Understanding Hair Loss: The Basics

Medically Reviewed by Zilpah Sheikh, MD on May 02, 2025
Written by Matt McMillen
13 min read

Hair loss can happen to anyone. Your genes, certain medical conditions, treatments for disease, and other factors can contribute to partial or, sometimes, total hair loss — not just on your head but on other parts of your body as well. Just as the types and causes of hair loss vary, so do the reactions of the many people who experience it.

“Some people may be very distressed by even minor hair loss, while others may be relatively unbothered by significant thinning or balding,” says dermatologist Brian Abittan, MD, director of Skin and Hair Rejuvenation, and director of Hair Transplantation at the Kimberly and Eric J. Waldman Department of Dermatology at Mount Sinai in New York City.

What Is Hair?

Hair grows everywhere on the human skin except on places such as the palms of our hands, the soles of our feet, our eyelids, and belly buttons, but many hair strands are so fine they’re virtually invisible.

Hair is made of a protein called keratin that is produced in hair follicles in the outer layer of skin. As follicles produce new hair cells, old cells are being pushed out through the surface of the skin at the rate of about 6 inches a year. The hair you can see is actually a string of dead keratin cells. The average adult head has about 100,000-150,000 hairs and loses up to 100 of them a day; finding a few stray hairs on your hairbrush is not necessarily cause for alarm.

At any time, about 90% of the hair on a person’s scalp is growing. Each follicle has its own life cycle that can be influenced by age, disease, and a wide variety of factors. This life cycle is divided into three phases:

  • Anagen. It is the active hair growth phase that generally lasts from two to eight years.
  • Catagen. It is the transitional hair growth phase that lasts two to three weeks.
  • Telogen. It is the resting phase that lasts about two to three months. At the end of the resting phase, the hair is shed and a new hair replaces it, and the growing cycle starts again.

As people age, their rate of hair growth slows.

Types of hair loss

There are many types of hair loss, also called alopecia:

Involutional alopecia. This is a natural condition in which the hair gradually thins with age. More hair follicles go into the resting phase, and the remaining hairs become shorter and fewer in number.

Androgenic alopecia. This is a genetic condition that can affect both men and women. Men with this condition, called male pattern baldness, can begin having hair loss as early as their teens or early 20s. It’s characterized by a receding hairline and gradual disappearance of hair from the crown and frontal scalp. Women with this condition, called female pattern baldness, don't have noticeable thinning until their 40s or later. Women experience a general thinning over the entire scalp, with the most extensive hair loss at the crown.

Alopecia areata. This often starts suddenly and causes patchy hair loss in children and young adults. This condition may result in complete baldness (alopecia totalis). But in about 90% of people with the condition, the hair returns within a few years.

Alopecia universalis. This causes all body hair to fall out, including the eyebrows, eyelashes, and pubic hair.

Trichotillomania. This, seen most frequently in children, is a psychological disorder in which a person pulls out their own hair.

Telogen effluvium. This is temporary hair thinning over the scalp that occurs because of changes in the growth cycle of hair. A large number of hairs enter the resting phase at the same time, causing hair shedding and subsequent thinning. 

Scarring alopecia. This results in permanent loss of hair. Inflammatory skin conditions (such as cellulitis, folliculitis, and acne) and other skin disorders (such as some forms of lupus and lichen planus) often result in scars that destroy the ability of the hair to regenerate. 

Traction alopecia. Hot combs and hair too tightly woven and pulled can also result in permanent hair loss.

Central centrifugal cicatricial alopecia. This is the most common type among Black women. This often manifests as a small bald patch in the center of the scalp that grows over time.

Doctors don’t know why certain hair follicles are programmed to have a shorter growth period than others. But several factors may influence hair loss, such as:

Hormones. Hormonal imbalance, such as abnormal levels of androgens (male hormones normally produced by both men and women), may lead to hair loss.

Genes. Genes from both male and female parents may influence a person’s predisposition to male or female pattern baldness.

Stress, illness, and childbirth. These can cause temporary hair loss. Ringworm, caused by a fungal infection, can also cause hair loss. 

Drugs. Drugs, including chemotherapy drugs used in cancer treatment, blood thinners, beta-adrenergic blockers used to control blood pressure, and birth control pills, can cause temporary hair loss.

Burns, injuries, and X-rays. These can cause temporary hair loss. In such cases, normal hair growth usually returns once the injury heals, unless a scar is produced. Then, hair will never regrow.

Autoimmune disease. This may cause alopecia areata. In alopecia areata, the immune system becomes more active for unknown reasons and affects the hair follicles. In most people with alopecia areata, the hair grows back, although it may temporarily be very fine and possibly a lighter color before normal coloration and thickness return.

Cosmetic procedures. Procedures such as shampooing too often, perms, bleaching, and dyeing hair can contribute to overall hair thinning by making hair weak and brittle. Tight braiding, using rollers or hot curlers, and running hair picks through tight curls can also damage and break hair. But these procedures don’t cause baldness. In most cases, hair grows back normally if the source of the problem is addressed. Still, severe damage to the hair or scalp sometimes causes permanent bald patches.

Medical conditions. Thyroid disease, lupus, diabetes, iron deficiency anemia, eating disorders, and anemia can cause hair loss. Most times, when the underlying condition is treated, the hair will return unless there is scarring as in some forms of lupus, lichen planus, or follicular disorders.

Diet. A low-protein diet or severely calorie-restricted diet can also cause temporary hair loss. 

Vitamin deficiencies. Deficiencies in vitamins A, B, C, D, and E, as well as iron and zinc, have been associated with hair loss.

Hair loss can be subtle and happen over a long period; it also can occur suddenly. The signs of hair loss depend on its causes and so does how you perceive your hair loss. These signs include:

Gradual thinning of hair on the top of your head. This occurs as a result of pattern baldness, the most common type of hair loss. In men and people assigned male at birth (AMAB), this thinning typically begins at the hairline. In women and people assigned female at birth (AFAB), the thinning frequently occurs where they part their hair, but their hair also commonly thins at the hairline.

Sudden hair loss, rather than gradual thinning, also can occur for a variety of reasons, such as alopecia areata and medical treatments, such as chemotherapy.

Patchy bald spots (which are often round or oval in shape) are signs of alopecia areata, which can develop suddenly. Such patches may be large or small, and they can develop on your beard, eyelashes, and eyebrows. Most of the time, alopecia areata does not cause widespread hair loss over your entire head. But it’s possible to lose all the hair on your head and, very rarely, the rest of your body.

Shedding,which can cause your hair to thin, can occur a few months after stressful or hormone-altering events, such as childbirth, illness, a divorce, or the death of a loved one. Some medications and nutritional deficiencies also may be to blame for this condition called telogen effluvium. You’ll likely notice a greater than normal amount of hair in your brush as your hair rapidly sheds.

Losing all the hair on your body can happen due to chemotherapy. The hair usually grows back after treatment ends.

Your hair loss may be accompanied by other symptoms, depending on the cause. These symptoms include:

  • Burning or stinging, which can occur if you have alopecia areata
  • Severe itching, burning, or tenderness in spots where the hair loss occurs, possibly due to infection
  • Scaly bald spots, with oozing sores or blisters, likely due to a fungal infection
  • Inflammation causing redness, swelling, and itchy, pus-filled sores, likely caused by folliculitis decalvans
  • Scaly patches caused by psoriasis, which can affect your scalp, though often only temporarily

Sometimes, hair loss occurs for obvious reasons, such as pattern baldness due to age, your genes, or chemotherapy. In some cases, though, identifying the cause of your hair loss can take some digging. Start the process as soon as you see signs of hair loss. The sooner your doctor identifies the cause, the sooner treatment can begin.

“It is imperative to be diagnosed and treated early to prevent further complications,” says Michael Pan, MD, a dermatologist who specializes in hair loss at Cedars Sinai in Los Angeles. “Besides the emotional impact, hair loss can also expose the scalp to UV radiation, which can lead to skin cancers on the scalp.”

Diagnosing hair loss will involve the following:

  • Physical exam and medical history to identify any underlying medical conditions
  • A scalp exam to look for signs of inflammation
  • Blood tests to evaluate your iron level, your vitamin D level, and your thyroid function
  • Family history to find out if any relatives also had hair loss and, if so, at what age
  • Trichoscopy, an imaging test to evaluate your scalp
  • Biopsy, the removal of skin samples from your scalp to evaluate for skin disease such as scarring alopecia

Your doctor will treat your hair loss based on what caused it. Addressing that cause should solve the problem. But some types of hair loss are permanent. Treating the cause may prevent additional hair loss, but it won’t restore hair that you have already lost. For example, if you have lost hair because you have kept it tightly tied back, you won’t be able to regrow it, though changing how you tie your hair will protect the hair you still have.

Most hair loss treatments target pattern baldness, or androgenic alopecia.

Medications

Your doctor may recommend first trying an over-the-counter topical medication, such as minoxidil (Rogaine), which you apply directly to your scalp. A prescription drug called finasteride (Propecia) has been approved for men and people AMAB with pattern baldness.

If you have hair loss due to alopecia areata, lichen planopilaris, and discoid lupus erythematosus, prescription corticosteroids may help to regrow your hair. 

Antiandrogens such as oral birth control, which suppress sex hormones, may help women and people AFAB with pattern baldness.

Procedures

Several procedures, including surgery, can help with certain types of hair loss. These include:

  • Grafting. In this, hair from another part of your body is transplanted to a bald spot on your head to encourage regrowth.
  • Scalp reduction. In this, a surgeon removes skin from the parts of your scalp where hair does not grow. Skin with hair is then stretched over the bald parts on the side and back of your scalp. It does not help a receding hairline.
  • Light therapy. This is also called low-level laser therapy. This therapy stimulates your hair follicles to regrow hair and may require several sessions. But it can be done at home, says Pan.
  • Platelet-rich plasma (PRP) therapy. This therapy uses your own blood plasma to stimulate hair growth.
  • Scalp micropigmentation. In this, your doctor covers bald patches with a permanent tattoo to make it look like your hair’s thicker than it actually is.

Remember, these treatments take time. You likely won’t see results for three to six months, and your treatments may require multiple sessions.

Finally, be wary of products that promise too much.

“Unfortunately, there are many products on the market that claim to promote hair growth but have very little scientific evidence to support their claims,” says Pan. “It is important that patients find a board-certified dermatologist who can help them navigate this process and find the best treatment.”

Not all hair loss can be prevented. It may be coded into your genes, for example, or it may be an unavoidable side effect of a necessary medical treatment, such as chemotherapy. But you can take steps to protect yourself from some types of hair loss.

“Eating a balanced, healthy diet, maintaining healthy hair care practices, and managing stress can assist,” says Abittan.

Here’s what you can do:

  • Reduce or avoid your exposure to damaging chemicals found in hair dyes, permanents, and relaxers.
  • Eat a diet that contains adequate amounts of biotin (vitamin B7), protein, iron, and zinc.
  • Go easy on your hair. Don’t style it in a way that pulls it too tightly; avoid wrapping it around your fingers; and be gentle when you brush and comb it.
  • Limit or avoid heating your hair with hot oil treatments and heating devices such as curling irons and blow dryers (unless set on low), as heat damages your hair.
  • Wash your hair properly. Use a gentle shampoo followed by a moisturizing conditioner, ideally a leave-in conditioner. Dry with a towel rather than a blow dryer.

Abittan also recommends seeing a dermatologist at the first sign of hair loss. Caught early, you may have more treatment options.

Hair loss affects different people in different ways. Some people may not be bothered by it, while others may find it traumatizing.

“Hair loss can be very difficult to deal with on many levels,” says Abittan.

It often depends on the type of hair loss as well as what caused it and where it occurs, says Abittan. He points to the loss of eyebrows and eyelashes as an example.

“This often signals illness and can have a strong psychological and social impact,” he says, adding that complete hair loss also can be quite distressing. Pattern baldness, on the other hand, does not have the same association with illness and is generally more accepted as normal.

The first thing to do if you experience hair loss is to see your doctor. Not only will this identify the cause and guide treatment, it also may help you emotionally.

“Understanding the cause can help patients feel more in control,” says Abittan.

If your hair loss does cause you emotional distress, talk to your doctor about this. Your treatment plan can address this as well as the hair loss itself.

“An early integrative approach considering both the physical and psychological components of hair loss often yields the greatest benefit,” says Abittan. “As such, combination treatments with medical and psychological interventions are usually more successful.”

If your hair loss is bothering you, there’s plenty that you can do to feel more confident about how you look.

  • Choose styling products that give your hair more volume.
  • Change your hair color.
  • Adjust your hairstyle to make the signs of hair loss, such as a widening part, less apparent.
  • Try a wig (which may be covered by insurance if you have hair loss due to a medical condition) or hair extensions.
  • Shave your head.

“My advice to patients who are struggling with hair loss is to not give up hope,” says Pan.

Hair loss can happen for many reasons. In some cases, you can treat the cause and regrow your hair. But hair loss can be permanent, too. 

See your doctor at the first signs of hair loss to figure out the cause and how it can be treated and, possibly, prevented in the future.

Does creatine cause hair loss?

This supplement, often used by athletes to provide energy to their muscles, likely does not cause hair loss. No studies have shown that hair loss is a side effect of creatine.

Do GLP-1 medications cause hair loss?

GLP-1 meds are popular weight loss drugs, but they do not cause hair loss. It does lead to rapid weight loss, which can trigger telogen effluvium, or stress shedding. This, in turn, can lead to thinning of your hair, but that typically does not last long before your hair regrows. Experts also think weight loss may affect your hormone levels (particularly in women) and that can cause hair loss, not necessarily the meds.

Can lifestyle cause hair loss?

Yes, certain lifestyle factors can lead to hair loss. Stress, for example, can contribute to telogen effluvium, which causes excess shedding of hair. A poor diet also can lead to hair loss. This includes eating too few calories as well as having too little iron, protein, biotin, and zinc in your diet. Smoking can cause inflammation that makes hair loss worse. The harsh chemicals in certain hair care products, as well as how you style your hair, can also make hair loss more likely.

How much hair loss is normal?

It’s normal to shed 50-150 hairs per day. That’s a tiny fraction of the approximately 100,000 hair follicles you have on your head.