Alopecia isn’t a single condition. It’s an umbrella term for hair loss, and you can “get” it through several distinct pathways: your immune system attacking hair follicles, genetics making follicles shrink over time, physical tension on the scalp, nutritional gaps, medication side effects, or a delayed reaction to bodily stress like surgery or childbirth. About 1 in 50 people will develop alopecia areata alone over their lifetime, and pattern baldness is even more common. Here’s how each type happens.
Autoimmune Attack on Hair Follicles
Alopecia areata, the type most people mean when they say “alopecia,” is an autoimmune condition. Your immune system mistakenly identifies hair follicle cells as a threat and sends specialized white blood cells to destroy them. The result is smooth, round patches of hair loss that can appear suddenly on the scalp, beard, eyebrows, or anywhere on the body.
Normally, hair follicles have a form of immune protection that keeps them hidden from the body’s defenses. In alopecia areata, that protection breaks down. A specific type of immune cell, called a CD8+ T cell, infiltrates the follicle and releases inflammatory signals that shut down hair growth. Those signals trigger the follicle to produce even more chemical messengers that attract additional immune cells, creating a feedback loop that amplifies the damage. Other immune players pile on too, including cells that release large amounts of inflammatory compounds directly into the skin around the follicle.
No one fully understands what flips this switch. It tends to run in families, and people with other autoimmune conditions (thyroid disease, vitiligo, type 1 diabetes) are at higher risk. Emotional or physical stress can sometimes precede a first episode, though stress alone doesn’t cause it. Some cases resolve on their own. A study of patients with rapid, widespread alopecia areata found that nearly all experienced close to complete regrowth within six months, regardless of treatment. But the condition is unpredictable, and some people experience repeated episodes or permanent loss.
Genetics and Hormone-Driven Hair Loss
Androgenetic alopecia, commonly called male or female pattern baldness, is the most widespread form of hair loss. It’s driven by a combination of inherited genes and a hormone called dihydrotestosterone, or DHT. Your body produces DHT by converting testosterone through an enzyme in the skin. Everyone has some DHT circulating, but people with androgenetic alopecia have follicles that are genetically programmed to be sensitive to it.
When DHT binds to receptors on these sensitive follicles, it shortens the hair’s growth cycle and causes the follicle to physically shrink, a process called miniaturization. Over successive growth cycles, the follicle produces thinner, shorter, lighter hairs until it eventually stops producing visible hair altogether. In men, this typically shows up as a receding hairline and thinning at the crown. In women, it usually appears as diffuse thinning along the part line while the hairline stays intact.
Because the underlying cause is genetic sensitivity to a normal hormone, this type of hair loss is progressive. It doesn’t arrive all at once. Most people notice gradual thinning over years or decades.
Physical Tension and Styling Damage
Traction alopecia comes from repeated pulling on the hair. It’s entirely mechanical: the follicle gets damaged not by hormones or immune cells, but by sustained physical force. People who regularly wear tight braids, cornrows, ponytails, buns, hair extensions, or dreadlocks are most at risk. The added weight of long braids or extensions compounds the problem.
It’s not limited to hairstyles. Nurses who wear tightly pinned caps, motorcyclists and skiers who spend long hours in helmets, Sikh men and women who tie hair tightly under turbans, and women who wear hijabs in ways that pull the hairline can all develop it over time. Chemical treatments like relaxers, dyes, or straighteners weaken the hair shaft, and when combined with tight styles, the damage accelerates.
Early traction alopecia is reversible if you change the hairstyle. But repeated tension over months or years scars the follicle permanently, and scarred follicles cannot regrow hair. The hair loss typically appears along the hairline, temples, or wherever the pulling is strongest.
Stress, Illness, and Delayed Shedding
Telogen effluvium is a temporary but alarming form of hair loss triggered by a shock to the body. Common triggers include high fever, severe infection, major surgery, childbirth, extreme psychological stress, thyroid disorders (both overactive and underactive), crash diets low in protein, and stopping birth control pills. Certain medications can also cause it, including some blood pressure drugs, anti-inflammatory medications, antidepressants, and retinoids.
The confusing part is the delay. The trigger pushes a large number of hair follicles into their resting phase all at once. Those hairs then fall out two to three months later, when the follicle releases the resting hair to start a new growth cycle. So if you notice sudden, diffuse thinning, the cause is often something that happened to your body roughly three months earlier. A dermatologist diagnosing telogen effluvium will typically ask about events from that timeframe.
Acute telogen effluvium usually resolves within six months once the underlying trigger is addressed. The hair comes back because the follicles themselves are undamaged. They were just temporarily knocked off schedule.
Nutritional Deficiencies
Hair follicles are among the fastest-dividing cells in your body, which makes them sensitive to nutritional shortfalls. Three deficiencies are most consistently linked to hair loss: iron, vitamin D, and zinc.
Low iron is the most studied. Hair loss specialists generally recommend ferritin (the body’s iron storage marker) levels above 50 to 70 on a standard blood test, which is higher than the threshold used to diagnose anemia. You can have iron levels that are technically “normal” but still too low to support healthy hair growth. Low vitamin D has been associated with both telogen effluvium and female pattern hair loss, with lower levels correlating with more severe shedding. Studies of patients with various types of hair loss, including alopecia areata, have found statistically lower zinc levels compared to people without hair loss.
Deficiency-related shedding typically looks like telogen effluvium: diffuse thinning rather than patches or bald spots. Correcting the deficiency through diet or supplementation usually allows regrowth, though it can take several months for new hair to become visible.
Medications and Chemotherapy
Some drugs cause hair loss by directly poisoning hair follicle cells during their active growth phase. This is called anagen effluvium, and it’s most associated with chemotherapy. Because chemo drugs target rapidly dividing cells throughout the body, hair matrix cells get caught in the crossfire. Hair typically begins falling out within two weeks of starting treatment and progresses rapidly over the next one to two months. The loss can be severe, affecting the scalp, eyebrows, eyelashes, and body hair.
Not all chemo drugs cause the same degree of hair loss. Some, like certain targeted therapies, cause minimal shedding, while others are almost guaranteed to cause significant loss. Anti-clotting medications are another class linked to hair thinning, though the mechanism and severity differ from chemotherapy. In most cases, hair regrows after the medication is stopped, because the follicle itself isn’t permanently destroyed.
How Dermatologists Tell Them Apart
Because these types of hair loss look different under close examination, a dermatologist can usually narrow down the cause during a physical exam. They’ll check your scalp for inflammation, redness, sores, or scarring. Scarring is a critical distinction: non-scarring types like alopecia areata and telogen effluvium leave follicles intact and are potentially reversible, while scarring types (from traction, burns, or certain inflammatory conditions) permanently destroy follicles.
If the pattern isn’t clear from a visual exam, a scalp biopsy can help. A small tissue sample is examined under a microscope to reveal whether the follicles are inflamed, miniaturized, scarred, or in an abnormal resting phase. Biopsies are especially useful for distinguishing between different types of scarring alopecia, where the treatment approach varies significantly. Blood work can also help identify thyroid problems, nutritional deficiencies, or hormonal imbalances contributing to the loss.