Hair falling out is almost always caused by one of a handful of common triggers: genetics, stress, nutritional gaps, hormonal shifts, medications, or physical damage to the follicles. Losing between 50 and 150 hairs per day is normal. If you’re finding clumps in the shower, noticing your part widening, or seeing more scalp than usual, something has likely disrupted your hair’s growth cycle. The key to figuring out what’s going on is matching your pattern of loss to the most likely cause.
How Your Hair’s Growth Cycle Works
Every hair on your head moves through three phases independently. The growth phase lasts 3 to 10 years, which is why scalp hair can get so long. After that, the hair enters a short transition phase of 2 to 3 weeks, then shifts into a resting phase that lasts 3 to 4 months. At the end of that resting phase, the hair falls out and a new one starts growing in its place.
At any given time, about 85 to 90 percent of your hair is in the growth phase. Most types of hair loss work by pushing too many hairs into the resting phase at once, or by shrinking the follicle so the growth phase gets shorter and shorter. Understanding this delay matters: whatever triggered your shedding probably happened two to three months before you noticed hair on your pillow.
Stress-Related Shedding
If your hair loss started suddenly and seems to be coming from all over your scalp rather than one spot, the most common explanation is a condition called telogen effluvium. This happens when a physical or emotional shock pushes a large batch of hairs into the resting phase all at once. Two to three months later, those hairs all fall out together, and the volume of shedding can be alarming.
Known triggers include high fever, severe infections, childbirth, major surgery, crash dieting (especially low-protein diets), and significant psychological stress. Stopping birth control pills can also set it off. The good news is that acute episodes typically resolve within six months, and your hair grows back on its own once the trigger is removed. If you can trace the shedding back to a specific event roughly three months earlier, stress-related shedding is the most likely explanation.
Genetic Hair Loss
Pattern hair loss, the most common form of permanent thinning, is driven by a hormone called DHT. DHT binds to receptors in genetically susceptible hair follicles, causing those follicles to gradually shrink. Over time, thick pigmented hairs are replaced by thinner, finer, nearly invisible ones. The growth phase gets shorter while the resting phase gets longer, so each new hair that grows in is smaller and falls out sooner.
In men, this typically shows up as a receding hairline and thinning at the crown. In women, it tends to appear as a widening part or overall thinning across the top of the scalp, with the hairline staying intact. This type of loss is gradual, happening over years rather than weeks, which is the main way to tell it apart from stress-related shedding. If you have a family history of thinning hair, genetics is likely playing a role.
Nutritional Deficiencies
Your hair follicles are metabolically demanding, and when your body is short on key nutrients, hair is one of the first things it deprioritizes. Two of the most studied deficiencies linked to hair loss are iron and vitamin D.
For iron, the marker to pay attention to is ferritin, the protein that stores iron in your body. In women of childbearing age, ferritin levels below 10 to 15 ng/mL are strongly associated with diffuse hair thinning. This is common in women with heavy periods or those following restrictive diets. For vitamin D, levels below 30 IU/dL are considered insufficient, and levels below 10 IU/dL are deficient. Both can be checked with a simple blood test and corrected with supplementation, though it can take several months for shedding to slow and new growth to become visible.
Hormonal and Thyroid Issues
Both an overactive and underactive thyroid can cause diffuse, all-over hair thinning. The loss tends to be uniform across the entire scalp rather than concentrated in patches, and the hair looks generally sparse. Thyroid-related hair loss is usually reversible once the thyroid condition is properly managed, though regrowth can be slow.
Polycystic ovary syndrome (PCOS) is another hormonal cause, particularly in women who also notice irregular periods, acne, or weight gain. PCOS involves elevated androgen levels, which can trigger the same follicle-shrinking process seen in genetic hair loss. PCOS also overlaps with autoimmune thyroid disease, so if you have one, it’s worth screening for the other.
Autoimmune Hair Loss
If your hair is falling out in smooth, round patches roughly the size of a quarter, the likely cause is alopecia areata. This is an autoimmune condition where the immune system mistakenly attacks hair follicles. The bald patches are typically smooth with no scarring, though you might notice tiny black dots where broken hairs sit just below the surface, or short “exclamation point” hairs that are thicker at the tip and narrow toward the scalp. These are distinctive signs.
Alopecia areata can also present as diffuse thinning rather than distinct patches, or as a band of hair loss around the lower back and sides of the scalp. It occurs more frequently in people who already have other autoimmune conditions, including thyroid disease. In many cases, hair regrows on its own, but the condition can be unpredictable, with cycles of loss and regrowth.
Medications That Cause Hair Loss
A surprisingly long list of common medications can trigger shedding. Blood thinners, blood pressure medications (especially beta-blockers), hormonal treatments including birth control and hormone replacement therapy, anticonvulsants, mood stabilizers, retinoids used for acne or skin conditions, cholesterol-lowering drugs, and even some NSAIDs like ibuprofen can all push hairs into the resting phase prematurely.
Some medications cause more hair loss than others. Valproic acid, an anticonvulsant, causes hair loss in 12 to 28 percent of people taking it, with higher doses carrying more risk. Lithium causes it in 12 to 19 percent. Chemotherapy drugs work differently, attacking hairs during the active growth phase, which is why that type of loss is more sudden and severe. If your hair loss started within a few months of beginning or changing a medication, that timing is worth noting and discussing with whoever prescribed it.
Damage From Styling and Tension
Traction alopecia is hair loss caused by repeated mechanical pulling on the hair. Tight ponytails, braids, cornrows, buns, weaves, and heavy extensions all put constant pressure on hair roots, especially along the hairline. Over time, this stress damages the follicles. Early signs include thinning or small bald spots around the hairline, tenderness at the roots, and small bumps on the scalp.
The critical thing about traction alopecia is that it’s reversible in the early stages but can become permanent if the pulling continues long enough to scar the follicles. Combining tight styles with chemical treatments like relaxers or harsh dyes accelerates the damage. Even headgear worn for long periods, such as tightly pinned caps or helmets, can cause it. If your thinning is concentrated where your hair experiences the most tension, loosening your styling habits is the single most important step.
How to Tell What Type You Have
The pattern and timing of your hair loss are the two most useful clues. Sudden, diffuse shedding that you can trace to an event two to three months ago points to telogen effluvium. Gradual thinning concentrated at the part or crown suggests genetic loss. Smooth round patches suggest alopecia areata. Thinning along the hairline where you style your hair tightest points to traction.
A dermatologist can narrow things down quickly. One standard test involves grasping about 40 strands and gently pulling. If six or more come out, you have active shedding. A separate test tugs on a strand from both ends to check if it snaps in the middle, which indicates brittle or structurally damaged hair rather than a follicle problem. Blood work for ferritin, vitamin D, thyroid function, and hormones can rule in or rule out the most common systemic causes. In many cases, hair loss has more than one contributing factor, so a thorough workup is more useful than guessing.