Alopecia isn’t a single condition with a single cause. It’s an umbrella term for hair loss, and the way you “get” it depends entirely on which type you have. Some forms are driven by your immune system attacking hair follicles, others by hormones gradually shrinking them, and still others by physical stress, styling habits, nutritional gaps, or medications. Understanding what’s behind your specific pattern of hair loss is the first step toward knowing whether it’s reversible.
Alopecia Areata: When Your Immune System Attacks
Alopecia areata is the form most people think of when they hear the word “alopecia.” It’s an autoimmune condition where your body’s own immune cells mistakenly target hair follicles. Normally, hair follicles have a kind of immune protection that keeps them hidden from the immune system. When that protection breaks down, immune cells recognize proteins on the follicle as foreign threats and launch an attack.
The assault is led primarily by two types of T cells. Cytotoxic CD8+ T cells directly damage the follicle, while CD4+ T cells coordinate the broader immune response. Skin biopsies from affected areas show a dense cluster of immune cells swarming around the follicle, a pattern pathologists describe as a “swarm of bees.” Natural killer cells, mast cells, and other inflammatory cells join in, and the follicle essentially shuts down hair production.
No one fully understands what triggers this immune collapse in the first place. Genetics play a clear role: if a close family member has alopecia areata, your risk is higher. Emotional or physical stress can also set it off, and the condition frequently overlaps with other autoimmune problems. In one study of 584 patients with alopecia areata, nearly 19% also had thyroid disease. Memory T cells that linger in the skin may explain why patches tend to recur in the same spots.
The good news is that the follicles aren’t destroyed, just dormant. About 90% of people with limited scalp involvement (less than a quarter of the scalp affected) experience spontaneous regrowth within two years, even without treatment.
Androgenetic Alopecia: Hormonal Thinning
This is the most common type of hair loss worldwide, often called male-pattern or female-pattern baldness. It’s driven by androgens, the group of hormones that includes testosterone. In people who are genetically susceptible, a hormone called dihydrotestosterone (DHT) overstimulates hair follicles on the scalp, particularly at the temples and crown.
That overstimulation does something counterintuitive: it shortens the growth phase of each hair cycle. Instead of growing for several years before naturally shedding, hairs grow for shorter and shorter periods. Each new cycle produces a thinner, finer strand. The follicle itself physically shrinks in a process called miniaturization, and the replacement of shed hairs slows down. Over years or decades, this results in visible thinning and eventually bare scalp.
Your likelihood of developing androgenetic alopecia depends heavily on family history on both sides. It can begin as early as the late teens in men, though it typically becomes noticeable in the 30s or 40s. In women, it usually appears as diffuse thinning along the part line rather than a receding hairline, and it often accelerates after menopause when the balance between estrogen and androgens shifts.
Telogen Effluvium: Stress-Related Shedding
If you’ve ever noticed alarming amounts of hair falling out two to three months after a major physical event, that’s likely telogen effluvium. It happens when a large percentage of your hair follicles are shocked into the resting (telogen) phase simultaneously, then shed all at once when new growth pushes them out.
The list of triggers is long: high fever, severe infections, major surgery, childbirth, rapid weight loss, and significant psychological stress. Thyroid problems, both overactive and underactive, are common culprits. Stopping birth control pills can trigger it. Crash diets that lack adequate protein are another frequent cause, because hair cells are among the fastest-dividing cells in the body and are sensitive to nutritional shortfalls.
Telogen effluvium is almost always temporary. Once the triggering event resolves, hair typically returns to its normal growth cycle within six to nine months, though it can feel like a painfully slow process.
Traction Alopecia: Damage From Pulling
This type of hair loss results from sustained tension on the hair follicles. Tight ponytails, braids, cornrows, buns, and hair extensions are the most common causes. Heavy dreadlocks and long braids can produce enough weight to gradually pull hair from the follicle. Wigs attached to tight braids, styles that require frequent re-tightening, and even headgear like tightly pinned nursing caps or helmets worn for long periods can contribute.
The hair loss typically shows up along the hairline or wherever the tension is greatest. Early on, it’s fully reversible: stop the pulling, and the follicles recover. But if the tension continues over months or years, repeated damage causes scarring in the follicle, and at that point, regrowth becomes impossible. The key variable is duration. Occasional tight styling is unlikely to cause permanent loss, but daily or near-daily tension over a long period crosses the line from temporary to permanent.
Scarring Alopecia: Permanent Follicle Destruction
In scarring (cicatricial) alopecia, inflammation destroys the stem cells and oil glands in the middle section of the hair follicle. Scar tissue replaces those structures, and the follicle can never produce hair again. This is what separates scarring alopecia from other forms: it’s irreversible once the damage is done.
Several conditions cause this. Lichen planopilaris is the most common primary scarring alopecia, mostly affecting women over 50. It can cause scaling, burning, itching, and inflamed bumps on the scalp. Central centrifugal cicatricial alopecia (CCCA) is the most common form of hair loss in Black women, typically starting as a bald patch on the crown around age 30 and spreading outward. Chronic cutaneous lupus, a skin manifestation of the autoimmune disease lupus, can also scar follicles. Even certain bacterial infections, particularly staph infections that cause a condition called folliculitis decalvans, can trigger ongoing inflammation severe enough to destroy follicles permanently.
Because the damage is irreversible, early detection matters more here than with any other type of alopecia.
Nutritional Deficiencies That Thin Hair
Low levels of certain nutrients can contribute to hair loss or make existing hair loss worse. Vitamin D deficiency is one of the most studied links. The Endocrine Society considers blood levels below 20 ng/mL deficient, and case reports consistently show patients with diffuse hair thinning at severely low levels, often around 9 to 12 ng/mL. Both abnormally high and abnormally low vitamin D levels have been associated with telogen effluvium, suggesting the body needs this hormone within a specific range to maintain healthy hair cycling.
Iron deficiency is another well-established contributor, particularly in women with heavy menstrual periods or restrictive diets. Low zinc and inadequate protein intake can also play a role. These deficiencies rarely cause dramatic bald patches on their own, but they can accelerate thinning or prevent regrowth in someone already dealing with another form of alopecia.
Medications That Cause Hair Loss
A surprisingly large number of common medications can trigger hair shedding, typically through telogen effluvium. Blood thinners like heparin and warfarin are well-known culprits. Blood pressure medications, particularly beta-blockers and ACE inhibitors, can do it. Hormonal medications, including birth control pills and hormone replacement therapy, sometimes trigger shedding during use or after stopping.
Anticonvulsants cause hair loss at notable rates: valproic acid triggers it in 12 to 28% of users depending on the dose, and carbamazepine in up to 6%. Mood stabilizers are frequent offenders, with lithium causing hair loss in 12 to 19% of patients. Retinoids (used for acne and skin conditions), cholesterol-lowering drugs, NSAIDs, and even some vaccines have been associated with shedding. Chemotherapy drugs work through a different mechanism, directly killing rapidly dividing hair cells, which is why they cause more dramatic and rapid hair loss than other medications.
In most cases, medication-related hair loss reverses after the drug is stopped or the dose is adjusted, though it can take several months for visible regrowth.