People get alopecia for several distinct reasons, and the cause depends on which type they have. The most common forms are pattern hair loss driven by hormones and genetics, autoimmune alopecia where the immune system attacks hair follicles, and stress-triggered shedding caused by illness or nutritional deficiencies. Less common types involve physical tension from hairstyles, scarring from inflammatory skin diseases, or side effects from medications. Understanding which type is at play matters because the causes, outlook, and options differ significantly.
Autoimmune Alopecia Areata
Alopecia areata is the form most people picture when they hear the word “alopecia”: round, smooth patches of hair loss that appear suddenly. It affects roughly 0.1% of the global population over a lifetime and is most common in adults between 19 and 50, though it can start in childhood.
The root cause is an immune system malfunction. Hair follicles normally have a kind of biological shield that hides them from immune cells. In alopecia areata, that shield breaks down, exposing proteins inside the follicle that the immune system doesn’t recognize as “self.” Immune cells, particularly a type called CD8+ T cells, swarm around the exposed follicle and attack it. Biopsies of affected skin show clusters of these immune cells surrounding each follicle, along with elevated levels of inflammatory signals like interferon-gamma and interleukin-15.
Genetics play a significant role. Many of the genes linked to alopecia areata belong to a family called the human leukocyte antigen (HLA) complex, which helps the immune system distinguish the body’s own tissue from foreign invaders. Specific variations in these genes appear to make the immune system more likely to misidentify hair follicle proteins as threats. Genes involved in inflammation and immune regulation outside the HLA system are also implicated, which is why alopecia areata often co-occurs with other autoimmune conditions like thyroid disease or vitiligo.
The good news is that the follicles are not destroyed. They’re suppressed. About 80% of people with limited patches regrow hair within a year without treatment. When more than half the scalp is involved, the odds of spontaneous recovery drop to around 8%, but smaller patches (under 25% of the scalp) carry better than a 68% chance of resolving on their own. Relapse, however, can happen at any stage.
Hormonal Pattern Hair Loss
Androgenetic alopecia, the gradual thinning most people call “male pattern” or “female pattern” hair loss, is the most widespread form of alopecia worldwide. It runs in families and is driven by a hormone called dihydrotestosterone, or DHT.
Here’s how it works: an enzyme in the scalp converts testosterone into DHT, which then binds to receptors inside the cells at the base of genetically susceptible hair follicles. Once activated, those receptors trigger changes that shrink the follicle over time. The growth phase of the hair cycle gets shorter while the resting phase gets longer, so each new hair that grows in is thinner and shorter than the last. Over years, thick terminal hairs are gradually replaced by fine, nearly invisible “vellus” hairs, and eventually the follicle may stop producing visible hair altogether.
Not every follicle on the scalp is equally sensitive to DHT. That’s why pattern hair loss follows predictable shapes: a receding hairline and thinning crown in men, diffuse thinning along the part line in women. The follicles at the back and sides of the head are largely resistant, which is why those areas tend to keep their hair.
Stress and Nutritional Triggers
Telogen effluvium is a type of diffuse shedding that happens when a large number of hair follicles are pushed into their resting phase at the same time. Normally, only about 10% of your hair is in the resting phase at any given moment. With telogen effluvium, that percentage spikes, and two to three months after the triggering event, those hairs fall out in noticeable quantities.
Common triggers include:
- Physical stress: high fever, severe infections, major surgery, or childbirth
- Hormonal shifts: thyroid disorders (both overactive and underactive) or stopping birth control pills
- Nutritional deficiencies: restrictive diets, particularly those low in protein (your body needs 40 to 60 grams per day to support normal hair cycling), and iron deficiency
- Psychological stress: prolonged emotional distress or acute shock
- Medications: certain blood pressure drugs, antidepressants, retinoids, and anti-inflammatory medications
Telogen effluvium is almost always temporary. Once the triggering stressor resolves, new growth typically begins within a few months, though it can take six months to a year for hair density to look noticeably restored.
Medications That Cause Hair Loss
Drug-induced hair loss falls into two main patterns. The first is the telogen effluvium pattern described above, where medications push resting hairs to shed. Endocrine therapies (including antiestrogen drugs used in breast cancer treatment), certain antibody-based drugs, and some diabetes medications have all been linked to this type of shedding.
The second is anagen effluvium, where a drug halts active hair growth so abruptly that hair falls out within days or weeks. Chemotherapy is the most well-known cause. The hair follicle’s rapidly dividing cells are collateral damage in drugs designed to target fast-growing cancer cells. Other, rarer causes include the antibiotic isoniazid, the immunosuppressant cyclosporine, and heavy metal exposure. In most cases, hair regrows once the medication is stopped.
Hairstyles and Physical Tension
Traction alopecia results from hairstyles that pull on follicles repeatedly over months or years. Cornrows, locs, tight braids, tightly pulled ponytails or buns, and hair extensions or weaves (especially on chemically relaxed hair) are the most common culprits. Rollers worn to bed regularly and the constant friction of hats or head coverings can also contribute.
There’s no precise measurement for how much tension is too much, but a useful rule from the American Academy of Dermatology: if your hairstyle feels painful, it’s too tight. The hair loss typically appears along the hairline or wherever the tension is greatest. Caught early, traction alopecia reverses once the pulling stops. Left too long, the follicles scar over, and the loss becomes permanent.
Scarring Alopecia
Scarring (cicatricial) alopecia is the most serious category because the hair follicles are permanently destroyed and replaced with scar tissue. It comes in two forms.
Primary scarring alopecia occurs when an inflammatory or autoimmune disorder directly targets the follicles. The most common subtype is lichen planopilaris, which primarily affects women over 50 and causes scaling, burning, and itching along with hair loss. Chronic cutaneous lupus erythematosus, a skin manifestation of lupus, can also destroy follicles, often alongside rashes and sun sensitivity. Folliculitis decalvans, possibly driven by a staph bacterial infection, causes ongoing inflammation that damages follicles. Central centrifugal cicatricial alopecia (CCCA), which disproportionately affects Black women, may be linked to chemical relaxers, heat styling, or tight styles, though some research suggests a hereditary component.
Secondary scarring alopecia is a side effect of skin damage from burns, radiation therapy, infections, or tumors. In both forms, the goal of treatment is to stop the inflammation before more follicles are lost, since scarred follicles cannot regrow hair.
Why Some People Are More Vulnerable
For most types of alopecia, genetics load the gun and environment pulls the trigger. In androgenetic alopecia, you inherit follicles that are sensitive to DHT. In alopecia areata, you inherit immune system genes that make autoimmune misfires more likely, but the disease often needs a triggering event, such as a viral infection or period of stress, to become active. Telogen effluvium, by contrast, can happen to anyone whose body encounters a strong enough stressor, regardless of genetic background.
Age, sex, and hormonal status all shift the odds. Pattern hair loss accelerates after menopause in women as estrogen levels drop. Alopecia areata peaks during the most active working years. Traction alopecia correlates directly with cultural hairstyling practices. In every case, the specific combination of inherited susceptibility, hormonal environment, and external triggers determines whether and when hair loss appears.