Alopecia has several distinct causes depending on the type, but they all come down to something disrupting the normal hair growth cycle. The most common form, pattern hair loss, is driven by hormones. Other types involve the immune system attacking hair follicles, physical stress pushing hair into a shedding phase, or sustained tension on the scalp from styling. Understanding which mechanism is behind your hair loss is the first step toward knowing what to do about it.
Pattern Hair Loss and Hormones
Androgenetic alopecia, the gradual thinning most people picture when they hear “hair loss,” is caused by a hormone called dihydrotestosterone (DHT). Your body makes DHT by converting testosterone through an enzyme called 5-alpha reductase. In areas of the scalp prone to thinning, this enzyme is more active, producing higher local concentrations of DHT. People with pattern hair loss also have more hormone receptors in those balding areas, making the follicles there especially sensitive.
DHT doesn’t kill the follicle outright. Instead, it shortens the growth phase of each hair cycle. With every cycle, the follicle produces a slightly thinner, shorter hair. Over years, what was once a thick terminal hair becomes a fine, nearly invisible vellus hair. This process, called follicular miniaturization, is why thinning tends to creep in gradually rather than falling out all at once. In men, it typically starts at the temples and crown. In women, it usually shows up as diffuse thinning along the part line.
Alopecia Areata: The Immune System Turns on Hair
Alopecia areata is an autoimmune condition in which the body’s own immune cells attack hair follicles. Two types of T cells, CD8 and CD4, cluster around the base of the follicle and release inflammatory signals. Research published in Science Advances showed that CD4 T cells from affected skin could transfer the disease to healthy mice, confirming their direct role. These cells follow a specific inflammatory pattern, producing a signaling molecule called interferon-gamma that is essential for the attack. When researchers blocked that molecule, the T cells lost their ability to cause hair loss.
The result is usually one or more smooth, round patches of complete baldness that can appear suddenly, often over days to weeks. 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 follicles aren’t permanently destroyed, which is why hair can regrow, sometimes on its own. But the unpredictable nature of the immune response means it can also recur.
Stress, Illness, and Telogen Effluvium
Telogen effluvium is a reactive type of hair shedding triggered by a shock to the body. Normally, only about 10% of your hair is in the resting (telogen) phase at any given time. When something stressful happens, a much larger percentage of hairs get pushed into that resting phase at once. Two to three months later, all those hairs fall out together, which is why the shedding seems sudden even though the trigger happened weeks or months earlier.
The list of known triggers is long: high fevers, severe infections, major surgery, crash dieting, low protein intake, iron deficiency, and thyroid problems. Certain medications are also common culprits, particularly beta-blockers, blood thinners, excess vitamin A, and some anticonvulsants. The good news is that telogen effluvium is almost always temporary. Once the trigger is removed or resolved, normal hair growth typically resumes within six to nine months.
Postpartum Shedding
One of the most common forms of telogen effluvium hits new mothers about three months after delivery. During pregnancy, elevated estrogen keeps more hairs in the growth phase than usual, which is why many pregnant women notice thicker hair. After childbirth, estrogen drops sharply and all those extra hairs enter the resting phase at once. The resulting shedding can look alarming, with clumps coming out in the shower or on your pillow. It typically lasts less than six months, and most women see their hair return to its normal fullness by their baby’s first birthday.
Nutritional Deficiencies
Low iron and low vitamin D are two of the most consistently identified nutritional factors in diffuse hair loss. In one study of patients with hair shedding, nearly 80% had vitamin D levels below the normal threshold of 20 ng/ml, with an average reading of just 14 ng/ml. About 20% had low ferritin, the protein that stores iron, with average levels of roughly 15 ng/ml. Both nutrients play roles in the hair follicle’s growth cycle, and deficiency can push hairs prematurely into the resting phase.
Zinc deficiency has also been linked to hair shedding, though it’s less common in people eating a varied diet. The practical takeaway is that if you’re experiencing unexplained diffuse thinning, checking your iron stores, vitamin D, and basic nutritional markers through a blood test can identify a correctable cause before pursuing other treatments.
Traction Alopecia From Styling
Traction alopecia is caused by repeated mechanical pulling on the hair. Any hairstyle that applies sustained tension to the follicle can cause it: tight ponytails, buns, braids, cornrows, weaves, extensions, dreadlocks, and even long-term use of hair rollers or clips. The risk nearly triples with cornrows in young girls, and it increases further when tight styles are combined with chemical relaxers.
Early on, traction alopecia is reversible. You might notice small bumps or tenderness along the hairline, or thinning at the temples and edges where tension is greatest. If the pulling continues over months or years, the damage becomes permanent. The general recommendation is to avoid leaving braids or extensions in place longer than two to three months, to loosen any style that causes pain or stinging at the time of placement, and to take breaks between tight hairstyles to let follicles recover.
Chemotherapy and Drug-Induced Hair Loss
Chemotherapy causes a distinct type of hair loss called anagen effluvium, which targets hair that’s actively growing rather than pushing it into a resting phase. Because cancer drugs are designed to attack rapidly dividing cells, they also hit the hair matrix, one of the fastest-dividing tissues in the body. The result is usually rapid, widespread shedding that begins within weeks of starting treatment.
The severity depends on the drug. Certain agents cause hair loss in up to 80% or even 100% of patients, while others affect only 10 to 20%. Targeted therapies and immunotherapies used in newer cancer treatments can also cause hair changes, including partial thinning, texture changes, and depigmentation, though these tend to be milder than traditional chemotherapy effects. Hormone-blocking therapies like tamoxifen can cause mild thinning as well. In most cases, hair regrows after treatment ends, though the texture or color may be different initially.
Scarring Alopecia: Permanent Follicle Destruction
Scarring (cicatricial) alopecia is the most serious category because the hair loss is irreversible. In these conditions, inflammation destroys a specific structure in the follicle called the bulge, which houses the stem cells responsible for regenerating the hair. Once those stem cells are gone, the follicle is replaced by scar tissue and can never produce hair again.
Several distinct conditions fall under this umbrella, each driven by different types of inflammation. Some are caused by an overactive immune response targeting the follicle’s oil glands or outer sheath. Others result from infections, burns, or radiation damage. The hallmark signs are patches of smooth, shiny scalp where no follicle openings are visible, sometimes with redness or scaling at the edges where the condition is still active. Early treatment focused on controlling the inflammation can save follicles that haven’t yet been destroyed, which is why getting a diagnosis quickly matters.