Hair loss has dozens of possible causes, but most cases trace back to a handful of common ones: genetics, hormonal changes, stress, nutritional gaps, medical conditions, or physical damage to the follicles. More than 50% of men over age 50 experience noticeable hair loss, and women are affected nearly as often, though the pattern usually looks different. Understanding what’s driving your hair loss is the first step toward knowing whether it’s reversible.
Genetic Hair Loss and DHT
The most common cause of hair loss is androgenetic alopecia, often called male- or female-pattern hair loss. It’s driven by a hormone called DHT (dihydrotestosterone), which is produced when an enzyme converts testosterone into a more potent form. DHT binds to receptors found exclusively in the dermal papilla cells of hair follicles. Once it attaches, it triggers changes in gene expression that gradually shrink the follicle in a process called miniaturization. Over time, each affected follicle produces thinner, shorter, lighter hairs until it barely produces visible hair at all.
People with androgenetic alopecia have higher levels of both the enzyme that creates DHT and the receptor complexes that respond to it, compared to people without the condition. This is genetically determined, which is why hair loss runs in families. In men, the pattern typically starts at the temples and crown. In women, it usually appears as a widening part or overall thinning across the top of the scalp. The follicles aren’t destroyed in this type of hair loss. They’re miniaturized but still alive, which is why treatments that block DHT or stimulate growth can sometimes reverse the thinning.
Stress and Sudden Shedding
Under normal conditions, about 9% of your hair follicles are in the resting phase (telogen) at any given time. The rest are actively growing. When your body goes through a major stressor, a much larger percentage of follicles get pushed into the resting phase all at once. This condition is called telogen effluvium, and it causes diffuse, widespread shedding that can feel alarming.
Common triggers include surgery, high fever, serious illness, childbirth, rapid weight loss, reduced protein intake, and intense psychological stress. The shedding doesn’t happen immediately. It typically shows up two to three months after the triggering event, which often makes it hard to connect cause and effect. The good news: once the trigger is removed, shedding usually stops within three to six months, and new growth becomes visible in a similar timeframe. Cosmetically significant regrowth, where your hair looks and feels noticeably thicker, can take 12 to 18 months.
Thyroid Imbalances
Both an underactive and overactive thyroid can cause hair loss, though the mechanisms differ. Thyroid hormones play a direct role in starting and maintaining the growth phase of your hair cycle. They stimulate cell division within hair follicles and regulate the transition from resting back to growing.
When your thyroid is underactive (hypothyroidism), hair growth slows down. The hair that does grow tends to be coarse, dry, and brittle. You may notice diffuse thinning across the scalp and, characteristically, loss of the outer third of your eyebrows. When your thyroid is overactive (hyperthyroidism), excess thyroid hormone creates oxidative stress that damages follicle cells. The hair becomes fine and silky but falls out in a diffuse pattern. In both cases, hair loss is typically non-scarring and reversible once thyroid levels are brought back to normal.
Nutritional Deficiencies
Hair follicles are among the fastest-dividing cells in your body, which makes them sensitive to nutritional shortfalls. Iron and vitamin D are the two nutrients most consistently linked to diffuse hair loss. In one study comparing people with hair loss to healthy controls, people experiencing shedding had average iron storage levels (measured by ferritin) of about 15 ng/mL, compared to 25 ng/mL in people without hair loss. Their vitamin D levels averaged 14 ng/mL, well below the normal threshold of 20 ng/mL.
Zinc, B vitamins, and protein also matter. Telogen effluvium can be triggered by crash diets, bariatric surgery, or any period of significantly reduced protein intake. If your shedding started after a major dietary change, nutritional gaps are worth investigating with bloodwork.
Autoimmune Hair Loss
Alopecia areata is an autoimmune condition in which your immune system attacks hair follicles directly. The key players are a specific subset of T cells (a type of white blood cell) that infiltrate the layers of the hair follicle and release inflammatory signals. These T cells produce a chemical that collapses the follicle’s natural immune protection, creating a self-reinforcing cycle of inflammation and damage.
Alopecia areata typically appears as smooth, round patches of complete baldness on the scalp. It can remain limited to a few patches or progress to total scalp hair loss (alopecia totalis) or loss of all body hair (alopecia universalis). Because the follicles aren’t permanently destroyed, regrowth is possible, sometimes spontaneously and sometimes with treatment that calms the immune response.
Medications That Cause Thinning
A number of medication classes can trigger hair shedding, usually by pushing follicles prematurely into the resting phase. The most commonly associated categories include anticonvulsants (seizure medications), oral retinoids (used for severe acne and other skin conditions), oral contraceptives, antithyroid drugs, cholesterol-lowering medications, beta-blockers (used for blood pressure and heart conditions), ACE inhibitors, and amphetamines. Chemotherapy drugs work differently: rather than shifting hair into the resting phase, they directly halt cell division during active growth, which is why the hair loss is faster and more dramatic.
If you notice increased shedding within a few months of starting a new medication, the timing is worth noting. Drug-induced hair loss is almost always reversible once the medication is changed or discontinued.
Physical Damage and Traction
Traction alopecia results from prolonged or repeated tension on hair follicles. Tight ponytails, braids, cornrows, weaves, extensions, dreadlocks, and even tight headscarves can place enough mechanical stress on the scalp to damage follicles over time. The condition is most visible along the hairline and at the temples, where tension tends to be greatest.
In the early stages, traction alopecia is fully reversible. Loosening hairstyles and reducing tension allows follicles to recover. But chronic, repetitive pulling leads to inflammation, then scarring around the follicle, and eventually permanent destruction of the follicle’s stem cells. At that point, the hair loss becomes irreversible. There’s no fixed timeline for when damage becomes permanent. It depends on how tight the style is, how frequently it’s worn, and individual susceptibility. The key signal that damage is progressing is if you notice broken hairs, scalp tenderness, or small bumps along the hairline.
Scarring vs. Non-Scarring Hair Loss
The distinction between scarring and non-scarring hair loss is the single most important factor in whether regrowth is possible. In non-scarring types (androgenetic alopecia, telogen effluvium, alopecia areata), the follicle is miniaturized or dormant but still intact. The follicle retains its ability to regenerate, which means effective treatment can restore growth.
In scarring alopecia, the follicle is permanently destroyed and replaced by scar tissue. The stem cells that would normally regenerate the hair are gone. Clinically, scarring alopecia looks like smooth, shiny patches where you can’t see any pore openings at all. Sometimes magnification is needed to confirm this. The most common form of scarring alopecia is lichen planopilaris, which accounts for at least 10% of cases and often presents with scalp itching, pain, and scaly patches. Other forms include conditions driven by bacterial infection, such as folliculitis decalvans, which causes crusting, and dissecting cellulitis, which causes pus-like discharge.
Because scarring alopecia is irreversible once established, early identification matters. If you’re losing hair in patches and the skin in those areas looks smooth and shiny with no visible pores, that warrants prompt evaluation.