Alopecia isn’t a single condition. It’s a broad term for hair loss, and you can develop it through several distinct pathways: your genetics, your immune system attacking your own hair follicles, physical damage to your scalp, hormonal changes, nutritional deficiencies, medications, or major physiological stress. Some forms are temporary and reverse on their own. Others cause permanent follicle destruction. Understanding which type you’re dealing with determines whether your hair can grow back.
Genetics and Hormones: Pattern Hair Loss
The most common form of alopecia is androgenetic alopecia, often called male- or female-pattern hair loss. It’s driven by a hormone called DHT, which your body produces from testosterone. In people who are genetically susceptible, DHT binds to receptors in the cells at the base of the hair follicle and gradually shrinks those follicles. Each growth cycle produces thinner, shorter, lighter hairs until the follicle eventually stops producing visible hair altogether. This process is called miniaturization.
DHT doesn’t just shrink the follicle directly. It also disrupts the signaling between cells at the follicle’s base and the surrounding tissue, altering the growth factors that keep hair cycling normally. In men, this typically shows up as a receding hairline and thinning at the crown. In women, it tends to appear as diffuse thinning across the top of the scalp. The genetic component involves multiple genes, and having a parent or sibling with pattern hair loss raises your likelihood significantly.
Autoimmune Attack: Alopecia Areata
Alopecia areata is fundamentally different from pattern hair loss. It’s an autoimmune condition where your immune system mistakenly targets hair follicles. Hair follicles normally have a kind of immune protection, a biological “no-go zone” that keeps immune cells from attacking them. In alopecia areata, that protection collapses. Specialized immune cells, primarily a type called CD8 T cells, swarm the follicle and shut down hair production. CD4 T cells play an important supporting role, particularly in more extensive cases where hair loss spreads across larger areas of the scalp or body.
The result is usually smooth, round patches of complete hair loss that appear suddenly. In some people it stays limited to a few patches. In others it progresses to total scalp hair loss or even loss of all body hair. The condition affects roughly 2% of the global population, with a lifetime risk estimated between 1.7% and 2.1%. Several genes in the immune system’s HLA complex, which helps your body distinguish its own cells from invaders, have been linked to the condition. Having a first-degree relative with alopecia areata raises your risk above the general population, though most people with the condition have no family history of it.
Physical Damage: Traction Alopecia
You can cause alopecia through repeated physical stress on your hair follicles. Traction alopecia develops when hairstyles pull on the hair root with sustained or repeated force. Common culprits include tight ponytails, cornrows, braids, hair weaves, extensions, dreadlocks, and prolonged use of hair rollers. Even frequent use of tight headbands, clips, or hats that press against the scalp can contribute over time. Chemical relaxers, which weaken hair structure and make it more vulnerable to breakage, compound the risk.
In early stages, you might notice small bumps on the scalp, redness around the hairline, or a widening part. At this point, the damage is usually reversible if you change your styling habits. But continuous pulling eventually destroys the follicle permanently. In later stages, the affected area develops scarring, and the follicles cannot regrow hair regardless of treatment. The hairline and temples are the most commonly affected areas because they bear the most tension from pulled-back styles.
Stress-Related Shedding: Telogen Effluvium
A major physical or emotional shock can push large numbers of hair follicles into their resting phase all at once. Normally, only about 10% of your hair is in this resting phase at any given time. With telogen effluvium, that percentage jumps dramatically, and two to three months after the triggering event, those hairs fall out in noticeable clumps. The delay is what makes this type confusing. By the time you’re losing hair, you may have forgotten the event that caused it.
Known triggers include high fever, severe infections, major surgery, childbirth, significant psychological stress, thyroid disorders (both overactive and underactive), and crash diets low in protein. Stopping birth control pills can also set it off. The acute form typically resolves within six months as your hair cycles back into its growth phase. This is one of the more reassuring forms of alopecia because the follicles themselves aren’t damaged, just temporarily disrupted.
Medications That Trigger Hair Loss
Certain medications can cause hair loss as a side effect, and the mechanism depends on the drug. Chemotherapy is the most well-known example. These drugs target rapidly dividing cells throughout the body, and hair follicle cells in their active growth phase divide faster than almost any other cell type. Chemotherapy damages these cells, triggering cell death in the hair bulb and causing hair to fall out, sometimes within weeks of starting treatment. This type, called anagen effluvium, is usually temporary, and hair typically regrows after treatment ends.
Beyond chemotherapy, a wide range of commonly prescribed medications have been linked to hair loss. Research examining 192 drugs found that 110 had at least some evidence of causing alopecia. Thirteen had strong clinical evidence supporting the connection, spanning drug classes used for autoimmune conditions, blood clotting, seizures, and certain cancers. Blood thinners, immune-suppressing drugs, some antidepressants, beta-blockers, calcium channel blockers, retinoids, and anti-inflammatory drugs have all been associated with hair shedding. In most of these cases, the hair loss follows the telogen effluvium pattern, appearing a few months after starting the medication.
Nutritional Deficiencies
Your hair follicles need a steady supply of nutrients to sustain their rapid growth cycle, and deficiencies in key nutrients can slow or stop hair production. Iron deficiency is the most studied nutritional cause. Your body stores iron as ferritin, and when those stores drop below normal levels (under 20 nanograms per milliliter for both men and women), hair loss can follow. The follicle needs iron to support the cell division that drives hair growth, and when supplies run low, hair production is one of the first non-essential functions your body scales back.
Protein deficiency, zinc deficiency, and severe calorie restriction from crash dieting can also trigger shedding. These nutritional causes are among the most treatable forms of hair loss because correcting the deficiency usually allows normal hair growth to resume within a few months.
How Doctors Tell the Types Apart
Because so many different conditions fall under the umbrella of “alopecia,” diagnosis starts with a careful scalp examination. A dermatologist looks for inflammation, redness, sores, or scarring, each of which points toward different causes. The pattern of loss matters too: smooth round patches suggest alopecia areata, thinning at the hairline and temples points toward traction alopecia or androgenetic alopecia, and diffuse shedding across the entire scalp is more typical of telogen effluvium or nutritional deficiency.
Most of the time, a physical exam and analysis of the hair itself are enough to identify the cause. When there’s ambiguity, particularly when scarring is present and could result from more than one condition, a small scalp biopsy can distinguish between types. Blood work can identify thyroid problems, iron deficiency, or hormonal imbalances that might be driving the loss. Getting the diagnosis right matters because the treatment path for each type is completely different, and some forms require early intervention to prevent permanent follicle destruction.