Baldness happens when hair follicles shrink, shut down, or get damaged to the point where they stop producing visible hair. The single biggest driver is your genetics, but hormones, stress, nutrition, medications, and even the way you style your hair all play a role. By age 35, about two-thirds of men will notice some degree of hair loss, and by 50, roughly 85% have significantly thinning hair. Women lose hair too, though the pattern looks different and full baldness is rare.
Genetics and Hormones
The most common form of hair loss, called pattern baldness, is written into your DNA. A key gene sits on or near the androgen receptor, which controls how your hair follicles respond to hormones called androgens. When follicles are genetically sensitive to androgens, those hormones gradually shrink the follicle over time. Each growth cycle produces a thinner, shorter, lighter hair until eventually the follicle produces nothing visible at all.
Because the androgen receptor gene sits on the X chromosome, people have long assumed baldness comes from your mother’s side. That’s partially true, but research shows it’s not the whole story. The genetic predisposition involves multiple genes from both parents, so looking at your father’s hairline matters too.
In men, this process typically starts above the temples, creating a receding hairline that forms an “M” shape while the crown thins toward full baldness. In women, the pattern is different: thinning starts along the part line and spreads outward from the top of the head. A woman’s hairline rarely recedes, and progression to complete baldness is uncommon. The role of androgens in female hair loss is also less clear-cut, though conditions that increase androgen levels (like certain ovarian or adrenal gland disorders) can accelerate it.
How the Hair Growth Cycle Breaks Down
Understanding why hair falls out starts with how it grows. Each hair on your head cycles through four phases independently. The growth phase lasts 2 to 8 years, which is why scalp hair can get so long. Then comes a brief transition phase of about 2 weeks, followed by a resting phase of 2 to 3 months where the hair sits in the follicle but isn’t actively growing. Finally, the hair sheds and a new one begins growing in its place.
In pattern baldness, each successive cycle gets shorter. The growth phase shrinks from years down to weeks, producing thinner, wispier hairs. Eventually, the follicle miniaturizes to the point where it can’t produce a visible strand at all. This is why balding is gradual: you don’t wake up one morning with bare patches. You slowly notice that your hair looks thinner, finer, and less dense before obvious bald spots appear.
Stress, Illness, and Temporary Shedding
Not all hair loss is permanent. A condition called telogen effluvium happens when a physical or emotional shock pushes a large number of hairs out of the growth phase and into the shedding phase all at once. Instead of losing the normal 50 to 100 hairs a day, you might lose double or triple that amount. It shows up as diffuse thinning all over rather than a receding hairline or bald patch.
Common triggers include high fever, severe infections, major surgery, childbirth, psychological stress, and thyroid problems (both overactive and underactive). Crash diets low in protein can cause it too. The shedding usually doesn’t start immediately. It often shows up two to three months after the triggering event, which can make it hard to connect cause and effect.
The good news: telogen effluvium typically resolves on its own within three to six months once the underlying stressor is gone. Your hair grows back without treatment in most cases.
Autoimmune Hair Loss
Alopecia areata is a fundamentally different kind of hair loss. Your immune system mistakenly identifies hair follicles as foreign invaders and attacks them, causing hair to fall out in small, round patches roughly the size of a quarter. In mild cases, you get one or two patches that may regrow on their own. In more severe forms, it can progress to total scalp hair loss or even loss of all body hair, including eyebrows and eyelashes.
Several patterns exist. Some people lose hair in a band along the lower back of the scalp. Others experience diffuse thinning rather than distinct patches, which can look similar to other types of hair loss and make diagnosis tricky. Alopecia areata can start at any age and affects men and women equally.
Medications That Cause Hair Loss
A surprisingly long list of medications can thin your hair. Chemotherapy is the most well-known culprit, and it works differently from other drug-related hair loss. Chemo targets rapidly dividing cells, and since hair follicle cells divide quickly, the drugs attack them directly. This can cause severe, rapid loss of scalp hair, eyebrows, eyelashes, and body hair.
Other medications cause a slower type of shedding. Blood thinners, including both older drugs like warfarin and newer ones, have been linked to hair loss. Certain seizure and mood-stabilizing medications are particularly notorious, with one common class causing noticeable hair loss in roughly 11% of people who take it. Retinoids, beta-blockers, calcium channel blockers, some antidepressants, and even NSAIDs can trigger shedding. Some antibiotics, antifungals, and antiviral medications have been reported to cause it as well.
If you’ve noticed increased hair loss after starting a new medication, that connection is worth exploring with whoever prescribed it. In most cases, hair grows back once the medication is stopped or switched.
Nutritional Deficiencies
Your hair follicles need a steady supply of nutrients to keep producing hair. Iron deficiency is one of the most common nutritional causes of thinning, particularly in women. Low levels of stored iron (ferritin), zinc, and B12 have all been associated with increased hair shedding. Research on pregnant women found that those with severe hair loss had significantly lower zinc, ferritin, and B12 levels compared to women without hair loss.
Protein matters too. Hair is made almost entirely of a protein called keratin, and diets severely lacking in protein can shift hairs into the resting phase prematurely. This is one reason extreme or fad diets sometimes lead to noticeable thinning a few months later.
Hairstyles and Physical Damage
Constant pulling on hair follicles causes a type of hair loss called traction alopecia. It’s most visible along the hairline and temples, where the tension is greatest. Hairstyles that carry the highest risk include tight cornrows, locs, braids, buns, ponytails, and hair extensions or weaves, especially on chemically relaxed hair. Even wearing rollers to bed regularly or pulling hair back tightly under a hat or headscarf can contribute.
Early warning signs include broken hairs around your forehead, a receding hairline, pain or stinging on your scalp, and small patches of loss where hair is pulled tightest. A useful rule from the American Academy of Dermatology: if your hairstyle feels painful, it’s too tight. Caught early, traction alopecia is fully reversible. But years of repetitive tension can permanently scar the follicle, making the loss irreversible. Checking your hairline monthly for early signs is worth the few seconds it takes.
Why Some People Lose Hair and Others Don’t
The frustrating reality is that baldness is rarely caused by one thing. Genetics loads the gun, but hormones, nutrition, stress, health conditions, and lifestyle pull the trigger in different combinations for different people. Two siblings with the same parents can have completely different hair loss trajectories because of how genes interact and which environmental factors each person encounters.
The type of hair loss also determines whether it’s reversible. Pattern baldness is progressive and permanent without intervention, though treatments can slow it or partially restore density. Telogen effluvium and nutritional deficiencies resolve once the cause is addressed. Traction alopecia reverses if caught early. Alopecia areata is unpredictable: hair may regrow spontaneously, fall out again, or both. Knowing which type you’re dealing with is the first step toward doing something about it.