Hair falls out for dozens of reasons, but they all come down to a few core mechanisms: your genetics, your hormones, your immune system, what you eat, physical damage to the follicle, or a reaction to stress or medication. Losing 50 to 100 hairs a day is completely normal. When shedding exceeds that, or when hair visibly thins, something has shifted in the growth cycle.
Your hair grows in phases. Each strand spends years in an active growth phase before transitioning to a resting phase and eventually falling out to make room for a new strand. Most causes of hair loss work by either shortening the growth phase, pushing too many hairs into the resting phase at once, or destroying the follicle entirely.
Genetic Pattern Baldness
The most common cause of hair loss worldwide is androgenetic alopecia, better known as male or female pattern baldness. It affects roughly half of all men by age 50 and a significant number of women as well, though the pattern looks different. Men typically see a receding hairline and thinning crown. Women notice a widening part and overall thinning across the top of the scalp.
The mechanism involves a hormone called DHT, a potent byproduct of testosterone. An enzyme in your scalp converts testosterone into DHT, which then binds to receptors in the hair follicle’s root cells. In people who are genetically susceptible, this binding gradually shrinks the follicle over successive growth cycles. Each new hair comes in thinner, shorter, and lighter than the one before it. Eventually the follicle produces only a tiny, nearly invisible strand, or stops producing visible hair altogether. This process is called follicle miniaturization, and it’s progressive without treatment.
Because DHT is the driving force, treatments for pattern baldness work by either blocking the enzyme that creates DHT or by preventing DHT from binding to follicle receptors. The earlier you catch it, the more follicles are still capable of recovering.
Stress and Shock to the Body
A type of hair loss called telogen effluvium happens when a physical or emotional shock pushes a large number of hair follicles into the resting phase all at once. Instead of the normal 50 to 100 hairs, you might lose handfuls in the shower or notice clumps on your pillow. The unsettling part is the delay: shedding typically starts two to three months after the triggering event, which makes it hard to connect cause and effect.
Known triggers include high fever, severe infections, major surgery, significant psychological stress, childbirth, crash diets low in protein, and sudden changes in medication (including stopping birth control pills). Thyroid disorders, both overactive and underactive, can also set it off.
The good news is that acute telogen effluvium usually resolves on its own within six months once the underlying stressor is gone. Hair returns to its normal growth cycle without any specific treatment. If shedding persists beyond six months, it may have become chronic, which warrants a closer look at ongoing health issues.
Postpartum Shedding
During pregnancy, elevated estrogen levels keep more hairs locked in the active growth phase than usual. Your hair may feel thicker and fuller than it’s ever been. After delivery, estrogen drops sharply back to pre-pregnancy levels, and all those hairs that were “held over” enter the resting phase simultaneously. The result is noticeable shedding that typically begins about three months after giving birth.
Postpartum hair loss usually resolves within 6 to 12 months as the hair cycle resets. It can feel alarming, especially when large clumps come out at once, but it represents a return to your normal baseline rather than true hair loss. Gentle hair care and a balanced diet can help minimize additional breakage during this window, but the timeline is largely set by biology.
Nutritional Deficiencies
Your hair follicles are metabolically demanding. When your body is short on key nutrients, hair growth is one of the first non-essential functions to be scaled back. The two deficiencies most strongly linked to hair thinning are iron and vitamin D.
Low iron is especially common in women with heavy periods and in people on restrictive diets. Ferritin, the protein that stores iron, is the marker doctors check. Levels below 10 ng/mL are considered depleted, but some dermatologists suspect that hair thinning can begin at levels well above that clinical cutoff. Vitamin D deficiency, typically defined as levels below 30 IU/dL, has also been associated with diffuse hair loss in women. Other nutrients that play a role include zinc, biotin, and protein. Crash diets and fad diets that eliminate entire food groups are a particularly common culprit.
The encouraging part is that hair loss from nutritional deficiency is usually reversible once levels are restored, though regrowth can take several months given the slow pace of the hair cycle.
Thyroid Problems
Both an overactive thyroid and an underactive thyroid can cause hair loss, and the pattern tends to be diffuse thinning across the entire scalp rather than bald patches. Thyroid hormones directly regulate the hair growth cycle, so when levels swing too high or too low, follicles can prematurely shift into the resting phase.
Because of the hair cycle’s built-in delay, thinning often doesn’t appear until several months after thyroid levels go off-track. This lag can make it tricky to diagnose. Once thyroid function is corrected with medication, hair typically regrows, but patience is required. Full recovery can take many months.
Autoimmune Hair Loss
Alopecia areata is an autoimmune condition in which your immune system mistakenly attacks hair follicles. It usually presents as round, smooth bald patches on the scalp, though it can affect eyebrows, eyelashes, and body hair as well. The condition affects roughly 2% of people at some point in their lives.
Normally, hair follicles enjoy a kind of immune protection that hides them from the body’s surveillance system. In alopecia areata, that protection breaks down. Immune cells, particularly a specific type of white blood cell, swarm the base of the follicle in a characteristic pattern pathologists describe as a “swarm of bees.” These cells attack the follicle and shut down hair production, but they don’t destroy the follicle’s stem cells. That’s why regrowth is possible, sometimes spontaneously and sometimes with treatment.
The condition is unpredictable. Some people experience a single episode that resolves completely. Others have recurring patches over years, and a smaller number progress to total scalp or body hair loss. Newer targeted therapies have dramatically improved outcomes for moderate to severe cases.
Medications That Cause Shedding
A surprisingly long list of common medications can trigger hair loss as a side effect. The mechanism varies. Some drugs push follicles into the resting phase (similar to telogen effluvium), while others, particularly chemotherapy drugs, attack rapidly dividing cells and halt hair growth directly.
Drug classes known to cause shedding include:
- Blood thinners (anticoagulants)
- Blood pressure medications, including beta-blockers, ACE inhibitors, and diuretics
- Cholesterol-lowering drugs
- Antidepressants and mood stabilizers
- Acne medications containing vitamin A (retinoids)
- Hormone-related treatments, including birth control pills and hormone replacement therapy
- Anti-inflammatory drugs (NSAIDs)
- Anticonvulsants
- Immunosuppressants
- Weight loss drugs
Chemotherapy is in a category of its own. It targets all fast-dividing cells, and since hair follicle cells are among the fastest-dividing in the body, hair loss is often rapid and widespread. Hair almost always regrows after chemotherapy ends, though the texture or color may temporarily change.
If you notice thinning after starting a new medication, the timing is your best clue. Drug-induced shedding usually begins within a few weeks to a few months of starting the medication, and it generally reverses after stopping or switching.
Physical Damage to the Follicle
Traction alopecia is hair loss caused by hairstyles that pull on the follicle over time. Tight cornrows, braids, locs, buns, ponytails, hair extensions, and weaves are the most common culprits, especially when combined with chemical relaxers that weaken the hair shaft. Even the constant friction of a hat or head covering can contribute if hair is pulled back tightly underneath.
Early signs include small bumps along the hairline, tenderness at the scalp, and broken hairs around the temples or edges. At this stage, the damage is reversible. Loosening the style and giving the follicles time to recover allows hair to regrow. But if the pulling continues, the follicles eventually scar over and stop producing hair entirely. Where you once had hair, you’ll see smooth, shiny skin. At that point, the loss is permanent.
Heat styling, aggressive brushing, and chemical treatments (bleaching, perming) cause a different kind of damage. They weaken the hair shaft itself, leading to breakage rather than true follicle loss. The hair snaps off at various lengths, creating a thinning appearance, but the follicle is still intact and will grow new hair if given a break from damage.
How to Tell What’s Causing Your Hair Loss
The pattern and timing of your hair loss are the two most useful clues. Diffuse thinning across the entire scalp points toward telogen effluvium, nutritional deficiency, or thyroid issues. Thinning concentrated at the crown or along the part suggests pattern baldness. Smooth, round patches are the hallmark of alopecia areata. Receding along the hairline or temples, especially in combination with tight hairstyles, points to traction alopecia.
Timing matters just as much. If you can trace shedding back to an event roughly three months earlier, a stressor, illness, surgery, medication change, or childbirth, telogen effluvium is the likely explanation. Gradual thinning over months to years is more consistent with pattern baldness or a chronic condition. A blood test checking iron, ferritin, vitamin D, and thyroid function can rule in or rule out several common causes at once.