Hair loss has dozens of possible triggers, but most cases come down to a handful of common causes: genetics, hormones, stress, nutritional gaps, autoimmune reactions, or physical damage to the follicle. Losing up to about 100 strands a day is normal. When shedding consistently exceeds that, or when hair visibly thins or patches appear, something deeper is usually going on.
Genetics and Hormones: The Most Common Cause
Pattern hair loss, known medically as androgenetic alopecia, accounts for more cases than any other type. Up to 80% of men and 50% of women develop it by age 70. It runs in families, and the mechanism centers on a hormone called DHT (dihydrotestosterone).
Your body converts testosterone into DHT using an enzyme called 5-alpha reductase. People with pattern hair loss have higher levels of both this enzyme and the receptors that DHT binds to in certain areas of the scalp. When DHT locks onto those receptors, it shortens the growth phase of the hair cycle. Over time, each cycle produces a thinner, shorter strand. Eventually, the follicle shrinks so much that the hair it produces can’t even break through the skin’s surface. In men, this typically shows up as a receding hairline and thinning at the crown. In women, it usually appears as gradual thinning along the part line.
Stress-Related Shedding
A major physical or emotional stressor can push large numbers of hair follicles into their resting phase all at once. When those follicles release their strands two to three months later, you notice sudden, diffuse thinning. This delayed timeline is important: the hair loss you see today often traces back to something that happened months ago.
Known triggers include high fever, severe infections, major surgery, childbirth, significant psychological stress, crash diets low in protein, thyroid disorders, and stopping birth control pills. Certain medications can also set it off, including some blood pressure drugs, antidepressants, and anti-inflammatory painkillers. This type of shedding is almost always temporary. Once the underlying stressor resolves, hair typically returns to its normal fullness within six months, though regrowth can feel slow since new hairs take time to reach noticeable length.
Nutritional Deficiencies That Thin Hair
Hair follicles are metabolically demanding. When your body is low on key nutrients, hair growth is one of the first things it deprioritizes. Three deficiencies show up repeatedly in research on hair loss patients: iron, vitamin D, and zinc.
Iron is the most studied. In one clinical comparison, people with diffuse hair loss had average ferritin levels (the protein that stores iron) of about 15 ng/ml, compared to 25 ng/ml in healthy controls. Both numbers fall within the technically “normal” lab range, which starts at 10, but the difference was significant enough to affect hair cycling. This is why some people with hair loss are told their bloodwork looks fine even when their iron stores are suboptimal for hair growth.
Vitamin D levels told a similar story: hair loss patients averaged around 14 ng/ml, while healthy individuals averaged 17 ng/ml. The accepted normal range starts at 20, meaning both groups were actually below optimal, but the hair loss group was notably lower. Zinc differences were subtler overall, but patients with zinc levels below 70 µg/dl were more likely to have active hair loss conditions.
If you suspect a nutritional cause, a blood panel measuring ferritin, vitamin D, and zinc can help clarify what’s going on. Correcting a deficiency through diet or supplementation often slows or reverses the shedding, though it can take several months to see results.
Autoimmune Hair Loss
Alopecia areata looks different from other types of hair loss. It produces small, round, well-defined bald patches that appear suddenly, often on the scalp but sometimes in the beard or eyebrows. The borders are sharp, with normal hair growing right up to the edge of the patch.
The cause is an immune system malfunction. Certain T cells, which normally fight infections, mistakenly identify proteins in the hair follicle as threats. They swarm around the follicle and attack it, forcing the hair into its shedding phase. Natural killer cells also join the assault. The patches are round because the autoimmune reaction spreads outward from follicle to follicle, radiating from a central point.
Alopecia areata is unpredictable. Some people have a single episode that resolves on its own. Others experience recurring patches or, in rarer cases, total scalp or body hair loss. It can occur at any age and often coexists with other autoimmune conditions like thyroid disease or vitiligo.
Thyroid Problems
Both an underactive and overactive thyroid can cause hair thinning, though the mechanisms differ slightly. Thyroid hormones play a direct role in starting and maintaining the growth phase of hair follicles. They stimulate cell division inside the follicle and help regulate how long hair stays in its resting phase before cycling back to active growth.
When thyroid hormone levels drop (hypothyroidism), follicle cells divide more slowly, and hairs that fall out naturally take longer to be replaced. The result is gradual, diffuse thinning rather than bald patches. Hyperthyroidism can also trigger widespread shedding by pushing follicles into the resting phase prematurely. In both cases, the hair loss is typically reversible once thyroid levels are brought back into a normal range with treatment, though regrowth takes time.
Tight Hairstyles and Physical Damage
Repeated mechanical pulling on hair follicles causes a type of loss called traction alopecia. It’s most common with tight ponytails, buns, cornrows, braids, dreadlocks, hair extensions, and weaves. The constant tension damages the follicle and triggers an inflammatory response: redness, small bumps, or even pustules around the hairline or wherever the tension is greatest.
In the early stages, the damage is reversible. Stop the tension, and the follicles recover. But chronic, repeated pulling over months or years destroys the follicle permanently. Terminal hairs are replaced by scar tissue, and no amount of treatment can bring them back. Research on Turkish women wearing tightly wrapped turbans showed frontline hair loss appearing within 10 years, spreading to the sides of the scalp after more than a decade. South Korean nurses who pinned caps to the same spot on their scalp daily for an average of nearly 10 years developed localized bald patches at the pin sites. Even a ballerina who wore a 1.5-pound hairpiece daily for two years experienced worsening hair loss from the weight alone.
The key takeaway is that any hairstyle pulling hard enough to cause scalp tightness or tenderness is pulling hard enough to cause damage over time.
Scalp Inflammation and Skin Conditions
Seborrheic dermatitis, the condition behind persistent dandruff and flaky, irritated scalp skin, can contribute to hair shedding. The culprit is a yeast called Malassezia that naturally lives on your skin. It feeds on the oils your scalp produces, breaking them down into fatty acids that trigger an inflammatory cascade. Your immune system responds with a surge of inflammatory signals, histamine, and immune cells.
This chronic low-grade inflammation doesn’t destroy follicles directly, but it can push hairs into the resting phase prematurely, causing diffuse thinning. It can also accelerate pattern hair loss in people who are already genetically predisposed to it. Treating the scalp condition with antifungal shampoos or anti-inflammatory treatments typically slows this type of shedding.
Multiple Causes Often Overlap
Hair loss rarely has a single clean explanation. Someone with a genetic tendency toward thinning may notice it accelerate after a stressful period, a nutritional dip, or the onset of a thyroid problem. A person with tight braids and low iron stores will lose hair faster than someone with just one of those factors. This is part of why hair loss can feel so frustrating to pin down: it’s often the sum of several contributing forces rather than one obvious trigger. Identifying and addressing even one or two of those forces can make a measurable difference in how much hair you keep.