Hair fall has dozens of possible causes, ranging from genetics and hormones to stress, nutrition, and everyday styling habits. Losing between 50 and 150 hairs per day is normal, part of your hair’s natural growth-and-rest cycle. When shedding consistently exceeds that range, or when new growth slows down, something is pushing your follicles off track. Here’s what that something usually is.
Genetics and Pattern Hair Loss
The most common cause of hair fall worldwide is androgenetic alopecia, also known as male or female pattern hair loss. It’s driven by a hormone called DHT (dihydrotestosterone), which is produced when an enzyme converts testosterone into a more potent form. DHT binds to receptors found exclusively inside the dermal papilla cells at the base of each hair follicle. In people who are genetically susceptible, this binding gradually shrinks the follicle, a process called miniaturization. Over time, each hair cycle produces a thinner, shorter, less visible strand until 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 tends to cause diffuse thinning along the part line while the hairline stays intact. The genetic component can come from either parent, and it becomes more noticeable with age as follicles accumulate more damage from DHT exposure.
Stress and Sudden Shedding
A major physical or emotional stressor can force up to 70% of your actively growing hairs into the resting phase all at once. This condition, called telogen effluvium, doesn’t show up immediately. You’ll typically notice dramatic shedding two to three months after the triggering event, which is why people often don’t connect the cause to the hair loss.
Common triggers include high fevers, severe infections, major surgery, psychological stress, and crash diets that lack enough protein. The acute form usually resolves within six months once the stressor is removed. Your hair re-enters its growth phase on its own, though the regrowth process can feel painfully slow since hair only grows about half an inch per month.
Nutritional Deficiencies, Especially Iron
Your hair follicles are among the fastest-dividing cells in your body, so they’re sensitive to nutritional shortfalls. Iron deficiency is the best-studied link. In one case-control study, women with telogen effluvium had average ferritin levels (your body’s stored iron) of about 16 ng/mL, compared to 60 ng/mL in women without hair loss. When ferritin drops to 30 ng/mL or below, the odds of experiencing this type of shedding increase roughly 21-fold.
Other nutrients that matter include zinc, biotin, vitamin D, and protein. Restrictive diets, heavy menstrual periods, and conditions that impair nutrient absorption (like celiac disease) all raise the risk. The good news is that hair loss from nutritional deficiency is usually reversible once levels are restored, though it can take several months for visible improvement.
Thyroid and Hormonal Imbalances
Both an overactive and underactive thyroid can cause hair to fall out. Thyroid hormones help regulate how quickly your cells turn over, including the cells in your hair follicles. When those hormones are too high or too low, your body may not only shed hair faster but also slow or stop growing new hair altogether. The pattern is usually diffuse thinning across the entire scalp rather than patchy bald spots.
Other hormonal shifts cause temporary shedding too. Postpartum hair loss is one of the most common examples. During pregnancy, elevated estrogen keeps hair in its growth phase longer than usual, which is why many women notice thicker hair while pregnant. After delivery, estrogen drops sharply, and all that “extra” hair enters the resting phase at the same time. Most women notice peak shedding three to six months after giving birth. It resolves on its own, though it can take up to a year to feel like your hair density has returned to normal. Stopping birth control pills can trigger a similar, smaller-scale version of this hormonal shift.
Autoimmune Hair Loss
Alopecia areata is an autoimmune condition where your immune system mistakenly attacks your own hair follicles. Normally, hair follicles maintain a kind of immune protection that shields them from your body’s defense system. In alopecia areata, that protection breaks down. Certain immune cells (cytotoxic T cells) recognize follicle proteins as foreign and mount an inflammatory attack, causing hair to fall out in round, smooth patches.
The severity varies widely. Some people lose a single coin-sized patch that regrows within months. Others lose all scalp hair or, in rarer cases, all body hair. The condition can start at any age and often runs in families with a history of autoimmune disorders. Newer treatments targeting specific immune pathways have significantly improved outcomes for people with moderate to severe forms.
Scalp Conditions and Inflammation
Chronic scalp inflammation can damage follicles and slow hair growth. Seborrheic dermatitis is a common culprit. It happens when excess oil production on the scalp creates an environment where a naturally occurring yeast called Malassezia overgrows. This triggers irritation, flaking, and itchiness. The inflammation itself can disrupt hair growth, and the intense scratching that follows can physically damage follicles.
Other inflammatory scalp conditions, including psoriasis and folliculitis, can have similar effects. The hair loss from these conditions is generally reversible once the underlying inflammation is controlled through antifungal or anti-inflammatory treatments. Left untreated for long periods, however, chronic inflammation can cause scarring that permanently damages follicles.
Medications That Trigger Shedding
A wide range of medications can cause hair to thin or fall out as a side effect. The most well-known are chemotherapy drugs, which target rapidly dividing cells and often cause dramatic, temporary hair loss. But many everyday medications can trigger a subtler form of shedding, including retinoids (used for acne and skin conditions), beta-blockers (used for blood pressure), calcium channel blockers, certain antidepressants, and some anti-inflammatory drugs.
Medication-related hair loss usually follows the same delayed pattern as stress-related shedding, appearing two to three months after starting or changing a dose. If you notice unusual thinning after starting a new medication, your prescriber can often adjust the dose or switch to an alternative. Hair typically recovers after the medication is stopped or changed, though the regrowth timeline varies.
Hairstyles and Physical Damage
Hairstyles that pull on the hair for extended periods cause a type of hair loss called traction alopecia. Cornrows, locs, tight braids, high ponytails, buns, and hair extensions or weaves are the most common offenders, especially when worn tightly for weeks at a time. The American Academy of Dermatology recommends wearing braids for no longer than six to eight weeks and avoiding tight styles as a daily habit.
In the early stages, traction alopecia is reversible. You might notice small bumps along the hairline, broken hairs, or thinning where the tension is greatest. If the pulling continues over months or years, the follicles eventually scar over. At that point, the affected areas show shiny, smooth skin where hair will no longer grow back. The constant friction from hats, head scarves, or other coverings can contribute to the same problem, particularly if hair is pulled back tightly underneath.
Multiple Causes Often Overlap
Hair fall rarely comes down to a single factor. Someone with a genetic predisposition to thinning might not notice it until a period of high stress, a nutritional gap, or a hormonal change tips the balance. A new mother experiencing postpartum shedding might also be iron-deficient from pregnancy. Identifying which causes are at play matters because the treatments are different for each one. Pattern hair loss responds to hormonal interventions, nutritional deficiencies respond to supplementation, and autoimmune hair loss requires immune-targeted therapy. A dermatologist can often narrow down the cause through a combination of scalp examination, blood work, and your medical history.