Hair loss is the partial or complete loss of hair from areas where it normally grows, most commonly the scalp. Everyone sheds between 50 and 150 hairs per day as part of the normal growth cycle, but hair loss becomes noticeable when shedding outpaces regrowth or when follicles stop producing new hair altogether. It affects an estimated 50 million men and 30 million women in the United States alone, and while genetics drive the most common form, causes range from hormonal shifts and autoimmune conditions to stress, nutrition, and medication side effects.
How Hair Grows and Sheds Normally
Hair goes through four distinct phases. The growing phase lasts 2 to 8 years for scalp hair and determines how long your hair can get. A short transition phase follows, lasting about 2 weeks, during which the follicle shrinks and detaches from its blood supply. Then comes a resting phase of 2 to 3 months where the hair sits in the follicle but is no longer growing. Finally, the shedding phase can last several months as the old hair falls out and a new one begins growing in its place.
At any given time, about 85 to 90 percent of your scalp hairs are in the growing phase. The daily shedding you see in your brush or shower drain is simply hairs completing this cycle. Hair loss becomes a problem when something disrupts this balance, either by shortening the growing phase, forcing too many hairs into the resting and shedding phases at once, or permanently damaging follicles so they can no longer produce hair.
Pattern Hair Loss: The Most Common Type
Androgenetic alopecia, commonly called male- or female-pattern hair loss, accounts for the vast majority of cases. More than 50 percent of men over age 50 have some degree of it, and it can start as early as the teenage years. In men, it typically begins with a receding hairline and thinning at the crown. In women, it usually shows up as a gradual widening of the part line, with overall thinning across the top of the scalp rather than a receding hairline.
The underlying mechanism involves a hormone called DHT, a potent form of testosterone. DHT binds to receptors on susceptible hair follicles and triggers a process called miniaturization: the follicle gradually shrinks with each growth cycle, producing thinner, shorter, lighter hairs until it eventually stops producing visible hair at all. Which follicles are susceptible is determined by your genetics, which is why the hair on the sides and back of the head often remains intact while the top thins. This process is gradual, typically unfolding over years or decades.
Stress-Related Hair Loss
Telogen effluvium is a form of temporary, diffuse hair loss triggered by a shock to the body. It happens when a large number of hairs are pushed out of the growing phase and into the resting phase all at once. Two to three months later, those hairs all fall out within a short window, which can be alarming. You might notice clumps in the shower or visibly thinner ponytails.
Common triggers include high fever, childbirth, severe infections, major surgery, significant psychological stress, thyroid disorders, stopping birth control pills, and crash diets that lack adequate protein. Certain medications can also cause it. The good news is that acute telogen effluvium typically resolves on its own within six months once the underlying trigger is addressed. The tricky part is that because of the two-to-three-month delay, people often don’t connect the shedding to the event that caused it.
Autoimmune Hair Loss
Alopecia areata is an autoimmune condition where the immune system attacks hair follicles directly. It typically appears as smooth, round patches of hair loss on the scalp, though it can affect eyebrows, eyelashes, and body hair as well. It can occur at any age and affects both men and women.
In a healthy follicle, the lower portion has a form of immune protection that keeps immune cells from recognizing it as a target. In alopecia areata, this protection breaks down. Certain immune cells then recognize proteins in the follicle as foreign and launch an attack, which shuts down hair production. The follicles aren’t destroyed, which is why hair can regrow, but the immune attack can be unpredictable. Some people experience a single episode, while others cycle through periods of loss and regrowth for years.
Treatment options have expanded significantly in recent years. A class of medications called JAK inhibitors, which work by calming the specific immune pathways involved, received FDA approval starting in 2022. Three different JAK inhibitors are now approved for severe alopecia areata, with the most recent approved in 2024, providing options for both adults and patients as young as 12.
The Role of Nutrition
Nutritional deficiencies can contribute to hair loss, though they’re less common than many supplement ads would have you believe. Iron is the most well-established nutritional factor. Your lab results might show a “normal” ferritin (iron storage) level at 15 to 30 ng/mL, but research suggests optimal hair growth occurs when ferritin reaches around 70 ng/mL. Many women with hair loss have ferritin levels in the 30 to 40 range: technically normal, but not enough for healthy hair production.
Vitamin D deficiency also shows a consistent association with hair loss in studies, though the exact relationship is still being worked out. Nutrients like zinc, B12, and folate appear in many hair supplements, but medical reviews consistently find insufficient evidence to recommend them unless you have a documented deficiency, which is uncommon in people eating varied diets. In other words, supplements are worth considering if you have a confirmed deficiency, but taking handfuls of vitamins won’t fix hair loss that’s driven by genetics or hormones.
How Hair Loss Is Diagnosed
A doctor can often identify the type of hair loss through a visual examination and your medical history. One simple clinical tool is the hair pull test: a provider grasps 50 to 60 hairs near the scalp and slowly pulls. If six or more hairs come loose, it suggests active hair loss. This helps distinguish between normal shedding and something more aggressive.
For pattern hair loss, the distribution of thinning is often enough to make the diagnosis. Your doctor may ask about family history, recent stressors, medications, and dietary habits. Blood work can check for thyroid problems, iron levels, and hormonal imbalances. When the clinical picture is unclear, or when more than one condition might be present, a small scalp biopsy can provide a definitive answer by revealing what’s happening at the follicle level under a microscope.
Treatment Options
Treatment depends entirely on the type of hair loss. For androgenetic alopecia, two FDA-approved medications have been available for years. The first is a topical solution (available in 2% and 5% concentrations) approved for both men and women, which works by increasing blood flow to the follicle and extending the growing phase. The second is an oral medication approved for men that blocks the conversion of testosterone to DHT, directly targeting the hormone responsible for follicle miniaturization.
Both treatments work best when started early, before significant miniaturization has occurred. They slow further loss and can produce some regrowth, but results vary and the medications need to be used continuously. Stopping treatment typically means the hair loss resumes. For more advanced loss, hair transplant surgery moves follicles from resistant areas (usually the back of the head) to thinning areas.
For telogen effluvium, treatment focuses on identifying and resolving the underlying trigger. If it’s a nutritional deficiency, correcting it will allow hair to recover. If it’s stress-related, the hair typically regrows on its own once the body stabilizes. For alopecia areata, JAK inhibitors are now the primary option for severe cases, while milder cases may be treated with steroid injections into the affected patches to suppress the local immune response.
Signs That Hair Loss May Need Attention
Some shedding is completely normal, and a few hairs on your pillow or in the drain don’t mean anything is wrong. The signals worth paying attention to include a noticeably wider part line, a receding hairline, visible scalp through thinning hair, smooth round bald patches, sudden dramatic shedding (especially a few months after an illness or stressful event), or hair loss accompanied by scalp redness, scaling, or pain.
The timing of when you address hair loss matters. Pattern hair loss responds best to treatment in its early stages, before follicles have fully miniaturized. Telogen effluvium can sometimes become chronic if the underlying cause persists. And autoimmune hair loss tends to be more responsive to treatment when caught early rather than after extensive progression. Identifying the type of hair loss is the first and most important step, because the wrong treatment for the wrong type won’t help.