Hair loss happens because of genetics, hormones, stress, nutrition, medical conditions, or physical damage to the follicles. Most people shed 50 to 100 hairs a day as part of the normal growth cycle, but when shedding outpaces regrowth, or follicles stop producing hair altogether, thinning becomes visible. More than 50 percent of men over age 50 have some degree of hair loss, and women experience it nearly as often, though it tends to follow different patterns.
Understanding what’s behind your hair loss matters because some causes reverse on their own, some need treatment, and some become permanent if left alone too long.
How the Hair Growth Cycle Works
Every hair on your head cycles through three phases independently, which is why you don’t lose all your hair at once. The growth phase lasts two to six years, and this is when the follicle actively pushes out new hair. A short transition phase follows, lasting about two to three weeks, during which the follicle shrinks and detaches from its blood supply. Then comes the resting phase, which lasts roughly three months. At the end of that rest period, the hair falls out and a new one starts growing in its place.
At any given time, about 85 to 90 percent of your scalp hairs are in the growth phase. When something disrupts this balance and pushes too many follicles into the resting phase at once, you notice clumps in the shower drain or thinning across your scalp. Nearly every cause of hair loss works by shortening the growth phase, extending the resting phase, or damaging the follicle itself.
Genetics and Pattern Baldness
The most common reason people lose hair is androgenetic alopecia, better known as male or female pattern baldness. It’s driven by a hormone called DHT (dihydrotestosterone), which is made from testosterone. In people who are genetically susceptible, DHT binds to receptors on hair follicles and gradually shrinks them. Over time, the follicles produce thinner, shorter, lighter hairs until they stop producing visible hair at all. This process is called miniaturization.
In men, this typically shows up as a receding hairline and thinning at the crown. In women, it’s more of a diffuse thinning across the top of the scalp, with the hairline usually staying intact. The genetic component comes from both parents, not just one side of the family. Pattern baldness is progressive, meaning it continues unless something intervenes to block DHT’s effect on the follicles or stimulate regrowth.
Stress and Sudden Shedding
A sudden, dramatic increase in hair loss that happens a few months after a stressful event is called telogen effluvium. The stress shocks a large number of follicles into the resting phase simultaneously, and about two to three months later, those hairs fall out all at once. The triggers are varied:
- Physical stress: high fever, severe infections, major surgery, childbirth
- Emotional stress: intense psychological distress, grief, anxiety
- Dietary stress: crash diets low in protein, rapid weight loss
- Hormonal shifts: stopping birth control pills, thyroid disorders
- Medications: blood thinners, beta-blockers, high doses of vitamin A, certain antidepressants, and some anti-inflammatory drugs
The reassuring thing about telogen effluvium is that it’s almost always temporary. Hair typically grows back within three to six months without any treatment. The follicles aren’t damaged; they were just pushed out of sync. Once the underlying trigger resolves, normal cycling resumes and new growth fills in.
Hormonal Changes
Hormones regulate the hair growth cycle, so any significant hormonal shift can trigger shedding. Postpartum hair loss is one of the most recognizable examples. During pregnancy, elevated estrogen keeps more hairs in the growth phase than usual, which is why many women notice thicker, fuller hair. After delivery, estrogen drops rapidly, and all those extra hairs that were held in the growth phase enter the resting phase at once.
The shedding typically begins around three months after delivery, peaks at about five months, and resolves by eight months postpartum. It can look alarming, with large clumps coming out during washing or brushing, but it represents a return to your pre-pregnancy baseline rather than true hair loss.
Thyroid disorders also commonly cause hair thinning. Both an overactive and underactive thyroid can push a higher percentage of follicles into the resting phase, leading to diffuse shedding across the entire scalp rather than in patches. Thyroid-related hair loss generally reverses once the hormone imbalance is corrected, though regrowth can take several months.
Nutritional Deficiencies
Your hair follicles are among the most metabolically active cells in your body, and they need a steady supply of certain nutrients to function. Two deficiencies stand out in the research: iron and vitamin D.
A study comparing women with hair loss to women without it found striking differences. Women experiencing shedding-type hair loss had average ferritin levels (a marker of iron stores) of about 15 micrograms per liter, compared to 44 in the control group. Their vitamin D levels were roughly four times lower than those of women with healthy hair. Researchers identified a ferritin level below about 28 micrograms per liter and low vitamin D as thresholds associated with increased hair loss risk.
Iron deficiency is especially common in women who menstruate heavily, and vitamin D deficiency is widespread in people who get limited sun exposure. If your hair loss is diffuse and you can’t point to an obvious trigger like stress or a medication change, nutritional deficiencies are worth investigating with a simple blood test.
Autoimmune Hair Loss
Alopecia areata is a condition where your immune system mistakenly attacks your own hair follicles. Healthy hair follicles maintain a kind of protective shield that prevents immune cells from recognizing them as targets. In alopecia areata, that shield breaks down. Inflammatory immune cells flood into the follicle and attack the hair bulb, cutting the growth phase short and pushing the follicle into a prolonged resting state.
What makes this condition distinct is how it typically appears: smooth, round, coin-sized bald patches that seem to pop up overnight. It can stay limited to a few patches, or in rarer cases, progress to loss across the entire scalp or body. The follicles aren’t destroyed, which means regrowth is possible, but the condition can be unpredictable, with cycles of loss and regrowth over years. Treatments that calm the immune response at the follicle level have become increasingly effective in recent years.
Hairstyles and Physical Damage
Constant pulling or tension on the hair can cause a type of loss called traction alopecia. The culprits are hairstyles that put sustained stress on the follicle: tight cornrows, braids, buns, ponytails, locs, weaves, and extensions. Rollers worn to bed regularly can do it too. Even the repeated friction from a tightly worn hat or headscarf can contribute, especially if the hair underneath is already pulled back.
Early warning signs include pain or stinging at the scalp, small crusts forming along the hairline, and a “tenting” effect where sections of scalp visibly lift when the hair is pulled. At this stage, the damage is reversible. Loosening or changing the style allows the follicles to recover. But if the tension continues, the follicles eventually scar over, leaving smooth, shiny patches where hair can no longer grow. At that point, the loss is permanent.
People of African descent face a higher risk because the shape of their hair follicles makes them more susceptible to damage from tight styling. Ballet dancers, military personnel, and anyone required to wear their hair tightly pulled for work are also at increased risk. The American Academy of Dermatology recommends changing your hairstyle immediately if you notice any scalp pain or stinging, as those are the earliest signals that damage is occurring.
Multiple Causes Can Overlap
Hair loss rarely fits into a single neat category. Someone with a genetic tendency toward thinning might notice it accelerate after a stressful period, a nutritional dip, or a hormonal change. A woman with low iron stores who also has a family history of thinning hair may experience more noticeable loss than either factor would cause alone. This is why identifying the trigger (or triggers) matters. Pattern baldness requires a different approach than a nutritional deficiency, and treating one while ignoring the other leaves part of the problem unsolved.
The first step in figuring out what’s behind your hair loss is looking at the pattern. Diffuse thinning across the entire scalp points toward telogen effluvium, nutritional deficiencies, or thyroid issues. A receding hairline or crown thinning suggests pattern baldness. Smooth round patches suggest an autoimmune process. Thinning along the hairline where tension is applied points to traction alopecia. A blood test can quickly rule in or out nutritional and hormonal factors, making it one of the simplest and most useful starting points.