Alopecia, the medical term for hair loss, has dozens of possible causes ranging from genetics and hormones to autoimmune attacks, physical stress, and nutritional gaps. The type of hair loss you’re experiencing depends on which mechanism is at work, and the causes differ enough that identifying the right one is the first step toward effective treatment. Here’s a breakdown of the most common causes and what’s actually happening inside the hair follicle in each case.
Pattern Hair Loss: Hormones and Genetics
Androgenetic alopecia, commonly called male or female pattern baldness, is the most widespread form of hair loss. It affects roughly half of all men by age 50 and a significant percentage of women as well, particularly after menopause. The driving force is a hormone called dihydrotestosterone (DHT), which is converted from testosterone in the body. DHT binds to androgen receptors on hair follicles and, over time, causes them to shrink in a process called miniaturization. Each growth cycle produces a thinner, shorter hair until the follicle eventually stops producing visible hair altogether.
Genetics play a central role in who develops pattern hair loss and how severely. The key gene identified so far is the AR gene, which provides instructions for building androgen receptors. Variations in this gene make androgen receptors more easily activated by DHT, meaning even normal hormone levels can trigger accelerated hair thinning. The inheritance pattern is complex. It doesn’t follow a simple mother’s-side or father’s-side rule. Having a close relative with patterned hair loss increases your risk, but multiple genes and environmental factors are involved.
In men, this typically starts at the temples and crown. In women, it usually shows up as a widening part or overall thinning on top of the scalp, with the hairline staying intact.
Alopecia Areata: The Immune System Attacks Hair
Alopecia areata is an autoimmune condition where the body’s own immune cells turn against hair follicles. It often appears suddenly as smooth, round bald patches on the scalp, though it can affect eyebrows, eyelashes, and body hair too. In severe forms, it can cause total scalp hair loss (alopecia totalis) or complete body hair loss (alopecia universalis).
The underlying process starts when the hair follicle loses its “immune privilege,” a kind of biological shield that normally protects it from immune system surveillance. Once that shield collapses, immune cells recognize proteins on the hair follicle as foreign threats. A swarm of immune cells, primarily a type of white blood cell called CD8+ T cells, surrounds and infiltrates the follicle. In skin biopsies from people with alopecia areata, pathologists describe this clustering of immune cells around the follicle as a “swarm of bees.” The attack is fueled by inflammatory signaling molecules, particularly interferon-gamma, which is found at elevated levels in the blood of people with the condition. This signaling creates a self-reinforcing loop: it triggers more immune cells to arrive, which produce more inflammatory signals, which recruit even more immune cells.
The good news is that alopecia areata doesn’t permanently destroy follicles the way some other types do. The follicles remain alive but dormant, which is why hair can regrow spontaneously or with treatment. A class of medications called JAK inhibitors, which block the inflammatory signaling pathways driving the attack, has shown significant results in clinical trials. Oral versions of these drugs improved hair regrowth rates several-fold compared to placebo, and some patients achieved near-complete regrowth.
Telogen Effluvium: Stress-Related Shedding
If you’re finding clumps of hair in the shower or on your pillow two to three months after a major physical or emotional event, telogen effluvium is the likely cause. This type of hair loss happens when a large number of hair follicles are pushed prematurely into the resting (telogen) phase of the growth cycle, then shed all at once.
Common triggers include high fever, severe infections, childbirth, major surgery, significant psychological stress, thyroid disorders (both overactive and underactive), stopping birth control pills, and crash diets low in protein. Certain medications can also cause it, including retinoids, beta-blockers, calcium channel blockers, antidepressants, and some anti-inflammatory drugs. The hallmark of telogen effluvium is that delay: the stressor happens, and the shedding doesn’t start until roughly three months later. This gap often makes it hard to connect the two events.
Acute telogen effluvium typically resolves within six months once the trigger is removed or the body recovers. It causes diffuse thinning rather than bald patches, and the hair follicles aren’t damaged, so full regrowth is expected. In rare cases, the shedding can become chronic if the underlying trigger persists.
Traction Alopecia: Damage From Tight Hairstyles
Traction alopecia is caused by repeated physical pulling on the hair. The highest-risk hairstyles include tight ponytails, buns, cornrows, braids, dreadlocks, and hair extensions or weaves. The risk increases when these styles are combined with chemical hair relaxers. It’s not limited to any single cultural practice. Sikh men who twist their hair into a frontal knot under a tightly wrapped turban develop hair loss along the tension line, and some also lose beard hair from tying it into tight knots. Ballerinas, nurses who pin their caps in the same spot daily, and anyone who consistently pulls hair taut in one direction can develop it.
The biological process starts with mechanical damage to the follicle from sustained tension. This triggers inflammation, which initially shows up as redness, small bumps, or pustules along the hairline or wherever the pull is strongest. At this stage, the damage is reversible. If the pulling continues over months or years, the repeated cycles of damage and inflammation eventually destroy the follicle and replace it with scar tissue. Once scarring occurs, the hair loss is permanent. The transition from reversible to irreversible makes early recognition critical. The most common warning signs are a receding hairline at the temples or edges, tenderness in the scalp, and tiny bumps where hair is under tension.
Scarring Alopecia: Permanent Follicle Destruction
Scarring (cicatricial) alopecia is a group of conditions where inflammation destroys the hair follicle entirely and replaces it with scar tissue. Unlike most other forms of hair loss, this type is irreversible because the stem cells and oil glands in the middle of the follicle, both essential for producing new hair, are permanently eliminated.
Primary scarring alopecia is caused by inflammatory or autoimmune disorders that directly target the follicle. Conditions like lichen planopilaris and frontal fibrosing alopecia fall into this category. Secondary scarring alopecia results from external damage to the skin, such as burns, radiation, or severe infections that destroy follicles as collateral damage. The affected scalp often looks smooth and shiny, with no visible follicle openings. Because the destruction is permanent, the treatment focus is on stopping the inflammation before more follicles are lost rather than regrowing what’s already gone.
Nutritional Deficiencies That Thin Hair
Your hair follicles are metabolically demanding, and they’re among the first structures to suffer when key nutrients run low. Iron deficiency is one of the best-studied nutritional causes of hair loss. In one study, people with diffuse, nonscarring hair loss had average ferritin levels (ferritin is the protein that stores iron) of about 15 ng/mL, compared to roughly 25 ng/mL in people without hair loss. Vitamin D levels followed a similar pattern, averaging 14 ng/mL in the hair loss group versus 17 ng/mL in controls.
These deficiencies don’t always cause the dramatic shedding you’d see with telogen effluvium. More often, they produce a gradual, diffuse thinning that’s easy to dismiss until it becomes noticeable. Other nutrients linked to hair health include zinc, biotin, and protein. Crash diets and restrictive eating patterns are a common culprit, particularly when protein intake drops too low. A blood test can identify most nutritional gaps, and correcting the deficiency typically allows hair to recover over several months as new growth cycles begin.
Medications That Cause Hair Loss
Drug-induced hair loss is more common than many people realize, and it can happen through two different mechanisms. Some medications, particularly chemotherapy drugs, attack rapidly dividing cells and interrupt hair growth directly, causing hair to fall out within weeks. This type (anagen effluvium) typically causes rapid, widespread shedding.
More commonly, medications trigger a telogen effluvium pattern, where hair shifts into the resting phase and sheds two to three months after starting the drug. The medication classes most frequently associated with this include blood thinners, retinoids (used for acne and skin conditions, with higher doses carrying greater risk), beta-blockers and calcium channel blockers for blood pressure, antidepressants, and anti-inflammatory drugs. Hormonal medications, including stopping or starting birth control, can also shift the hair cycle. In most cases, hair regrows after the medication is discontinued or the dose is adjusted, though this can take several months.