Finding more hair than usual in your brush is unsettling. The first question most people have is what causes hair loss and whether it'll stop. Here we’ll take a look at what causes hair loss in men and women and how to make thinning hair look fuller.

The 15 Most Common Causes Of Hair Loss

The 15 most common causes of hair loss are below, roughly in order of how often they happen.

1. Genetics (Hereditary Hair Loss)

Hereditary hair loss is the most common cause of thinning hair in both men and women.

Its medical name is androgenetic alopecia, and it is commonly referred to as male pattern baldness and female pattern baldness. MedlinePlus estimates it affects 50 million men and 30 million women in the United States, and more than half of men over 50 have some degree of it.

It's driven by DHT, a hormone made from testosterone. In people with the genes for it, DHT shrinks hair follicles over time. Each new hair grows finer and shorter, until some follicles stop producing visible hair at all.

You can inherit the genes from either parent—not just your mother's side. Hereditary hair loss can start as early as your teens, and the risk increases with age.

What It Looks Like

Hereditary hair loss usually develops gradually, following a recognizable pattern:

• In men: Your hairline may start moving back at the temples, creating an M shape. You may also notice a thinning spot at the crown. As hair loss progresses, these areas can expand and meet, while the sides and back stay fuller.
• In women: Your part may look wider, with more scalp showing through the hair across the top and crown. Your ponytail may feel thinner, too. The front hairline usually stays intact, and complete baldness is uncommon.

We compare these patterns in more detail in female versus male pattern baldness. These signs can suggest hereditary hair loss, but a dermatologist can confirm the cause.

2. Age

Hair naturally gets thinner with age, even without hereditary hair loss.

The AAD notes that hair growth slows as we get older, and some follicles stop producing hair altogether. Strands also tend to get finer. 

Age-related thinning can happen alongside hereditary hair loss, so the two aren’t always easy to separate.

What It Looks Like

You may notice a gradual loss of fullness over the years. Your hair may feel finer, have less volume, or provide less scalp coverage than it once did. These changes can be spread across the scalp rather than concentrated in one bald spot.

Appearance alone can’t confirm that aging is the cause. A receding hairline or widening part may also point to hereditary hair loss, which also becomes more common with age.

3. Stress, Illness, or Surgery (Telogen Effluvium)

Stress, illness, and surgery can cause hair loss by pushing a large number of hairs into the shedding phase at once.

This is called telogen effluvium. The trigger can be emotional, like a death in the family or a breakup. It can also be physical, like surgery, a high fever, a bad case of the flu or COVID-19, or a crash diet.

The catch is the delay. According to JAMA, shedding usually starts two to four months after the trigger and settles within six to nine months. By the time your hair starts falling out, you may have stopped thinking about the event that caused it.

What It Looks Like

The biggest clue is a noticeable increase in shedding. You may see:

• Much more hair than usual in your shower drain, brush, or on your pillow.
• Thinning across your whole scalp, rather than distinct bald patches.
• Less overall fullness or a smaller ponytail as shedding continues.

Think back over the previous few months: a major illness, stressful event, or surgery may help explain the timing. These clues can suggest telogen effluvium, but they don’t confirm the cause.

The good news is that the follicles aren't damaged, so the hair grows back. (Read more in our posts on hair loss caused by stress and anxiety and why your hair is shedding so much.)

4. Pregnancy and Childbirth

Many women shed a lot of hair a few months after having a baby.

High estrogen during pregnancy keeps hair in its growing phase longer, and many women notice thicker hair while pregnant. After birth, hormone levels drop, and all that extra hair sheds at once. It can be alarming, but it's temporary. 

What It Looks Like

You may notice much more hair in your brush, shower drain, or on your clothes, along with less fullness across your scalp. The key clue is the timing: shedding starts a few months after childbirth and usually peaks around four months.

According to the American Academy of Dermatology, most women regain their normal fullness by their baby’s first birthday, or even sooner. If yours hasn’t returned after a year, a dermatologist can check whether something else is contributing. (See our guide to postpartum hair loss to learn more.)

5. Menopause and Other Hormone Changes

Falling estrogen during perimenopause and menopause is another common cause of thinning hair in women.

As estrogen drops, hair along the part and the top of the head often thins, and the texture can change too. This is also when hereditary hair loss becomes most noticeable in women. (Our post on menopause and hair loss covers it in more detail.)

Birth control can play a part as well. Stopping an estrogen-containing pill can trigger shedding a few months later, the same way childbirth does. 

For men, testosterone therapy can speed up hereditary hair loss in those prone to it, because the body converts some of that testosterone into DHT. 

What It Looks Like

The signs depend on the hormone change:

• Around menopause: Your part may gradually widen, with more scalp showing across the top of your head. Your hair may also feel finer, drier, or less full. The front hairline usually stays largely intact.
• After stopping an estrogen-containing pill: You may notice increased shedding and thinning across the scalp a few months later.
• With testosterone therapy: Hair loss typically follows the male pattern, with recession at the temples, thinning at the crown, or both.

Timing can provide a clue, but appearance alone can’t tell you whether hormones are the cause.

6. Polycystic Ovary Syndrome (PCOS)

PCOS can cause hair thinning on the scalp because it raises androgen levels. These hormones can contribute to thinning hair on the scalp while encouraging extra hair growth on the face or body.

What It Looks Like

Hair thinning usually follows the female pattern: a widening part and less coverage across the top of the head, while the front hairline generally stays intact.

Other symptoms can provide clues, including:

• Irregular or missed periods
• Acne
• Increased facial or body hair, such as coarse hairs on the chin or chest

A widening part alone doesn’t mean you have PCOS. If you’re noticing thinning alongside these other changes, it’s worth asking your doctor whether PCOS could be contributing.

The AAD notes that treating the hormone imbalance may help reverse the hair loss.

7. Thyroid Disease

Both an underactive thyroid (hypothyroidism) and an overactive thyroid (hyperthyroidism) can cause hair loss, particularly when the condition is severe or goes untreated for a long time.

What It Looks Like

Your hair may look thinner across your whole scalp, with less overall fullness rather than one distinct bald spot. Other symptoms can provide clues:

• An underactive thyroid: Feeling unusually cold or tired, weight gain, dry skin, and dry or thinning hair.
• An overactive thyroid: Feeling unusually hot or sweaty, unexplained weight loss, shakiness, or a racing heartbeat.

If these changes sound familiar, your doctor can check your thyroid function with blood tests.

The upside is that thyroid-related hair loss is one of the most fixable types of hair loss. Once your thyroid levels are treated and stable, the AAD says the hair loss usually reverses.

8. Alopecia Areata 

Alopecia areata is an autoimmune condition where the immune system attacks the hair follicles, causing round, smooth bald patches. MedlinePlus says it affects about 1 in every 500 to 1,000 people in the U.S., and it usually starts in the teens or early adulthood. People who have it are also more likely to have other autoimmune conditions, like thyroid disease or vitiligo.

What It Looks Like

The most recognizable sign of alopecia areata is one or more round or oval bald patches, often about the size of a coin. These patches can appear quickly, and the exposed skin usually looks smooth.

You may notice patches on your scalp or beard, or gaps in your eyebrows or eyelashes. Some people also develop small dents, called pits, in their fingernails. Less commonly, hair loss affects the whole scalp or body.

Hair often grows back on its own, but it can fall out again. JAMA notes that regrowth happens without treatment in up to 30% of milder cases. Steroid injections can help patches regrow, and a newer class of pills called JAK inhibitors is used for severe cases.

9. Nutritional Deficiencies

Low iron, low vitamin D, and too little protein are the deficiencies most often linked to hair loss. Nutrient deficiencies, rapid weight loss, and very restrictive diets can trigger the same kind of shedding as stress. 

Nutrients that may play a part include:

• Iron. Low iron is a common, treatable cause of shedding, especially in women with heavy periods.
• Vitamin D. Low levels are linked to several types of hair loss, including alopecia areata.
• Protein and zinc. Not getting enough of either can cause noticeable thinning.

What It Looks Like

You may notice more hair in your brush or shower drain and less fullness across your scalp. If a restrictive diet or rapid weight loss triggered the shedding, it may not begin until two to four months later.

There’s no specific thinning pattern that tells you which nutrient you’re missing. A doctor can review your diet and symptoms and decide which blood tests would be useful.

Biotin gets most of the marketing, but a review in Dermatology and Therapy found that real biotin deficiency is rare, and that large studies don't support biotin supplements for hair. Taking too much of some nutrients (like vitamin A or selenium) can even cause hair loss. Ask for a blood test before you start supplementing. That way you're treating a real shortfall, not guessing.

10. Medications

Hair loss is a side effect of a long list of common medications.

JAMA lists the blood thinner warfarin, the blood pressure drug propranolol, the antidepressant fluoxetine, lithium, the seizure drug valproate, and retinoids like isotretinoin. Mayo Clinic adds that some drugs for arthritis, gout, and heart problems can also contribute to hair loss. Fast weight loss can also trigger shedding, including weight loss from the newer GLP-1 weight loss injections.

What It Looks Like

For many medications, the main sign is increased shedding and thinning across the scalp. You may notice more hair in your brush or shower drain and less overall fullness.

The timing can provide a clue: shedding may begin two to four months after starting a medication or changing the dose. Some drugs, particularly chemotherapy medicines, can cause much faster and more extensive hair loss.

If your shedding started a few months after a new prescription, talk to your doctor. Don't stop the medication on your own. There's often an alternative that doesn't affect your hair.

11. Chemotherapy and Radiation

Some chemotherapy drugs damage rapidly dividing cells in hair follicles as they target cancer cells. Radiation can also damage hair follicles, but only in the area being treated.

What It Looks Like

• With chemotherapy: Hair may begin falling out within a few weeks of starting treatment. You might notice gradual thinning or hair coming out in clumps. Depending on the treatment, eyebrows, eyelashes, and body hair can also be affected.
• With radiation: Hair loss is limited to the treatment area. Radiation to the scalp can cause patches of hair loss, while radiation elsewhere doesn’t directly cause scalp hair to fall out.

Before starting chemotherapy, ask your care team about a cooling cap, which can reduce hair loss during chemo. (Read more on hair loss and regrowth after cancer treatment.)

12. Tight Hairstyles and Harsh Hair Care

Hairstyles that pull on the hair for long periods can cause a type of hair loss called traction alopecia. Tight ponytails, braids, cornrows, buns, extensions, and weaves all put steady tension on the roots. Over time, hair at the hairline and temples starts to thin. 

Relaxers, perms, bleach, and frequent high heat also weaken the hair shaft and can damage the follicles. 

What It Looks Like

• From tight hairstyles: Thinning edges, a receding hairline, or sparse areas where the style pulls most strongly. You may notice short, broken hairs around your forehead. Pain, stinging, or scalp tenderness can signal that a style is too tight.
• From chemical or heat damage: Brittle, frizzy hair with broken strands of different lengths. Your hair may look less full because strands are snapping before they can grow longer.

If your edges are thinning, remove the tension first. Early traction alopecia can improve once the pulling stops, but repeated pulling over time can permanently damage the follicles. Reducing heat and chemical treatments can help prevent further breakage while healthier hair grows in.

13. Scalp Infections and Skin Conditions

Scalp infections and inflammatory skin conditions can cause hair loss. Scalp ringworm (tinea capitis) is a contagious fungal infection that affects the hair and scalp. Psoriasis is an inflammatory condition that causes scaly plaques, where scratching or forcefully removing the scales can lead to temporary hair loss.

What It Looks Like

Changes to the skin alongside the hair loss can provide clues:

• Scalp ringworm: Itchy, scaly patches with broken hairs or small black dots where hairs have snapped near the scalp. The affected areas may look inflamed.

• Scalp psoriasis: Thick, scaly patches that may itch, burn, or feel sore, with thinning where you’ve been scratching or picking at the scales.  

If hair loss comes with scaling, itching, or soreness, have your scalp checked. Scalp ringworm needs prescription antifungal medicine taken by mouth. Hair usually regrows after the infection is treated or the psoriasis is controlled, provided the follicles haven’t scarred.                                                                               

14. Scarring Alopecia

Scarring alopecia is a group of conditions where inflammation destroys hair follicles and replaces them with scar tissue. It's the one type where speed matters most. Once a follicle scars, it can't grow hair again. 

What It Looks Like

The pattern depends on the condition. Two forms to know are:

• Central centrifugal cicatricial alopecia (CCCA). It starts as a small round patch at the crown and spreads outward. The AAD says it's the most common cause of scarring hair loss in Black women and usually begins in middle age.
• Frontal fibrosing alopecia (FFA). The hairline slowly recedes along the front and sides. It mostly affects women after menopause, and the AAD notes that 80% to 95% of women with it lose some or all of their eyebrows too.

Burning, itching, or tenderness where the hair is thinning can be another warning sign. Early treatment can stop the spread and sometimes regrow some hair.

15. Hair Pulling and Friction

Repeatedly pulling, rubbing, or twisting hair can cause hair loss in the areas you touch. Trichotillomania is a condition where people feel an urge to pull out their own hair, often during stress and sometimes without noticing. 

What It Looks Like

• Hair pulling: Irregular patches with hairs of different lengths, often feeling short and stubbly. You may also notice gaps in the eyebrows or eyelashes.
• Friction: A patch of short, broken hairs or thinning that matches the area being repeatedly rubbed.

Hair can grow back once the pulling or rubbing stops, provided the follicles haven’t been permanently damaged. If you find yourself pulling and struggling to stop, a doctor or therapist can help. Habit reversal training is a commonly used treatment that helps you recognize the urge and practise a different response.

How To Find Out What's Causing Your Hair Loss

To find the cause of your hair loss, start with three questions: where is it, how fast is it happening, and what changed a few months ago:

1. Check the pattern. Photograph your part, crown, and hairline in the same light every few weeks. Patterned, all-over, or patchy will point you in the right direction.
2. Look at the timeline. Slow hair loss over years points to genetics. Sudden hair loss over weeks points to a trigger.
3. Look back three months. Illness, surgery, a baby, a new medication, a new diet, or a big loss can all cause hair loss a few months down the road.
4. Check your family. Thinning hair in parents, grandparents, aunts, or uncles on either side raises your odds of hereditary loss.
5. Check your scalp. Redness, scaling, itching, pain, or shiny, smooth skin are signs to see a doctor.

If you’re struggling with hair loss, a doctor or dermatologist will usually look closely at your scalp and may gently pull on a few hairs to see how many come out. They may order blood tests for thyroid function, iron stores, and vitamin D. If scarring alopecia is a possibility, a small scalp biopsy can confirm it.

Where You're Losing Hair Says A Lot About Why

The pattern of your hair loss is the single best clue to its cause.

A JAMA patient guide sorts most hair loss into three patterns. Look in the mirror, or ask someone to take a photo of the top of your head, and see which one fits.

• Patterned thinning. Slow, even thinning at the front, top, and sides. This is almost always hereditary hair loss.
• Diffuse thinning. Hair thins evenly all over, often with heavy shedding. This points to telogen effluvium, thyroid problems, low iron, or a medication.
• Patches. Round, defined bald spots. This is usually alopecia areata, though a scalp infection or hair pulling can look similar.

When should you see a doctor about hair loss?

See a doctor about hair loss if it's sudden, patchy, painful, or comes with other symptoms like:

• Hair is coming out in handfuls or in round patches.
• Your scalp is itchy, scaly, red, burning, or sore.
• Your hairline is receding, and you're a woman, or your eyebrows are thinning.
• You also feel unusually tired or cold, or your weight has changed without explanation.
• You've just started a new medication, and the shedding followed.
• The thinning is bothering you, and you want to treat it.

Don't wait it out if any of these apply. Most treatments for hair loss work best when they're started early, and scarring types can't be reversed once the follicle is gone.

How To Make Thinning Hair Look Fuller While You Find The Cause

You can make thinning hair look fuller right away with a cosmetic concealer, whatever the cause.

Treatments take months to show results, and telogen effluvium can take most of a year to settle. In the meantime, a concealer doesn't change your hair. It changes how much scalp shows through. Here are some options:

• Hair fibers work best where you still have some hair, such as a thinning crown or a widening part. SureThik Hair Fibers are made with keratin, cling to your existing hair, and come in eight shades.
• Hairline powder works on very fine or sparse areas, like a thinning hairline or temples, because it tints the scalp itself. SureThik Hairline Powder is talc-free and sweat-resistant.
• A gentle shampoo keeps the scalp clean without stripping fragile hair. Our Hair Thickening Shampoo is sulfate-free.

If your scalp is red, scaly, or sore, have it checked before using any product on it. You can learn more about how hair fibers can help cover your thinning areas in our piece on how hair fibers work.

FAQs

What is the difference between hair shedding and hair loss?

Losing 50 to 100 hairs a day is normal shedding. Hair loss is when hair stops growing back in its place.

Every hair on your head grows for a few years, rests, then falls out and gets replaced. New hair is growing in as the old hair falls out.

If far more hair than usual sheds at once, or the hair that comes back grows thinner and shorter until it stops, you’re likely dealing with hair loss.

Can hair loss be reversed?

Yes, hair loss can often be reversed, depending on the cause. Shedding from stress, childbirth, thyroid disease, low iron, or a medication usually recovers once the cause is fixed. Hereditary hair loss can be slowed and partly regrown with treatment. Scarring alopecia can't be reversed, but it can be stopped.

Can you prevent hair loss?

No, you can't prevent hereditary hair loss, but you can avoid several other causes. Loosen tight hairstyles, go easy on heat and chemical treatments, avoid crash diets, and don't share hats or brushes. For hereditary thinning, treatments like minoxidil and, for men, finasteride work best when started early.

Do hats cause hair loss?

No, normal hats don't cause hair loss. A hat tight enough to rub the same spot every day can thin the hair there. Choose a looser fit if you wear one all day.