Hair falls out because every strand has a limited lifespan. Each follicle cycles through growth, rest, and shedding phases, and losing 50 to 150 hairs a day is completely normal. When shedding exceeds that range or hair visibly thins, something has usually disrupted the cycle: hormones, stress, nutritional gaps, immune dysfunction, or scalp conditions.
The Hair Growth Cycle
Your hair goes through four distinct phases. The anagen (growth) phase lasts two to seven years and determines how long each strand can get. The catagen phase is a short transition period where the follicle shrinks. Then comes the telogen (resting) phase, which lasts about two to three months. Finally, in the exogen phase, the old hair detaches and falls out while a new strand begins growing underneath. At any given time, most of your hair is in the growth phase, while a small percentage is resting or shedding.
This staggered timing is why you don’t notice normal daily shedding. Problems start when something pushes a large number of follicles into the resting or shedding phase at the same time, or when follicles shrink so much they can only produce fine, barely visible hair.
Genetic Hair Thinning
The most common cause of progressive hair loss is pattern baldness, which affects both men and women. Your body converts testosterone into a more potent hormone called DHT, and in people with a genetic predisposition, DHT binds to receptors on scalp follicles and gradually shrinks them. Over time, each growth cycle produces thinner, shorter strands until the follicle stops producing visible hair altogether. In men, this typically shows up as a receding hairline and thinning crown. In women, it tends to appear as overall thinning along the part line.
This process is slow, unfolding over years or decades. Blocking DHT’s effects on the follicle can slow or partially reverse it. Two FDA-approved medications target this type of hair loss: a topical solution called minoxidil, which increases blood flow to follicles and extends the growth phase, and finasteride, an oral medication that reduces DHT production (approved for men only).
Stress-Related Shedding
A condition called telogen effluvium is behind the dramatic shedding many people experience after a major physical or emotional event. High fever, severe infection, surgery, childbirth, crash dieting, or intense psychological stress can shock a large number of follicles out of the growth phase and into the resting phase simultaneously. Two to three months later, all those resting hairs fall out at once.
The good news is that this type of shedding is almost always temporary. Acute telogen effluvium resolves within six months, and the follicles are undamaged. Hair grows back on its own once the underlying stressor passes. If you’re pulling out more than one or two hairs when you run your fingers through clean, dry hair, that’s a sign shedding has become excessive.
Hormonal Shifts
Hormones regulate the hair growth cycle, so any significant hormonal change can trigger shedding. Two of the most common scenarios are thyroid disorders and postpartum hair loss.
Thyroid Problems
Both an underactive and overactive thyroid can disrupt hair growth. Thyroid hormones help regulate how quickly cells turn over, including the cells in hair follicles. When production is too low or too high, a higher percentage of hairs enter the resting phase and eventually fall out. The shedding is typically diffuse, meaning it happens all over the scalp rather than in patches. Once thyroid levels are brought back to a normal range with treatment, hair growth usually resumes.
Postpartum Hair Loss
During pregnancy, elevated estrogen extends the growth phase, which is why many women notice thicker, fuller hair in their second and third trimesters. After delivery, estrogen drops back to normal levels, and all the hair that was held in the growth phase enters the shedding phase at once. This typically starts about three months after giving birth and resolves on its own within six to twelve months. It can look alarming, with clumps coming out in the shower, but it’s a normal hormonal reset rather than true hair loss.
Nutritional Deficiencies
Hair follicles are among the fastest-dividing cells in the body, which makes them sensitive to nutritional shortfalls. Iron is one of the most important nutrients for hair growth. Your body needs adequate iron stores (measured by a protein called ferritin) to supply oxygen to follicle cells. When ferritin drops, hair can thin noticeably. Zinc plays a supporting role in cell division and tissue repair within the follicle, and low levels have been linked to increased shedding.
Research on pregnant women found that those with lower levels of zinc, ferritin, hemoglobin, and vitamin B12 during their second and third trimesters experienced significantly more hair loss compared to women in their first trimester or women who weren’t pregnant. While pregnancy amplifies the demand for these nutrients, the same deficiencies can cause shedding in anyone. Restrictive diets, particularly those low in protein, are a recognized trigger for telogen effluvium.
Autoimmune Hair Loss
Alopecia areata is an autoimmune condition where the immune system mistakenly attacks hair follicles. Normally, follicles have a kind of immune protection that keeps them hidden from the body’s defense system. In alopecia areata, that protection breaks down, and a specific type of white blood cell recognizes proteins on the follicle as foreign threats. These immune cells cluster around the base of the follicle, forcing it out of the growth phase and preventing it from restarting the cycle.
The result is sudden, often coin-sized patches of complete hair loss that can appear on the scalp, eyebrows, or beard. In some cases it progresses to total scalp or body hair loss. Unlike pattern baldness, the follicles themselves aren’t destroyed, which means regrowth is possible if the immune attack is controlled. A newer class of medications called JAK inhibitors has been FDA-approved for severe cases. These drugs work by calming the specific immune signaling pathways that drive the attack on follicles.
Scalp Inflammation and Skin Conditions
The environment on your scalp matters more than most people realize. Seborrheic dermatitis, a chronic condition that causes flaky, red, itchy patches, can damage follicles and interfere with hair growth. Excess oil production on the scalp feeds a naturally occurring yeast called Malassezia, which triggers inflammation when it overgrows. That inflammation irritates the follicles directly, and the intense itching leads to scratching, which causes further mechanical damage.
The hair loss from seborrheic dermatitis is reversible. Once the inflammation is treated with antifungal and anti-inflammatory therapies, the follicles recover and hair grows back. Persistent scalp itching, redness, or flaking that coincides with increased shedding is worth investigating for this reason.
Medications That Cause Shedding
Certain medications can push follicles into the resting phase as a side effect. Retinoids, beta-blockers, calcium channel blockers, some antidepressants, and even common anti-inflammatory drugs have all been linked to telogen effluvium. Stopping hormonal birth control is another well-documented trigger, as the body adjusts to the sudden change in hormone levels. This type of shedding follows the same two-to-three-month delay as other forms of telogen effluvium, which can make it hard to connect the hair loss to the medication that caused it.
How to Tell If Your Shedding Is Normal
A simple way to check is the pull test. Run your fingers through clean, dry hair and tug gently. One or two hairs coming away is normal. If you’re consistently pulling out more than that with each pass, shedding has likely become excessive. Pay attention to where the thinning occurs, too. Diffuse thinning across the whole scalp points toward telogen effluvium, nutritional deficiencies, or thyroid issues. A receding hairline or thinning crown suggests genetic pattern loss. Distinct patches of bare scalp are more consistent with alopecia areata or scarring conditions.
The cause determines the treatment, and many causes are fully reversible. Nutritional deficiencies can be corrected, thyroid levels can be managed, stress-related shedding resolves on its own, and scalp conditions respond to targeted therapy. Even genetic thinning can be slowed significantly when caught early. The pattern and timing of your hair loss are the most useful clues for figuring out what’s driving it.