Alopecia isn’t a single condition you “catch.” It’s an umbrella term for hair loss, and different types have very different causes, from your immune system attacking hair follicles to tight hairstyles physically damaging them over time. Some causes are genetic and unavoidable, others are triggered by stress, medications, or nutritional gaps. Understanding what drives each type helps clarify who’s at risk and why.
Alopecia Areata: When Your Immune System Turns on Hair Follicles
Alopecia areata is an autoimmune condition where a specific type of immune cell, cytotoxic T cells, infiltrates the hair follicle and destroys it. These T cells produce an inflammatory signal that collapses the follicle’s natural immune protection, creating a feedback loop: the more the follicle is exposed, the more immune cells swarm in. The global prevalence sits between 0.1% and 2.1%, with a lifetime risk near 2%. It affects women at a higher rate than men, and among children and adolescents, those aged 15 to 19 carry the heaviest burden.
Nobody fully controls whether they develop alopecia areata. Genetic susceptibility plays a central role. Genome-wide studies have identified specific immune-related genes that make certain people’s T cells more likely to recognize hair follicles as a threat. If you have a family history of autoimmune diseases (not just hair loss, but conditions like thyroid disease or vitiligo), your risk is elevated. Triggers that tip the immune system into action can include viral infections, major physical stress, or emotional trauma, but many people develop patches of hair loss with no identifiable trigger at all.
Pattern Baldness: Genetics and Hormones
Androgenetic alopecia, the most common form of hair loss, is driven by a hormone called DHT (dihydrotestosterone). Your body converts testosterone into DHT using an enzyme in the scalp. In people genetically predisposed to pattern baldness, hair follicles in certain areas have more androgen receptors and produce higher local levels of DHT. The hormone essentially chokes off blood supply to the follicle by killing tiny blood vessels in the tissue surrounding it. Over time, the follicle shrinks, produces thinner and shorter hairs, and eventually stops growing visible hair altogether.
The first gene linked to pattern baldness sits on the X chromosome, which means you inherit it from your mother’s side. But that’s not the whole story. Researchers have identified risk genes on chromosomes 2, 3, 5, and 12 as well, with the strongest association on chromosome 2 in a gene involved in hair follicle signaling. This means pattern baldness can come from either parent. In women, a gene called CYP19A1, which controls how the body converts androgens to estrogen, has variants that increase risk of female pattern hair loss. Women with polycystic ovary syndrome (PCOS) often share a similar hormonal imbalance profile with men who develop early androgenetic alopecia, which is why thinning hair is a common PCOS symptom.
Stress and Illness: Telogen Effluvium
Telogen effluvium is a temporary, diffuse shedding that happens when a large number of hair follicles are simultaneously pushed into their resting phase. It typically shows up two to three months after the triggering event, which can make it hard to connect cause and effect. Common triggers include high fevers, severe infections, major surgery, significant trauma, and crash dieting. Postpartum hormone shifts, particularly the drop in estrogen after delivery, are one of the most recognized causes.
Thyroid dysfunction is another major contributor. Thyroid hormone receptors exist directly on the outer root sheath of hair follicles and help regulate how often follicles cycle into active growth. Hypothyroidism slows that cycle down, while hyperthyroidism produces abnormally thin hairs. Low protein intake, iron deficiency, and heavy metal exposure can also push follicles into premature rest.
Nutritional Deficiencies That Thin Your Hair
Iron, zinc, vitamin D, and B vitamins are the micronutrients most consistently linked to hair loss. In a large cross-sectional study, people diagnosed with hair loss had lower median zinc levels (96 µg/dL compared to 99 µg/dL in controls) and lower ferritin, the protein that stores iron (median of 30 versus 33). Their hemoglobin levels were also slightly lower. These differences are modest on an individual level, but across a population they’re statistically significant and point to a real relationship.
Iron deficiency doesn’t have to reach the point of full-blown anemia to affect your hair. Ferritin can be technically “normal” on a lab report while still being low enough to contribute to shedding. If you’re experiencing unexplained hair loss, having your ferritin, zinc, vitamin D, and thyroid levels checked gives you a practical starting point. Correcting deficiencies through diet or supplementation often slows or reverses this type of loss.
Medications That Cause Hair Loss
Drug-induced hair loss is more common than most people realize, and it usually takes the form of telogen effluvium rather than permanent baldness. Beta-blockers (used for blood pressure and heart conditions) are among the most frequent culprits. Roughly 15% of patients taking lithium develop noticeable hair thinning. Valproic acid, used for seizures and mood disorders, is the most commonly implicated anticonvulsant.
All anticoagulant (blood-thinning) drugs can trigger shedding, as can antithyroid medications. Oral contraceptives can cause hair loss both while you’re taking them and after stopping. Retinoids, including high-dose vitamin A supplements, are well-documented causes. Less common but documented triggers include certain cholesterol-lowering drugs, NSAIDs like ibuprofen (in a small percentage of users), and cimetidine, an older heartburn medication. If you notice increased shedding after starting a new medication, the timing is worth flagging with your prescriber.
Traction Alopecia: Physical Damage From Hairstyling
Traction alopecia develops when hairstyles repeatedly pull on the same follicles over months or years. The highest-risk styles are tight buns, ponytails, weaves with extensions, cornrows, and dreadlocks. One study found that cornrows nearly tripled the risk. The danger compounds when tight styling is combined with chemical relaxers, which weaken the hair shaft and make it more vulnerable to breakage and follicle damage.
The critical thing to understand about traction alopecia is that it’s reversible in early stages but becomes permanent if the tension continues. Follicles that are repeatedly traumatized eventually scar over and stop producing hair entirely. Practical steps to reduce risk include alternating hairstyle direction and tightness, avoiding leaving braids or weaves in for more than two to four weeks, keeping dreadlocks trimmed to reduce weight on the follicle, skipping bonding glue for weaves, and replacing rubber bands with covered elastic ties. If you notice thinning along your hairline or at the sites of clips and partings, loosening your styling routine early can prevent permanent loss.
Scarring Alopecia: Permanent Follicle Destruction
Scarring (cicatricial) alopecia is the most serious category. Unlike other forms, the hair follicle is irreversibly destroyed and replaced by scar tissue. The most common cause is lichen planopilaris, an inflammatory condition that accounts for at least 10% of scarring alopecia cases. Frontal fibrosing alopecia, which causes a slowly receding hairline primarily in postmenopausal women, is a variant of the same condition.
Central centrifugal cicatricial alopecia, the second most common form in North America, starts at the crown and expands outward. It disproportionately affects Black women and may be related to both genetic susceptibility and hair care practices. Cutaneous lupus and a bacterial condition called folliculitis decalvans are other recognized causes. These conditions are classified by the type of inflammatory cell found in a scalp biopsy: lymphocyte-driven, neutrophil-driven, or mixed. Because scarring alopecia is permanent, early identification matters. The presence of visible follicle openings on the scalp (seen under magnification) helps distinguish reversible hair loss from scarring types where follicles have already been replaced by fibrous tissue.
What Determines Your Personal Risk
Most forms of alopecia result from a combination of genetic predisposition and environmental triggers. You can’t change your genes, but many of the modifiable risk factors are surprisingly mundane: maintaining adequate iron and zinc intake, managing thyroid function, avoiding extreme diets, choosing lower-tension hairstyles, and being aware of medication side effects. For autoimmune alopecia areata, the triggers are less controllable, but recognizing it early (typically as smooth, round patches without scarring) allows for treatment before it progresses. Pattern baldness is largely determined by your genetics and hormone profile, with onset and severity varying widely even among family members carrying the same risk genes.