Hair falls out because every strand on your head has a limited lifespan, and losing 50 to 150 hairs a day is completely normal. Your body is constantly cycling old hairs out and new ones in. But when shedding speeds up, or new growth slows down, the balance tips and you start noticing thinning, clumps in the shower drain, or a widening part. The reasons range from genetics and hormones to stress, nutrition, and even the way you style your hair.
How the Hair Growth Cycle Works
Every hair follicle on your scalp moves through four phases independently. The growth phase (anagen) lasts 2 to 8 years and is when the hair strand actively lengthens. At any given time, 85% to 90% of your scalp hairs are in this phase. Next comes a short transition phase (catagen) lasting about 2 weeks, where the follicle shrinks and detaches from its blood supply. Only 1% to 3% of hairs are in this stage at once.
After that, the hair enters a resting phase (telogen) for 2 to 3 months, where it sits in the follicle but is no longer growing. About 9% of your scalp hairs are resting at any moment. Finally, during the shedding phase, the old strand falls out, often helped along by washing or brushing, while a new hair begins growing beneath it. This overlapping cycle is why you don’t go bald all at once, and why finding loose hairs on your pillow or in the shower is not, by itself, a reason to worry.
Genetic Pattern Hair Loss
The most common cause of progressive hair loss is inherited sensitivity to a hormone called DHT (dihydrotestosterone). Your body converts testosterone into DHT using an enzyme in the scalp, and DHT binds to receptors inside hair follicles far more strongly than testosterone itself. In people genetically prone to pattern baldness, the follicles in certain areas of the scalp, particularly the hairline, temples, and crown, have a higher density of these receptors.
When DHT locks onto a susceptible follicle, it disrupts the signals that keep the follicle regenerating. Over time, the growth phase gets shorter and the resting phase gets longer. Each cycle produces a thinner, shorter strand until the follicle is only capable of growing fine, nearly invisible “peach fuzz” hair. This gradual shrinking process is called miniaturization, and it happens over years or decades. Because the enzyme that produces DHT stays active with age, the process doesn’t reverse on its own. This type of hair loss affects both men and women, though the pattern differs: men typically lose hair at the temples and crown, while women usually see diffuse thinning across the top of the scalp.
Stress-Related Shedding
A major physical or emotional stressor can push an abnormally large number of hair follicles into the resting phase all at once. This condition, called telogen effluvium, causes widespread shedding across the entire scalp rather than in one specific area. The tricky part is timing: the shedding doesn’t start until 2 to 4 months after the triggering event, so it can be hard to connect cause and effect.
Common triggers include high fevers, serious infections (including COVID-19), major surgery, significant blood loss, crash diets, and severe emotional stress. One study of 100 patients with diffuse hair loss found that fever accounted for 33% of cases, psychological stress for 30%, and other systemic illnesses for 23%. The reassuring news is that telogen effluvium is usually self-limiting, typically resolving within about 6 months once the underlying trigger is gone. The follicles aren’t damaged; they just need time to re-enter the growth phase.
Hormonal Shifts After Pregnancy
Pregnancy-related hair loss is one of the most dramatic examples of telogen effluvium. During pregnancy, elevated estrogen levels extend the growth phase, keeping hairs that would normally shed locked in place. This is why many pregnant women notice their hair looking thicker and fuller. After delivery, estrogen drops back to its pre-pregnancy level, and all those hairs that overstayed their growth phase enter the resting and shedding phases simultaneously. The result is noticeable hair loss that typically peaks around 3 months postpartum. It looks alarming but is temporary, as the hair growth cycle gradually resets.
Iron and Nutritional Deficiencies
Hair follicles are among the fastest-dividing cells in the body, and they need a steady supply of nutrients to keep producing new strands. Iron deficiency is one of the best-studied nutritional causes of hair loss, particularly in women who haven’t yet reached menopause. Research suggests that ferritin (your body’s stored iron) levels need to be between 40 and 60 ng/mL to adequately support hair growth. That’s notably higher than the threshold used to diagnose anemia, which means you can have “normal” blood work by standard lab ranges and still have iron levels too low for healthy hair. Some researchers have proposed that a ferritin level below 60 ng/mL should be considered insufficient when evaluating hair loss. Other nutrients linked to shedding include zinc, vitamin D, biotin, and protein, though iron remains the most clinically significant.
Thyroid Problems
Your thyroid gland produces hormones that directly influence hair follicle stem cells. These hormones activate the signals that tell resting follicles to wake up and start producing new hair. When thyroid hormone levels are too low (hypothyroidism), those stem cells stay dormant longer than they should, follicle cell division slows, and hair becomes thin and brittle. The result is diffuse thinning across the scalp rather than bald patches. Interestingly, having too much thyroid hormone (hyperthyroidism) can also cause hair problems by overstimulating follicle stem cells, eventually depleting them. Both conditions cause hair loss that typically improves once thyroid levels are brought back to a normal range.
Autoimmune Hair Loss
Alopecia areata is a condition where your immune system mistakes hair follicles for a threat and attacks them. The culprits are a specific type of immune cell: cytotoxic T cells that cluster around the base of the hair follicle, right where new hair is produced. These T cells are activated by danger signals that the follicles themselves mistakenly produce, creating a cycle of immune attack that shuts down hair growth. The hallmark is smooth, round bald patches that appear suddenly, often on the scalp but sometimes affecting eyebrows, eyelashes, or body hair. Unlike pattern baldness, the follicles aren’t permanently destroyed in most cases, which is why hair can regrow, sometimes on its own, sometimes with treatment.
Damage From Tight Hairstyles
Hairstyles that pull on the hair for extended periods can cause a type of hair loss called traction alopecia. Braids, cornrows, tight ponytails, weaves, extensions, dreadlocks, and even frequently worn hair curlers all place mechanical stress on the follicles. The earliest signs are small bumps around the follicles (folliculitis), hair breakage, and thinning along the areas where tension is greatest, often the hairline and temples.
If caught early, switching to looser styles allows the follicles to recover. But chronic, repetitive tension causes the follicles to shrink and scar tissue to replace them. Once scarring sets in, the hair loss becomes permanent because the stem cells within the follicle are irreversibly damaged. This makes traction alopecia one of the few types of hair loss that is entirely preventable.
How to Tell Normal Shedding From a Problem
The range of normal daily shedding is wide, from 50 to 150 hairs, so counting individual strands isn’t particularly useful. A more practical indicator is change: if you’re suddenly finding significantly more hair on your pillow, in the drain, or on your clothes than you used to, something has likely shifted. Dermatologists use a simple test where they grasp about 50 to 60 hairs between their fingers and pull gently from root to tip. If more than 5 or 6 hairs come out easily, it suggests active hair loss beyond normal shedding.
The pattern of loss offers important clues. Thinning concentrated at the hairline, temples, or crown points toward genetic pattern loss. Diffuse shedding across the whole scalp suggests telogen effluvium, nutritional deficiency, or thyroid issues. Smooth, round patches are characteristic of autoimmune alopecia areata. And thinning along the margins of the scalp, especially where hairstyles pull tightest, suggests traction. Knowing which pattern you’re seeing helps narrow down the cause and determine whether the loss is likely reversible.