Hair falls out for dozens of reasons, but most cases trace back to a handful of common causes: genetics, hormonal shifts, stress, nutritional gaps, autoimmune conditions, medications, or physical damage to the follicle. Losing between 50 and 150 hairs a day is completely normal. When shedding climbs well above that range, or when hair visibly thins or develops bald patches, something specific is usually driving it.
Genetics and Pattern Hair Loss
The most common cause of hair loss worldwide is hereditary pattern baldness, known clinically as androgenetic alopecia. It affects both men and women, though the pattern differs. Men typically see a receding hairline and thinning crown, while women notice a gradual widening of their part line.
The process starts when a hormone called DHT builds up in the scalp. Over time, DHT contributes to thickening of tissue around each follicle, along with a progressive buildup of fibrous tissue and calcium deposits. These changes slowly restrict the space a follicle has to grow, while also reducing its blood supply, oxygen, and nutrients. The result is follicle miniaturization: hairs grow thinner, shorter, and lighter with each cycle until they eventually stop appearing at all. This happens gradually over years or even decades, which is why many people don’t notice it until significant thinning has already occurred.
Stress and Sudden Shedding
If you’ve noticed clumps of hair coming out in the shower or on your pillow, and it started seemingly out of nowhere, the most likely explanation is a condition called telogen effluvium. It happens when a large number of hair follicles are pushed into the resting phase at the same time, then shed together a few months later.
The tricky part is the delay. Hair loss from telogen effluvium typically shows up two to three months after the triggering event. That gap makes it hard to connect the dots. Common triggers include high fever, severe infection, major surgery, significant psychological stress, and crash diets that cut protein intake too sharply. The acute form usually resolves within six months as the body recalibrates and new hair enters the growth phase.
Hormonal Changes
Hormones regulate the hair growth cycle, so any major hormonal shift can disrupt it. Thyroid problems are one of the most frequent culprits. Both an overactive thyroid and an underactive thyroid can cause hair to stop growing and eventually fall out. People with thyroid imbalances often notice excessive shedding when washing, brushing, or styling, because a higher percentage of hairs have entered the resting phase all at once.
Pregnancy creates another dramatic hormonal swing. During pregnancy, elevated estrogen extends the growth phase, which is why many women enjoy thicker hair while pregnant. After delivery, estrogen drops back to normal levels, and all the hair that was held in the growth phase enters the shedding phase simultaneously. This postpartum hair loss can feel alarming, but it typically resolves on its own within 6 to 12 months after giving birth.
Stopping or starting birth control pills can trigger a similar effect, since oral contraceptives alter hormone levels that influence the hair cycle.
Nutritional Deficiencies
Your hair follicles are metabolically active and need a steady supply of nutrients to function. Iron is one of the most important. Low iron stores, measured by a blood protein called ferritin, are a well-documented contributor to hair shedding. Normal ferritin ranges are roughly 20 to 200 nanograms per milliliter for women and 20 to 500 for men, but some dermatologists believe hair growth suffers when ferritin drops below 30 to 40 even if it technically falls within the “normal” range.
Beyond iron, deficiencies in zinc, biotin, vitamin D, and protein can all impair hair growth. Fad diets and extreme calorie restriction are common offenders because they deplete these nutrients rapidly. In most cases, correcting the deficiency through diet or supplementation allows normal growth to resume within several months.
Autoimmune Hair Loss
Alopecia areata is an autoimmune condition in which the body’s own immune cells, specifically T cells, mistakenly attack hair follicles. It typically causes round, smooth bald patches on the scalp, though it can affect eyebrows, eyelashes, and body hair as well. The patches can appear suddenly and may come and go unpredictably.
In milder cases, hair regrows on its own within months. In more severe forms, the condition can progress to complete scalp hair loss or total body hair loss. It affects roughly 2% of the population at some point in their lives, and it can start at any age, though onset before 30 is common.
Medications That Cause Shedding
A wide range of medications can trigger hair loss as a side effect. Chemotherapy drugs are the most well-known example, but the list extends much further. Blood thinners like warfarin and heparin, seizure medications, lithium for bipolar disorder, beta-blockers used for blood pressure, gout medications, certain arthritis drugs, and amphetamines can all contribute to shedding. Even high doses of vitamin A or stopping birth control pills can tip the balance.
Medication-related hair loss usually follows the same telogen effluvium pattern, appearing weeks to months after starting or changing a prescription. In most cases, hair regrows once the medication is adjusted or discontinued, though that decision should always involve the prescribing doctor weighing the benefits of the drug against the side effect.
Physical Damage to the Follicle
How you style your hair matters more than most people realize. Traction alopecia occurs when hairstyles pull on follicles repeatedly over time. Cornrows, tight braids, locs, tightly pulled ponytails and buns, hair extensions, weaves on chemically relaxed hair, and even rollers worn overnight can all cause it. The constant rubbing of hats or head scarves over tightly pulled-back hair adds to the problem.
Early on, the damage is reversible. You might notice thinning along the hairline or wherever tension is greatest. But if the pulling continues long enough, the follicle scars over and the skin becomes smooth and shiny. At that stage, the hair loss is permanent. People of African descent face a higher risk because the natural shape of the hair follicle makes it more susceptible to tension damage.
Scalp Infections
Fungal infections, particularly ringworm of the scalp, can cause patchy hair loss along with redness, scaling, and itching. The fungus invades hair shafts and weakens them, causing breakage close to the scalp. These infections are more common in children but can affect adults, especially those with weakened immune systems. Treatment with antifungal medication typically resolves the infection and allows hair to regrow.
How the Cause Gets Identified
Because so many conditions cause hair loss, pinpointing the right one requires a structured approach. A dermatologist will typically start with a detailed history: when the loss started, the pattern it follows, your usual hairstyles, family history, recent illnesses or stressors, diet, and any medications you take. Details that seem unrelated, like a recent pregnancy or a crash diet three months ago, often turn out to be the key clue.
The physical exam involves inspecting the scalp for inflammation, redness, sores, or scarring, and evaluating the pattern of thinning. Simple in-office tests help narrow things down. A pull test, where the doctor grasps about 40 strands and gently tugs, reveals whether an abnormal number of hairs are in the shedding phase. A tug test checks whether strands break in the middle, pointing to brittleness or damage rather than root-level loss. Magnification tools can enlarge the view of follicles up to 100 times to assess their health and density.
If the cause isn’t clear from the exam, blood tests can check for thyroid dysfunction, iron and ferritin levels, vitamin deficiencies, and hormonal imbalances. In cases where scarring alopecia is suspected, a small punch biopsy of the scalp can distinguish between different types under a microscope. And when a fungal infection is possible, a scraping or swab of the scalp is sent for laboratory culture to confirm or rule out ringworm.