With age, most people notice some hair loss because hair growth slows.

Alopecia areata

Alopecia areata is a disease that develops when the body’s immune system attacks hair follicles (what holds the hair in place), causing hair loss. You can lose hair anywhere on your body, including your scalp, inside your nose, and in your ears. Some people lose their eyelashes or eyebrows.

Cancer treatment

If you receive chemotherapy or have radiation treatment to your head or neck, you may lose all (or most of) your hair within a few weeks of starting treatment.

Childbirth, illness, or other stressors

A few months after giving birth, recovering from an illness, or having an operation, you may notice a lot more hairs in your brush or on your pillow. This can also happen after a stressful time in your life, such as a divorce or death of a loved one.

Hair care

If you color, perm, or do too much experimentation with your hair, you could be damaging your hair. Over time, this damage can lead to hair loss.

Hairstyle pulls on your scalp

If you often wear your hair tightly pulled back, the continual pulling can lead to permanent hair loss. The medical name for this condition is traction alopecia.

Hormonal imbalance

A common cause of this imbalance is polycystic ovary syndrome (PCOS). It leads to cysts on a woman’s ovaries, along with other signs and symptoms, which can include hair loss. Stopping some types of birth control pills can cause a temporary hormonal imbalance. Women who develop a hormonal imbalance can develop thinning hair (or hair loss) on their scalp.

Scalp infection

A scalp infection can lead to scaly and sometimes inflamed areas on your scalp. You may see what look like small black dots on your scalp. These are actually stubs of hair. Some people develop a bald spot.

Medication

A possible side effect of some medications is hair loss. If you think a medication is causing your hair loss, ask the doctor who prescribed it if hair loss is a possible side effect. It’s essential that you do not stop taking the medication before talking with your doctor. Abruptly stopping some medications can cause serious health problems.

Scalp psoriasis

Many people who have plaque psoriasis develop psoriasis on their scalp at some point. This can lead to hair loss.

Pulling your hair

Some people pull on their hair, often to relieve stress. They may be unaware that they’re pulling their hair. The medical term for this is trichotillomania.

Scarring alopecia

This condition develops when inflammation destroys hair follicles. Once destroyed, a hair follicle cannot grow hair. Diverse conditions can cause this. The medical name for this group of conditions is cicatricial alopecia.

Sexually transmitted infection

Left untreated, a sexually transmitted infection (STI) can lead to hair loss. Syphilis is such an STI. Left untreated, syphilis can cause patchy hair loss on the scalp, eyebrows, beard, and elsewhere. Other STIs can also cause hair loss.

Thyroid disease

If you have a problem with your thyroid, you may see thinning hair. Some people notice that their hair comes out in clumps when they brush it.

Too little biotin, iron, protein, or zinc

If you’re not getting enough of one or more of these, you can have noticeable hair loss.

Friction

People can develop hair loss were boots, socks, or tight clothing frequently rubs against their skin. The medical term for this is frictional alopecia.

Poison

Being slowly poisoned can lead to hair loss. Poisons that can cause hair loss include arsenic, thallium, mercury, and lithium. If you ingest a large amount of warfarin, which is found in rat poisons, it can also cause hair loss. Taking large amounts of vitamin A or selenium is also toxic and can cause hair loss.