Alopecia doesn’t come from a single source. It’s an umbrella term covering several distinct types of hair loss, each with its own origin. Some forms are genetic, some are autoimmune, some are triggered by stress or nutrition, and some result from physical damage to the hair follicle. Understanding which type you’re dealing with changes everything about what to expect and what can be done about it.
Pattern Baldness Starts With Your Genes
Androgenetic alopecia, the most common form of hair loss in men, women, and adolescents, is driven by a combination of genetics and hormones. It doesn’t follow a simple inheritance pattern like eye color. Instead, it’s polygenic, meaning dozens of genes across multiple chromosomes contribute to whether and when you’ll experience it. The most significant gene identified so far is the androgen receptor gene, located on the X chromosome. Since men inherit their X chromosome from their mother, this is the basis for the old idea that baldness “comes from your mother’s side,” but that’s only part of the picture.
Researchers have found significant risk genes on chromosomes 2, 3, 5, 6, 12, and 20, all of which contribute independently of the X chromosome. For early-onset baldness, roughly 56% of the genetic risk comes from non-sex chromosomes and 23% from the X chromosome. For later-onset baldness, the balance shifts even more toward those other chromosomes. In women specifically, variations in a gene on chromosome 15 that encodes an enzyme involved in hormone processing may influence susceptibility.
The hormonal mechanism is straightforward. Testosterone gets converted into a more potent form called DHT, which binds to receptors in genetically sensitive hair follicles. Over time, DHT shrinks those follicles and shortens the growth cycle, producing thinner, shorter hairs until the follicle eventually stops producing visible hair altogether. This process, called miniaturization, typically begins between ages 12 and 40 and progresses slowly.
Alopecia Areata Is an Immune System Attack
Alopecia areata looks completely different from pattern baldness. It usually appears as sudden, round patches of hair loss on the scalp, though in rare cases it can progress to total scalp hair loss or even loss of all body hair. It affects children, adolescents, and adults of both sexes. The lifetime risk is roughly 2%, making it the most common autoimmune disorder affecting appearance and quality of life. Women are about 1.4 times more likely to develop it than men, and peak incidence occurs in the 30 to 34 age range.
The cause is an immune system error. Hair follicles normally enjoy a kind of immune protection, a biological shield that prevents immune cells from recognizing and attacking them. In alopecia areata, that shield collapses. Inflammatory signals cause the follicle to become visible to the immune system, and two types of white blood cells swarm in. One type surrounds the follicle from the outside, while a more aggressive type infiltrates the follicle itself and produces chemicals that shut down hair production. These immune cells also release signaling molecules that recruit even more immune cells to the area, creating a self-reinforcing cycle of inflammation.
The encouraging part: the follicles themselves aren’t destroyed. About 34% to 50% of people with patchy alopecia areata experience spontaneous regrowth within one year without any treatment. However, the condition is chronic and tends to recur, so a single episode of regrowth doesn’t necessarily mean it’s gone for good. Newer treatments that block the specific inflammatory pathways involved (called JAK inhibitors) have shown real effectiveness by interrupting the immune signals that drive the attack.
Stress and Illness Trigger Temporary Shedding
Telogen effluvium is the medical term for widespread, temporary hair shedding triggered by a physiological shock to the body. The hair growth cycle has an active growth phase and a resting phase. Normally, only a small percentage of your hair is resting at any given time. When the body experiences significant stress, a large number of follicles get pushed into the resting phase simultaneously, and about three months later, all that hair falls out at once.
The triggers are varied but share a common thread of putting the body under strain: high fever, surgery, hemorrhage, severe infection, childbirth, chronic illness, and endocrine disorders can all set it off. Nutritional causes include severe protein deficiency, zinc deficiency, fatty acid deficiency, chronic caloric restriction, and crash dieting. Certain medications, including some anticonvulsants, antidepressants, blood thinners, oral contraceptives, and vitamin A derivatives, can also trigger it.
Post-childbirth shedding, sometimes called telogen gravidarum, is one of the most common forms. It typically begins about three months after delivery and resolves on its own. In general, telogen effluvium is self-limiting and lasts about six months once the triggering event is resolved. The follicles are fully intact, so complete regrowth is the expected outcome.
Nutritional Gaps That Affect Hair Growth
Iron deficiency is one of the most well-documented nutritional contributors to hair loss, even when it hasn’t progressed to full anemia. Dermatologists typically look at ferritin levels (the body’s stored iron) and aim for levels above 50 to 70 micrograms per liter to support healthy hair growth. Many people, particularly women with heavy menstrual periods, fall below this threshold without realizing it.
Zinc plays a critical role in hair follicle function, but it’s tricky to measure accurately. Blood zinc levels can appear normal even when the body is functionally deficient, meaning hair loss from low zinc can show up before a blood test catches it. Severe protein restriction, common in extreme dieting, also starves follicles of the building blocks they need to produce hair.
Physical Damage From Hairstyling
Traction alopecia comes from repeated mechanical pulling on hair follicles over months or years. It affects roughly one-third of women of African descent who regularly wear high-tension hairstyles, though it occurs across all ethnicities. Tight braids, cornrows, dreadlocks, weaves, hair extensions, and consistently tight ponytails or buns are the highest-risk styles.
The timelines for damage vary. A study of South Korean nurses found that wearing a cap secured with bobby pins for eight hours daily caused localized hair loss after an average of nearly 10 years. A ballerina who wore a tight bun four days a week developed symmetrical patches of temporal hair loss after 13 years. Turkish women who wore tight turbans for more than 10 years showed hair loss extending further across the scalp than those who wore them for shorter periods. Sikh men who begin wearing tightly knotted turbans in adolescence can develop frontal hair loss at the site of greatest tension.
Early traction alopecia is reversible if the pulling stops. But if the same hairstyling practices continue for years, chronic inflammation destroys the follicle permanently, converting it into a scarring form of hair loss where regrowth is no longer possible.
Scarring Alopecia Destroys Follicles Permanently
All the types discussed above are non-scarring, meaning the follicle survives and regrowth remains possible. Scarring alopecias are different. In these conditions, inflammation destroys the hair follicle and replaces it with scar tissue, making the hair loss irreversible.
One of the most studied forms is central centrifugal cicatricial alopecia (CCCA), which primarily affects women of African descent. Its exact cause remains unclear, but the mechanism involves a specific type of immune cell infiltrating and destroying hair follicles from the inside. Other scarring alopecias, such as lichen planopilaris and frontal fibrosing alopecia, involve a different subset of immune cells, which helps dermatologists distinguish between them under a microscope.
Scarring alopecias tend to progress slowly and often go unnoticed until significant permanent loss has occurred. Early detection matters because treatment can stop further destruction, even though it can’t restore what’s already been lost.