inflammation destroys the stem cells and oil glands at the top of the follicle, making the loss permanent. Most common forms of hair loss are non-scarring.
Genetic Hair Loss and Hormones
The most widespread form of alopecia is pattern hair loss, also called androgenetic alopecia. It affects both men and women and is driven by a hormone called DHT (dihydrotestosterone). DHT binds to receptors in genetically sensitive hair follicles, causing them to shrink over time. As follicles miniaturize, the thick terminal hairs they once produced are gradually replaced by finer, shorter strands until the follicle produces almost nothing visible. In men, this typically shows up as a receding hairline and thinning crown. In women, it appears as a widening part and overall thinning across the top of the scalp.
Your genes determine which follicles are sensitive to DHT and how early the process begins. Blocking DHT’s activity at the follicle can partially reverse miniaturization, which is the basis for the most common medical treatments for pattern hair loss.
Autoimmune Attack in Alopecia Areata
Alopecia areata affects roughly 2% of the global population, with a lifetime risk between 1.7% and 2.1%. It’s more common in children, young adults, and women. Unlike pattern hair loss, alopecia areata isn’t about hormones. It’s an autoimmune condition where the body’s own immune cells attack healthy hair follicles.
Hair follicles normally have a form of immune protection that shields them from the body’s surveillance system. In alopecia areata, that protection breaks down. Once exposed, the follicle’s proteins are flagged as foreign, and immune cells swarm the area. Biopsies of affected skin show dense clusters of T cells and other inflammatory cells packed around the follicle base, a pattern pathologists describe as a “swarm of bees.” These immune cells halt hair growth and push follicles into a dormant state, producing the characteristic smooth, round bald patches that can appear suddenly on the scalp, eyebrows, beard, or anywhere on the body.
Because the follicle itself isn’t destroyed, regrowth is possible once the immune attack subsides. But the condition is unpredictable. Some people have a single episode, while others experience repeated cycles of loss and regrowth over years.
Stress-Related Shedding (Telogen Effluvium)
Telogen effluvium is a temporary but often alarming form of hair loss triggered by a physical or emotional shock to the body. The trigger pushes a large number of hair follicles into their resting phase all at once. Two to three months later, those hairs fall out in clumps, often noticed in the shower drain or on a pillowcase.
Common triggers include:
- High fever or severe infection
- Childbirth
- Major surgery
- Significant psychological stress
- Thyroid disorders (both overactive and underactive)
- Stopping birth control pills
- Crash diets low in protein
The shedding phase typically lasts three to six months. After that, new growth appears in the affected areas, and most cases resolve fully within six to eight months once the underlying trigger is addressed. No specific treatment is usually needed beyond correcting the cause.
Hormonal Shifts During Menopause and Postpartum
Estrogen supports hair follicle health, so any significant drop in estrogen levels can change how hair grows. After menopause, ovarian estrogen production stops, and the relative balance of hormones shifts toward androgens. This combination directly affects hair follicles. Women often notice decreased hair density, thinner individual strands, and changes in texture. The follicle’s blood supply and metabolism also decline without estrogen’s protective effects, compounding the problem.
Postpartum hair loss works through a related mechanism. During pregnancy, elevated estrogen keeps more follicles in their active growth phase than usual. After delivery, estrogen levels drop sharply, and all those follicles that were held in place enter the shedding phase at once. This is a form of telogen effluvium and typically resolves on its own within several months.
Nutritional Deficiencies
Hair follicles are metabolically active and require a steady supply of nutrients. When key vitamins and minerals are lacking, hair growth slows or stops.
Iron is one of the best-studied connections. For optimal hair regrowth, ferritin levels (your body’s iron stores) generally need to be above 40 to 70 ng/mL, with some research suggesting levels above 70 ng/mL produce the best results. Many people with unexplained hair thinning have low ferritin even when their standard blood count looks normal.
Vitamin D deficiency, defined as levels below 20 ng/mL, has also been linked to hair loss. The optimal level for overall health is around 30 ng/mL. Zinc plays a role in cell division within the follicle, and low levels can contribute to shedding. Biotin deficiency is frequently discussed online, but it’s actually rare in people eating a varied diet, and standard blood tests for biotin aren’t reliable markers of true deficiency.
Medications That Cause Hair Loss
A wide range of drugs can trigger hair loss through different mechanisms. The most dramatic example is chemotherapy, which halts rapidly dividing cells, including those in the hair follicle. This causes anagen effluvium, where hair falls out within days or weeks of treatment. Other medications linked to this rapid type of shedding include certain antibiotics used for tuberculosis and drugs associated with heavy metal toxicity.
More commonly, medications cause a slower telogen effluvium pattern, with shedding appearing two to three months after starting the drug. Classes implicated include hormonal therapies, blood pressure medications (beta-blockers and calcium channel blockers), antidepressants, anti-inflammatory drugs, and some diabetes medications. In rare cases, certain biologic drugs used for autoimmune conditions can paradoxically trigger alopecia areata. And a small number of targeted cancer therapies have been linked to scarring alopecia, which causes permanent follicle damage.
If you notice increased shedding after starting a new medication, the timing alone can be a strong clue. Hair typically recovers after the drug is stopped or switched.
Traction Alopecia From Hairstyles
Traction alopecia results from repeated physical tension on the hair root. It’s entirely preventable but often overlooked until the damage is significant. Hairstyles most commonly responsible include tight cornrows, locs, braids, buns, ponytails, and hair extensions or weaves, especially when applied to chemically relaxed hair. Even wearing rollers to bed regularly or pulling hair back tightly under a hat or headscarf can contribute.
Ballet dancers, military personnel, and anyone whose job requires tightly pulled-back hair face higher risk. The hair loss typically appears along the hairline and temples, where tension is greatest. Early on, the condition is non-scarring and reversible. But years of sustained pulling can permanently destroy follicles, converting it into a scarring form of hair loss. The key is recognizing the pattern early and loosening the hairstyle before permanent damage sets in.
Scarring Alopecia
Scarring (cicatricial) alopecias are less common but more consequential. In all forms, inflammation targets the upper portion of the hair follicle where stem cells reside. Once those cells are destroyed, the follicle cannot regenerate, and the skin scars over.
These conditions are classified by the type of inflammatory cell driving the damage: lymphocytes, neutrophils, or a mix of both. Two of the more recognized forms are frontal fibrosing alopecia, which causes a slowly receding hairline with scarring, and central centrifugal cicatricial alopecia (CCCA), the most common cause of scarring hair loss in Black women. CCCA typically begins at the crown and expands outward.
Because scarring alopecia causes permanent loss, early diagnosis matters. The goal of treatment is to stop the inflammation before more follicles are lost, since regrowth in already-scarred areas isn’t possible.