Hair shedding is a normal part of the hair growth cycle, with most people losing between 50 and 150 hairs per day. When shedding exceeds that range or increases noticeably, it typically means something has pushed a large number of hair follicles out of their active growth phase and into a resting phase prematurely. The causes range from everyday stress and nutritional gaps to hormonal shifts and medications.
How the Hair Growth Cycle Works
Every hair on your head cycles through three phases: a growth phase (which lasts two to six years), a brief transition phase, and a resting phase that ends with the hair falling out. At any given time, roughly 85 to 90 percent of your hair is actively growing. The remaining 10 to 15 percent is resting and will shed naturally over the coming weeks. This constant rotation is why you find loose hairs on your pillow and in the shower without noticing any thinning.
Excessive shedding, called telogen effluvium, happens when a physiological stress pushes a large wave of hairs from the growth phase into the resting phase all at once. Instead of the usual 10 to 15 percent of follicles resting, the percentage climbs significantly, and two to three months later those hairs fall out in noticeable quantities. The delay between the trigger and the shedding is what makes it so hard to connect cause and effect.
Stress and Cortisol
Chronic stress is one of the most common triggers. When your body stays in a prolonged stress state, it produces elevated levels of cortisol. A 2021 study published in Nature found the specific mechanism: cortisol acts on the dermal papilla, a signaling structure at the base of each hair follicle, and suppresses a key growth signal called GAS6. When GAS6 drops below a critical threshold, hair follicle stem cells stay dormant instead of cycling back into active growth. The result is an extended resting phase and visible thinning.
This isn’t limited to emotional stress. Major surgery, a high fever, a severe infection, rapid weight loss, or any event that puts significant strain on the body can trigger the same cascade. The shedding typically shows up two to three months after the stressful event, which is why people often don’t connect the two.
Nutritional Deficiencies
Hair follicles are among the most metabolically active cells in the body, and they’re sensitive to nutrient shortages. Iron is the most well-studied link. Your body stores iron as ferritin, and while standard lab ranges often flag ferritin as “normal” above 12 ng/mL, research in the Tzu-Chi Medical Journal suggests that ferritin levels need to reach 40 to 60 ng/mL to adequately support hair growth. Many women with ferritin in the 15 to 30 range, technically within the normal lab reference, still experience increased shedding.
Beyond iron, deficiencies in vitamin D, zinc, biotin, and protein can all contribute. Crash diets and highly restrictive eating patterns are a particularly common culprit because they create multiple nutrient gaps at once while also putting the body under caloric stress.
Hormonal Shifts
Postpartum Shedding
During pregnancy, elevated estrogen keeps a higher-than-normal percentage of hairs in the growth phase. Your hair may feel thicker and fuller than usual. After delivery, estrogen levels drop sharply, and all those hairs that overstayed their growth phase transition to resting at once. The shedding typically starts about three months after giving birth and resolves on its own within 6 to 12 months, according to Johns Hopkins Medicine. It can be alarming, with clumps coming out in the shower, but it represents a correction back to your baseline rather than permanent loss.
Thyroid Dysfunction
Both an overactive and underactive thyroid can cause diffuse hair shedding. Thyroid hormones regulate metabolism in every cell, including hair follicles, and when levels swing too high or too low, follicles can stop growing altogether or prematurely enter the resting phase. If you’re shedding more than usual and also experiencing fatigue, unexplained weight changes, or sensitivity to temperature, a thyroid panel is worth requesting.
Other Hormonal Causes
Starting or stopping hormonal birth control, entering perimenopause, and conditions like polycystic ovary syndrome (PCOS) can all shift the hormonal environment enough to affect the hair cycle. Androgens, the group of hormones that includes testosterone, play a particularly important role. Elevated androgen levels can miniaturize hair follicles over time, though this pattern tends to cause thinning at specific areas of the scalp rather than diffuse shedding.
Medications That Trigger Shedding
A number of commonly prescribed medications list hair shedding as a side effect. The main categories include retinoids (used for acne and skin conditions), certain antifungal drugs, mood stabilizers, blood thinners, and some blood pressure medications. The shedding follows the same telogen effluvium pattern, showing up weeks to months after starting the medication. If you notice increased shedding after beginning a new prescription, the timing is worth mentioning to your prescriber. In most cases, switching to an alternative resolves the problem.
Seasonal Patterns
There’s a biological rhythm to human hair shedding that most people don’t realize exists. A study tracking seasonal changes in hair growth found that the proportion of scalp follicles in the active growth phase peaks in March at over 90 percent and drops to its lowest point in September. Hair shedding peaks around August and September, with average daily loss reaching about 60 hairs per day, more than double the winter rate. If you notice extra hair in your brush every fall, this seasonal cycle is the likely explanation, and it resolves on its own.
How to Tell If Your Shedding Is Excessive
Losing 50 to 150 hairs per day falls within the normal range, but counting individual hairs isn’t practical. A more useful signal is a noticeable change from your own baseline. If your shower drain is suddenly catching more hair than usual, if your ponytail feels thinner, or if you see more scalp than you’re used to, those are meaningful signs.
Clinicians use a simple test: grasping a small bundle of about 50 to 60 hairs and pulling gently from the scalp to the ends. If more than five or six hairs come out easily, it suggests active excessive shedding. You can try a version of this at home to gauge whether what you’re experiencing is beyond the normal range.
Recovery Timeline
The reassuring reality of telogen effluvium is that it’s almost always reversible once the underlying trigger is addressed. After the cause is removed or resolved, shedding typically slows within three to six months. New growth becomes visible in a similar timeframe, but cosmetically significant regrowth, the point where your hair looks and feels noticeably fuller, usually takes 12 to 18 months. Those short, wispy hairs that stick up around your hairline are a good sign: they’re new growth cycling back in.
Chronic telogen effluvium, where shedding continues for longer than six months, can occur when the underlying trigger persists. Ongoing nutritional deficiencies, untreated thyroid disease, or sustained high stress can keep the hair cycle disrupted. In these cases, identifying and correcting the root cause is the only reliable path to recovery. Topical treatments and supplements can support the process, but they won’t override a trigger that’s still active.