Alopecia has no single cause. It can result from genetics, autoimmune attacks on hair follicles, physical stress on the body, nutritional gaps, medications, or even the way you style your hair. The type of hair loss you’re experiencing determines what’s behind it, whether it’s reversible, and how it’s treated.
Androgenetic Alopecia: The Genetic Type
Androgenetic alopecia is the most common form of hair loss in both men and women. It’s driven by a combination of genetics and hormones, specifically how your hair follicles respond to androgens (a group of hormones that includes testosterone). Over time, sensitive follicles gradually shrink and produce thinner, shorter hairs until they stop producing visible hair altogether.
The genetic component is strongly tied to variations in the androgen receptor gene, located on the X chromosome. One landmark study found that a specific genetic marker on this gene was present in 98% of young men with significant balding, compared to about 77% of men with no hair loss. Shorter repeat sequences within the same gene were also more common in bald men. These variations likely make follicles more sensitive to normal levels of androgens, which is why hair loss can happen even when hormone levels are completely normal.
The pattern of thinning differs by sex. Men typically lose hair at the temples and crown first, sometimes progressing to near-complete loss on top while hair at the sides and back remains. Women usually experience diffuse thinning across the top of the scalp, with the frontal hairline staying intact. Both patterns tend to develop gradually over years or decades.
Alopecia Areata: The Autoimmune Type
Alopecia areata affects up to 2% of the population and occurs equally in men and women. It causes sudden, patchy hair loss, typically appearing as smooth, round, flesh-colored patches on the scalp or other parts of the body. In more severe forms, it can progress to total loss of scalp hair (alopecia totalis) or total loss of all body hair (alopecia universalis).
The underlying cause is autoimmune. Hair follicles normally have a kind of immune protection, a biological shield that keeps the immune system from attacking them. In alopecia areata, that protection collapses. Once it does, the immune system’s killer T-cells recognize proteins on the hair follicle as foreign and launch an attack. These T-cells swarm the follicle and shut down hair production. Elevated levels of certain immune signaling molecules drive the T-cells to become more aggressive, intensifying the assault.
The follicles themselves aren’t destroyed, which is why hair can regrow, sometimes spontaneously. But the immune attack can recur unpredictably. What initially triggers the immune system to turn on hair follicles isn’t fully understood, though people with alopecia areata are more likely to have other autoimmune conditions like thyroid disease or vitiligo.
Telogen Effluvium: Stress-Related Shedding
Telogen effluvium is a temporary form of hair loss triggered by a physical or emotional shock to the body. It happens when a large number of hair follicles simultaneously shift into their resting phase and then shed. The hallmark feature is a noticeable delay: hair typically starts falling out two to three months after the triggering event, which often makes it hard to connect the cause to the effect.
Common triggers include high fever, severe infections, childbirth, major surgery, significant psychological stress, and thyroid disorders (both overactive and underactive). Certain medications can also set it off, including some blood pressure drugs, anti-inflammatory medications, antidepressants, and retinoids. Crash diets that lack adequate protein are another well-known trigger. Stopping birth control pills can cause it as well.
People with telogen effluvium often notice clumps of hair coming out in the shower or collecting in their hairbrush. The acute form typically resolves within six months once the underlying stressor is addressed. The hair follicles aren’t damaged, so regrowth is expected.
Nutritional Deficiencies That Cause Hair Loss
Several nutrient shortfalls can contribute to hair thinning or shedding, though they rarely cause dramatic bald patches the way autoimmune or genetic factors do.
- Iron carries oxygen to hair follicles. Without adequate iron, follicles can’t fuel the rapid cell division needed for hair growth. Iron deficiency is one of the more common nutritional causes of hair loss, particularly in women with heavy menstrual periods or people on restrictive diets.
- Vitamin D plays a direct role in creating the cells that form new hair follicles. Low vitamin D levels have been associated with several types of hair loss, including alopecia areata.
- Vitamin C is necessary for absorbing iron from food. A deficiency in vitamin C can worsen iron-related hair loss even if dietary iron intake seems adequate.
Nutritional hair loss is typically reversible once the deficiency is corrected, but supplementation works only when a genuine deficiency exists. Taking extra iron or vitamin D when your levels are already normal won’t improve hair growth and can cause other problems. Blood testing is the only reliable way to know.
Traction Alopecia: Damage From Styling
Traction alopecia results from repeated physical tension on hair follicles. It’s caused by hairstyles that pull, not by any internal disease process. Tight cornrows, braids, buns, ponytails, locs, and hair extensions or weaves are all common culprits, especially when worn consistently over months or years.
The early warning signs include pain or stinging at the scalp, broken hairs around the forehead, a gradually receding hairline, and small patches of thinning where the hair is pulled tightest. Some people develop crusting on the scalp or notice a “tenting” effect where sections of skin are visibly pulled upward.
People of African descent are at higher risk because the shape of their hair follicles makes strands more vulnerable to damage from tight or rough styling. The constant friction of hats, head scarves, or other coverings worn over tightly pulled hair can also contribute. If caught early, traction alopecia is fully reversible by changing hairstyles. But once it advances far enough, the follicles are permanently damaged and hair cannot regrow.
Scarring Alopecia: Permanent Follicle Loss
Scarring alopecia, also called cicatricial alopecia, is a group of conditions where inflammation destroys hair follicles and replaces them with scar tissue. Unlike most other types of hair loss, this is permanent. The follicles don’t just go dormant; they’re gone.
The key target is the middle portion of the hair follicle, where stem cells and oil glands live. Both are essential for regenerating new hair. When inflammation destroys this area and scar tissue fills in, the follicle loses its ability to produce hair entirely. The affected skin typically looks smooth and shiny, and the tiny pore openings where hair once emerged close up completely. Some people also experience redness, flaking, or blisters around affected areas.
This distinguishes scarring alopecia from non-scarring types. In conditions like alopecia areata or telogen effluvium, the damage occurs at the base of the follicle rather than the stem cell region, which is why regrowth remains possible. In scarring alopecia, the construction is irreversible. The causes are typically autoimmune or inflammatory disorders that directly target follicles, though the specific conditions vary widely and some remain poorly understood.
Medications That Trigger Hair Loss
Drug-induced hair loss is more common than many people realize. A comprehensive review of prescription medications found 110 commonly used drugs with evidence linking them to alopecia. Of those, 13 had strong clinical evidence supporting the connection, spanning several drug categories: immune-suppressing and immune-modifying drugs, blood thinners, certain cancer treatments, a seizure medication, and even a vaccine.
Medication-related hair loss usually follows the telogen effluvium pattern, with shedding starting weeks to months after beginning a new drug. In most cases, hair regrows after the medication is stopped or switched, though recovery can take several months. If you notice increased shedding after starting a new prescription, it’s worth checking whether hair loss is a known side effect.