Alopecia develops through several distinct pathways depending on the type. Some forms are driven by your immune system attacking hair follicles, others by hormones shrinking them over time, and still others by physical damage, nutritional gaps, or medications. Understanding which mechanism is at work matters because it determines whether the hair loss is temporary or permanent, and what can be done about it.
Autoimmune Attack on Hair Follicles
Alopecia areata, the patchy form of hair loss that affects roughly 2% of the global population, is an autoimmune condition. Your hair follicles normally have a form of immune protection, a kind of biological “do not disturb” signal that keeps immune cells from targeting them. When that protection breaks down, your immune system treats the follicle’s own proteins as foreign invaders.
The main attackers are a specific type of white blood cell called CD8+ T cells. These cells swarm the base of hair follicles in such dense clusters that dermatologists who look at skin biopsies describe the pattern as a “swarm of bees.” Along with these T cells come natural killer cells, mast cells, and other inflammatory cells, all releasing signals that force the follicle to shut down and stop producing hair. Memory T cells that linger in the skin after an episode may explain why alopecia areata often recurs in the same spots.
What triggers this immune collapse in the first place varies from person to person. Oxidative stress, an imbalance where damaging molecules overwhelm your body’s ability to neutralize them, appears to play a significant role. UV light exposure can ramp up inflammatory enzyme activity in the skin and attract immune cells to follicles. Viral infections and bacterial toxins can also set off the chain reaction. Prolonged psychological stress, particularly chronic stress experienced during childhood such as academic pressure or the death of a close family member, has been linked to higher risk of developing alopecia areata.
How Hormones Shrink Hair Follicles
Androgenetic alopecia, the most common form of hair loss in both men and women, works through an entirely different mechanism. Here, the culprit is a hormone called DHT (dihydrotestosterone), which your body produces from testosterone. High levels of DHT shrink hair follicles and shorten the hair growth cycle. Over successive cycles, the follicle produces thinner, shorter, lighter hairs until it eventually stops producing visible hair altogether. This process is called miniaturization.
Genetics play a central role. The condition clusters in families, and having a close relative with patterned hair loss is a clear risk factor. Researchers have confirmed that variations in a gene called the AR gene are directly involved. This gene controls how your body builds androgen receptors, the cellular docking stations that respond to DHT. People with certain AR gene variants have receptors that are more easily activated by DHT, meaning their hair follicles are more sensitive to normal hormone levels. The inheritance pattern is complex and likely involves multiple genes along with environmental factors, so it doesn’t follow a simple “from your mother’s side” or “from your father’s side” rule.
Stress and the Hair Growth Cycle
Your hair naturally cycles through phases: a growth phase that lasts years, a brief transition phase, and a resting phase that ends with the hair falling out to make room for a new one. Normally, only about 10% of your hair is in the resting phase at any given time. In telogen effluvium, a shock to the body pushes up to 70% of growing hairs prematurely into the resting phase, causing dramatic shedding two to three months later.
The list of triggers is long: high fever, childbirth, severe infections, major surgery, thyroid disorders (both overactive and underactive), stopping birth control pills, and significant psychological stress. Crash diets that lack adequate protein can also set it off. The good news is that telogen effluvium is usually temporary. Once the triggering event resolves, follicles re-enter the growth phase and hair density gradually returns to normal over six to twelve months.
When Nutrition Plays a Role
Iron deficiency is one of the most underrecognized contributors to hair loss, partly because you don’t need to be anemic for it to affect your hair. Research suggests that hair follicles need ferritin (your body’s stored iron) levels above 70 ng/mL to maintain a normal growth cycle. Many people, particularly women with heavy periods or those on restricted diets, fall below this threshold while still having normal blood counts. At ferritin levels between 21 and 70 ng/mL, iron stores are technically “adequate” by standard lab ranges but may not be sufficient to support hair growth.
This is why hair loss sometimes responds to iron supplementation even when blood tests don’t show outright anemia. If you’re experiencing diffuse thinning without an obvious cause, checking your ferritin level (not just your hemoglobin) is worth discussing with your provider.
Physical Damage From Hairstyles
Traction alopecia develops when hairstyles repeatedly pull on the same follicles over months or years. Tight ponytails, cornrows, braids, buns, and certain head coverings all create constant tension at the hairline or part line. In the early stages, you might notice small bumps, redness, or soreness at the scalp where the pulling is strongest. Hair may still grow back if you change the style.
But when the pulling continues long enough, the follicles sustain permanent damage. Where hair once grew, the scalp becomes smooth and shiny. At that point, regrowth is no longer possible because the follicle itself has been destroyed. This makes traction alopecia one of the most preventable forms of permanent hair loss: alternating hairstyles and reducing tension early can stop the progression entirely.
Scarring Alopecia and Permanent Follicle Loss
Scarring (cicatricial) alopecias are a group of conditions where inflammation destroys the stem cells that hair follicles need to regenerate. These stem cells normally sit in a protected niche within the follicle. When that protective environment breaks down, immune cells, particularly CD8+ T cells and natural killer cells, can access and destroy the stem cells directly. Once those cells are gone, the follicle is replaced by scar tissue and can never produce hair again.
Several conditions fall under this umbrella, including lichen planopilaris and frontal fibrosing alopecia. They tend to progress slowly, and early treatment focuses on stopping the inflammation before more follicles are permanently lost.
Medications That Cause Hair Loss
Certain medications can trigger hair loss through different mechanisms depending on the drug. The most dramatic example is chemotherapy, which halts active hair growth so abruptly that hair falls out within days or weeks. This type, called anagen effluvium, is usually reversible once treatment ends.
A broader range of everyday medications can cause a slower, more diffuse shedding similar to telogen effluvium. Endocrine therapies, some diabetes medications, beta-blockers, calcium channel blockers, retinoids, certain antidepressants, and even NSAIDs have all been implicated. In rarer cases, specific immune-modulating drugs, including some monoclonal antibodies and immune checkpoint inhibitors used in cancer treatment, can actually trigger autoimmune alopecia areata. If you notice unusual shedding after starting a new medication, that timing is worth flagging to your prescriber, since switching to an alternative may resolve it.