Hair falling out is almost always caused by one of a handful of common triggers: stress, hormonal shifts, nutritional gaps, genetics, or damage from styling. Losing between 50 and 150 hairs a day is normal, so finding strands on your pillow or in the shower drain doesn’t necessarily signal a problem. But if you’re noticing clumps in your brush, a widening part, thinning patches, or visibly less volume than you had a few months ago, something is likely disrupting your hair’s growth cycle.
Understanding which type of hair loss you’re dealing with matters because the causes, timelines, and outcomes are very different. Here’s a breakdown of the most common reasons.
Stress-Related Shedding
The most common cause of sudden, dramatic hair loss is a condition called telogen effluvium. Your hair grows in cycles. At any given time, most of your hair is in the active growth phase, while a small percentage is in a resting phase before it falls out. When your body goes through a major stressor, up to 70% of your actively growing hair can prematurely shift into the resting phase all at once. A few months later, all of that hair falls out in what feels like an alarming wave.
The trigger list is long: high fever, severe infection, major surgery, childbirth, psychological stress, crash dieting (especially low-protein diets), thyroid disorders, stopping birth control pills, and certain medications including some antidepressants and blood pressure drugs. The tricky part is that the shedding typically starts two to three months after the triggering event, so by the time your hair is falling out, you may have forgotten what caused it.
The good news is that telogen effluvium is almost always temporary. Once the stressor resolves, hair regrows on its own. Full recovery can take six to twelve months.
Postpartum Hair Loss
If you recently had a baby, this deserves its own explanation. During pregnancy, elevated hormones keep hair in the growth phase longer than usual, which is why many people enjoy thicker hair while pregnant. After delivery, those hormone levels drop and all that “extra” hair enters the resting phase at once.
Postpartum shedding usually begins about three months after giving birth and resolves on its own within six to twelve months. It can be startling, with large clumps coming out in the shower, but it’s a normal hormonal reset rather than a sign of permanent loss.
Genetic Pattern Thinning
The most common cause of gradual, progressive hair loss is genetic. In both men and women, a hormone called DHT (a byproduct of testosterone) binds to hair follicles on the scalp and slowly shrinks them. Over time, each growth cycle produces thinner, shorter, less visible hairs until the follicle stops producing hair altogether.
In men, this typically shows up as a receding hairline and thinning at the crown. In women, it usually appears as a widening part or overall thinning across the top of the scalp, with the hairline staying intact. The process is slow, often unfolding over years or decades, which is why many people don’t notice it until a significant amount of volume is already gone. Family history on either side can increase your risk, but the pattern and pace vary widely from person to person.
Thyroid Problems
Both an overactive thyroid and an underactive thyroid can cause hair loss, and the pattern is distinctive. Rather than bald patches or a receding hairline, thyroid-related hair loss tends to show up as diffuse thinning, a general decrease in density and volume spread across the entire scalp. Your hair texture may also change, becoming dry, coarse, and more prone to breakage.
One telling sign: thyroid-related hair loss doesn’t stop at your scalp. You may also notice thinning eyebrows (especially the outer third), less body hair, or sparser pubic hair. If you’re experiencing hair loss alongside fatigue, unexplained weight changes, feeling unusually cold or hot, or mood shifts, a thyroid issue is worth investigating with a simple blood test.
Iron and Vitamin Deficiencies
Low iron is one of the most underdiagnosed causes of hair loss, particularly in women. Your body needs iron to fuel the rapidly dividing cells in hair follicles. When iron stores drop, hair is one of the first things your body deprioritizes. In one study, women with diffuse hair shedding had average iron storage levels (measured as ferritin) of about 15 ng/mL, compared to roughly 60 ng/mL in women without hair loss. Women with ferritin below 30 ng/mL were 21 times more likely to experience this type of shedding.
Heavy periods, vegetarian or vegan diets, frequent blood donation, and digestive conditions that impair absorption are all common reasons for low ferritin. Standard blood work sometimes flags ferritin as “normal” even at levels that are low enough to cause hair loss, so it’s worth asking specifically about your ferritin number rather than just accepting a pass/fail result.
Vitamin D also plays a role. Receptors for vitamin D are found in the cells that make up hair follicles, and they appear to be essential for initiating the growth phase of the hair cycle. Without adequate vitamin D signaling, follicles can stall. If you live in a northern climate, spend little time outdoors, or have darker skin, your levels may be lower than optimal.
PCOS and Excess Androgens
Polycystic ovary syndrome is a common hormonal condition that raises levels of androgens (male-type hormones) in women. Those excess androgens get converted into DHT, the same hormone responsible for genetic pattern baldness, and it attacks scalp follicles in the same way. Women with PCOS often notice thinning along the top of the scalp in a male-pattern distribution, sometimes alongside excess facial or body hair, acne, and irregular periods.
Treatment focuses on reducing DHT’s contact with hair follicles, either by blocking its production or preventing it from binding to follicle receptors. If you’re losing hair and also dealing with any of the other hallmark symptoms of PCOS, a hormonal workup can clarify whether androgens are driving the problem.
Autoimmune Hair Loss
Alopecia areata is a condition where your immune system mistakenly attacks hair follicles, treating them like a foreign invader such as a virus or bacteria. The result is usually one or more smooth, round bald patches about the size of a quarter, though patches can be larger or smaller. The hair loss can happen quickly, sometimes within days or weeks.
This type of hair loss behaves differently from the others. The follicles are not destroyed, just suppressed, which means hair can regrow even after months or years of being bare. But it can also recur unpredictably. Alopecia areata sometimes runs alongside other autoimmune conditions like thyroid disease or vitiligo.
Damage From Styling
Hairstyles that pull on the hair can cause a type of hair loss called traction alopecia. Tight cornrows, locs, braids, buns, ponytails, hair extensions, weaves (especially on chemically relaxed hair), and even rollers worn to bed regularly can all create enough sustained tension to damage follicles. Headwear like hats and scarves worn over tightly pulled hair can add to the problem.
Early on, the damage is reversible. If you loosen the style and give the area a break, hair typically grows back. But if the pulling continues over months or years, the follicles eventually scar over and stop producing hair entirely. At that point, you’ll see smooth, shiny skin where hair used to be, and regrowth is no longer possible. The key is catching it early: if you notice soreness, small bumps, or thinning along your hairline or part where tension is greatest, that’s your signal to change your routine.
How to Narrow Down Your Cause
The pattern and timeline of your hair loss are the biggest clues. Sudden shedding all over your scalp, especially two to three months after a major event, points to telogen effluvium. Gradual thinning at the crown or part line suggests genetic loss or a hormonal issue like PCOS. Smooth round patches point to alopecia areata. Thinning at the hairline where you style or pull your hair suggests traction damage. Diffuse thinning paired with dry, brittle texture and changes in body hair raises the possibility of a thyroid problem.
A dermatologist can often diagnose the type of hair loss by examining your scalp and reviewing your history. Blood work checking thyroid function, ferritin, vitamin D, and hormone levels can rule in or out several of the most treatable causes. In many cases, hair loss that feels alarming turns out to be temporary or highly manageable once the underlying trigger is identified and addressed.