Alopecia isn’t a single disease. It’s an umbrella term for hair loss, and the mechanism behind it depends entirely on which type you have. Some forms involve your immune system attacking hair follicles, others are driven by hormones shrinking them over decades, and still others are temporary reactions to physical stress. What they share is a disruption of the hair growth cycle, but how that disruption happens varies dramatically.
How the Hair Growth Cycle Works
Every hair on your head cycles through four phases independently, which is why you don’t shed all your hair at once. The growth phase (anagen) lasts two to eight years and determines how long your hair can get. A short transition phase (catagen) follows, lasting about two weeks, during which the follicle shrinks and detaches from its blood supply. Then comes a resting phase (telogen) of two to three months, where the old hair sits in place while a new hair begins forming underneath. Finally, the old strand is pushed out and shed (exogen), and the cycle restarts.
At any given time, roughly 85 to 90 percent of your scalp hairs are in the growth phase. The proportion of follicles in anagen naturally declines with age, which is one reason hair tends to thin over time even without a specific condition. Every type of alopecia interferes with this cycle in its own way, either by shortening the growth phase, forcing follicles into rest prematurely, or destroying them outright.
Pattern Hair Loss: A Hormonal Shrinking Process
Androgenetic alopecia, the most common form of hair loss in both men and women, is driven by a hormone called DHT (dihydrotestosterone). An enzyme in your scalp converts testosterone into DHT, which binds to receptors on the cells at the base of your hair follicles. DHT has a much stronger grip on these receptors than testosterone does, making it the dominant force behind pattern baldness.
Once DHT attaches to those receptors, it disrupts the signaling pathways that keep follicles healthy and regenerating. It shortens the growth phase and extends the resting phase, so each cycle produces a slightly thinner, shorter hair. Over years or decades, follicles that once produced thick terminal hairs gradually shrink until they only produce fine, nearly invisible vellus hairs. This process is called miniaturization, and it’s why pattern baldness is progressive rather than sudden.
Not all follicles are equally vulnerable. Hair follicles along the front, temples, and crown of the scalp have a much higher density of hormone receptors, which is why pattern baldness follows a predictable shape. The follicles on the sides and back of the head are largely resistant, which is also why those areas are used as donor sites in hair transplants.
Genetics determine how sensitive your follicles are to DHT. The androgen receptor gene on the X chromosome is the single strongest genetic risk factor, which is why people often look to their mother’s side of the family for clues. But pattern baldness follows a polygenic inheritance pattern, meaning multiple genes across several chromosomes contribute. At least eight independent genetic locations have been linked to baldness risk. Thinning typically begins between ages 12 and 40 in genetically susceptible people.
Alopecia Areata: An Immune System Attack
Alopecia areata works through an entirely different mechanism. It’s an autoimmune condition affecting roughly 2 percent of the global population, with the highest incidence in North America, Southeast Asia, and Australia. Women and people aged 30 to 34 are most commonly affected.
Healthy hair follicles enjoy a form of immune protection, sometimes called “immune privilege,” that shields them from the body’s own defense system. In alopecia areata, that protection breaks down. When it does, a specific type of immune cell (CD8 T cells carrying the NKG2D receptor) identifies proteins in the hair follicle as foreign threats and launches an inflammatory attack. These T cells swarm the follicle, disrupting the growth cycle and forcing hairs into the resting phase prematurely.
The attack is self-amplifying. The T cells release inflammatory signaling molecules, which cause the follicle’s own cells to produce more chemical signals that attract and activate even more T cells. This positive feedback loop sustains the inflammation and keeps follicles from resuming normal growth. CD4 T cells, a different subset of immune cells, also accumulate around affected follicles and appear to help the CD8 cells mount a more extensive attack. Research in mouse models suggests CD4 involvement may be essential for the condition to spread across larger areas of the scalp.
The good news is that alopecia areata doesn’t destroy follicles. The stem cells at the base of the follicle survive, which means regrowth is possible if the immune attack subsides. This is the key distinction from scarring forms of hair loss.
Telogen Effluvium: Stress-Triggered Shedding
Telogen effluvium is the body’s reaction to a major physiological shock. A high fever, severe infection, surgery, crash dieting, childbirth, thyroid problems, iron deficiency, or certain medications (beta-blockers, blood thinners, and retinoids are common culprits) can push a large number of follicles from the growth phase into the resting phase all at once.
The tricky part is the delay. The follicles that were forced into rest sit quietly for one to six months, with three months being typical. You don’t notice anything during this period. Then, when those follicles finally restart growth, the old resting hairs are pushed out simultaneously, causing what feels like alarming, sudden hair loss. By the time you’re shedding handfuls, the triggering event is months in the past, which makes it hard to connect the two without thinking back carefully.
Because the follicles themselves aren’t damaged, telogen effluvium is almost always temporary. Once the triggering stress resolves, normal cycling resumes over the following months.
Scarring vs. Non-Scarring Hair Loss
All types of alopecia fall into one of two categories based on what happens to the follicle itself. Non-scarring types, including pattern baldness, alopecia areata, and telogen effluvium, preserve the follicle opening. The follicle may be miniaturized, inflamed, or dormant, but it’s structurally intact and potentially capable of regrowth.
Scarring (cicatricial) alopecia is fundamentally different. Inflammatory or traumatic processes permanently destroy the follicle and replace it with scar tissue. Once that happens, no treatment can coax hair from that spot because the structure that produces hair no longer exists. Scarring alopecia is less common but more urgent to diagnose, since early treatment can preserve the follicles that haven’t yet been destroyed.
How Alopecia Is Diagnosed
Dermatologists use a combination of physical examination and simple bedside tests. The hair pull test involves grasping about 50 to 60 hairs and pulling gently from root to tip. If more than five or six hairs come out easily, that’s a positive result indicating active hair loss. This test is done across different areas of the scalp to map where loss is occurring. A tug test checks for hair fragility by pulling along the middle of the shaft; breakage during this test points to structural hair problems rather than follicle issues. Dermatoscopy (a magnified view of the scalp) and, in uncertain cases, a small scalp biopsy help distinguish between types.
How Treatments Target Each Mechanism
Because the mechanisms differ, treatments are type-specific. For pattern hair loss, the goal is reducing DHT’s effect on follicles. Topical minoxidil works differently: it’s a vasodilator that, when converted to its active form in the scalp, appears to stimulate follicles by opening potassium channels in cells, though the exact pathway is still not fully mapped. It helps extend the growth phase and can partially reverse miniaturization in some people.
For alopecia areata, a newer class of drugs called JAK inhibitors targets the immune feedback loop directly. These medications block the specific signaling pathway that T cells and follicle cells use to amplify inflammation. By interrupting this loop, JAK inhibitors reduce the production of the inflammatory signals that sustain the immune attack, allowing follicles to re-enter the growth phase. They also promote the activation of hair follicle stem cells. The first oral JAK inhibitor for alopecia areata was approved in 2022, marking a significant shift from older approaches that relied on broad immune suppression.
Telogen effluvium generally doesn’t require specific treatment beyond addressing the underlying trigger. Correcting a nutritional deficiency, managing thyroid function, or simply allowing the body to recover from illness or surgery is usually enough for the hair cycle to normalize on its own within six to twelve months.