Hair falls out for dozens of reasons, but they all come down to a handful of core mechanisms: hormones shrinking your follicles, your body diverting resources away from hair growth, your immune system attacking follicles, or physical damage to the scalp. Losing 50 to 150 hairs a day is completely normal. Beyond that range, something is usually disrupting the natural growth cycle.
Every hair on your head cycles through three phases. The growth phase lasts two to eight years, followed by a short transition phase of a few weeks where the follicle shrinks, then a resting phase lasting two to four months before the hair sheds and a new one takes its place. Most causes of abnormal hair loss work by pushing too many hairs into that resting phase at once, or by gradually shrinking the follicle so each new hair grows thinner and shorter than the last.
Genetic Hair Loss
The most common cause of hair loss worldwide is pattern baldness, which affects both men and women. It’s driven by a hormone called DHT, a potent byproduct of testosterone. An enzyme in your scalp converts testosterone into DHT, which then binds to receptors inside the cells at the base of your hair follicle. In people who are genetically susceptible, this binding gradually miniaturizes the follicle. Each growth cycle produces a finer, shorter, lighter hair until eventually the follicle stops producing visible hair altogether.
In men, this typically shows up as a receding hairline and thinning at the crown. In women, it’s more often a widening part or overall thinning across the top of the scalp. The process is slow, unfolding over years or decades, which is why many people don’t notice it until significant thinning has already occurred. Genetics from both parents play a role, and the process can begin as early as your late teens or twenties.
Stress-Related Shedding
A condition called telogen effluvium is behind the dramatic hair shedding many people experience after a major physical or emotional event. It happens when a stressor pushes a large percentage of your hair follicles into the resting phase simultaneously. The tricky part is timing: the shedding typically shows up two to three months after the triggering event, which makes it hard to connect the dots.
Common triggers include high fever, severe infections (including COVID-19), major surgery, childbirth, psychological stress, crash diets low in protein, and sudden changes in medication like stopping birth control pills. Thyroid disorders, both overactive and underactive, can also trigger this type of shedding. The good news is that acute telogen effluvium usually resolves on its own within six months once the underlying cause is addressed. Your follicles aren’t damaged, just temporarily disrupted.
Hormonal Shifts
Pregnancy is a perfect example of how hormones control hair growth. During pregnancy, elevated estrogen keeps more hairs locked in the growth phase, which is why many women notice thicker hair. After delivery, estrogen levels plummet, and all those hairs that were “held back” enter the resting phase at once. Postpartum hair loss usually starts around three months after giving birth and resolves within 6 to 12 months as hormone levels stabilize.
Menopause triggers a similar shift. Declining estrogen and progesterone levels mean that the relative influence of androgens (like DHT) increases, which can lead to gradual thinning. Polycystic ovary syndrome, which raises androgen levels, is another hormonal driver of hair loss in women of reproductive age.
Autoimmune Hair Loss
Alopecia areata is an autoimmune condition where your immune system mistakenly attacks hair follicles. It typically appears as smooth, round bald patches on the scalp, though it can progress to complete scalp hair loss or even loss of all body hair. The underlying problem is a breakdown in the follicle’s natural immune protection. Normally, hair follicles have a kind of biological shield that keeps immune cells from recognizing them as targets. When that shield fails, immune cells swarm the base of the follicle, forcing hairs into the resting phase and preventing new growth from starting.
Alopecia areata can strike at any age, and its course is unpredictable. Some people experience a single episode with full regrowth, while others have recurring patches or progressive loss. Because it’s immune-driven rather than caused by follicle damage, the potential for regrowth remains even after prolonged hair loss. Newer treatments that calm the specific immune pathways involved have shown strong results in clinical trials over the past few years.
Nutritional Deficiencies
Your hair follicles are among the fastest-dividing cells in your body, which makes them sensitive to nutritional shortfalls. Iron deficiency is one of the most well-documented nutritional causes of hair loss. Low ferritin levels (your body’s iron storage protein) are frequently found in people experiencing diffuse thinning, even when they aren’t technically anemic. Zinc deficiency is another established contributor. Research on pregnant women with severe hair loss found significantly lower levels of zinc, ferritin, and vitamin B12 compared to controls.
Protein is also essential. Crash diets or very restrictive eating patterns that cut protein intake can trigger telogen effluvium within a few months. Vitamin D deficiency has been linked to hair loss in some studies, though the evidence is less consistent than for iron and zinc. If you’re losing hair and can’t identify another cause, a blood panel checking ferritin, zinc, vitamin D, thyroid function, and B12 is a reasonable starting point.
Thyroid Problems
Both an overactive and underactive thyroid can cause hair loss, and the pattern is often diffuse thinning across the entire scalp rather than bald patches. Thyroid hormones regulate the rate at which your body produces new cells, including the cells in hair follicles. When levels are off in either direction, hair follicles can stop growing new hair entirely or shift prematurely into the shedding phase. Thyroid-related hair loss is usually reversible once hormone levels are brought back into the normal range with treatment, though regrowth can take several months to become noticeable.
Medications That Cause Hair Loss
A surprisingly long list of common medications can trigger hair shedding as a side effect. Chemotherapy drugs are the most well-known, causing rapid, widespread hair loss by targeting fast-dividing cells. But many everyday medications can cause more gradual thinning, including blood thinners, beta-blockers used for blood pressure, certain seizure medications, lithium for bipolar disorder, gout medications, arthritis drugs, and high doses of vitamin A. Amphetamines prescribed for ADHD are another lesser-known trigger.
Drug-related hair loss is almost always reversible once the medication is stopped or changed, though it can take several months for shedding to slow and new growth to appear. If you suspect a medication is behind your hair loss, don’t stop taking it on your own. Talk to your prescriber about alternatives.
Physical Damage to the Scalp
Traction alopecia is hair loss caused by repeated physical pulling on the hair. Tight ponytails, braids, cornrows, buns, and hair extensions are the most common culprits, especially when worn consistently over months or years. The loss tends to concentrate around the hairline and temples, where tension is greatest. Heavy styles like long dreadlocks or weighty extensions compound the problem by adding constant downward force on the roots.
Occupational and cultural factors matter too. Nurses who pin caps tightly, people who wear helmets for extended periods, and anyone whose headwear creates consistent friction or pulling on specific areas of the scalp can develop traction alopecia over time. Chemical treatments like relaxers and harsh dyes weaken the hair shaft, so combining them with tight styles accelerates the damage. The key distinction with traction alopecia is that early cases are fully reversible if you change the hairstyle, but prolonged tension can scar the follicle permanently, making the loss irreversible.
Signs That Need Attention
Some patterns of hair loss warrant prompt evaluation. Sudden hair loss, where large clumps come out over days or weeks, can signal an underlying medical condition. Patches of scaling, redness, swelling, or oozing on the scalp may point to a fungal infection like ringworm rather than typical hair loss. A receding hairline in women, known as frontal fibrosing alopecia, benefits from early treatment because the scarring it causes can become permanent. And any hair loss accompanied by pain, burning, or visible scarring on the scalp suggests an inflammatory process that could destroy follicles if left untreated.
Patchy hair loss in children is worth investigating quickly, as it could be alopecia areata, ringworm, or a hair-pulling habit. In most other cases, especially diffuse thinning that develops gradually, the cause is identifiable through a combination of medical history, blood work, and scalp examination.