Hair falls out as a normal part of its growth cycle, and most people shed between 50 and 150 strands every day without noticing. When shedding ramps up noticeably, though, something has usually shifted in your body: a hormonal change, a nutritional gap, a stressful event, or an underlying health condition pushing more hair follicles into their resting phase at once.
How the Hair Growth Cycle Works
Understanding why hair falls out starts with how it grows. Each hair on your head cycles independently through four phases. The growing phase lasts 2 to 8 years, and at any given moment roughly 85% to 90% of your scalp hair is actively growing. A short transition phase follows, lasting about two weeks, during which the follicle shrinks and detaches from its blood supply. Then comes the resting phase, which lasts 2 to 3 months. About 9% of your scalp hair is resting at any time. Finally, during the shedding phase, the old hair drops out and a new strand begins growing in its place.
This staggered timing is why you don’t lose all your hair at once. But when something disrupts the cycle, a larger-than-normal percentage of hairs can get pushed into the resting phase simultaneously, and two to three months later they all fall out together. That’s the mechanism behind most types of sudden, diffuse hair loss.
Stress and Illness: The Most Common Trigger
The single most common reason for a sudden increase in shedding is a condition called telogen effluvium. A physical or emotional stressor pushes a large batch of growing hairs prematurely into the resting phase. Because that resting phase lasts two to three months, you typically don’t notice the shedding until well after the triggering event has passed, which makes it hard to connect the dots.
Common triggers include high fevers, surgery, severe infections, crash diets, significant emotional stress, and stopping or starting certain medications. The good news is that acute telogen effluvium usually resolves on its own within six months once the trigger is removed. Your hair re-enters the growth phase naturally, though it can take another several months before you notice visible regrowth and fullness returning.
Postpartum Hair Loss
If you recently had a baby, the timing lines up perfectly with this same mechanism. During pregnancy, elevated estrogen keeps more hairs in the growing phase than usual, which is why many people feel their hair is thicker while pregnant. After delivery, those hormone levels drop and all those extra hairs shift into the resting phase at once. The shedding typically starts about three months after giving birth and can feel alarming, with clumps coming out in the shower or on your pillowcase. It usually resolves 6 to 12 months postpartum without any treatment.
Thyroid Problems
Both an underactive and overactive thyroid can cause hair loss, and the pattern looks different from other types. Instead of bald patches, thyroid-related hair loss tends to show up as diffuse thinning, a general decrease in density and volume across your entire scalp rather than in one spot. You may also notice changes in hair texture: strands that feel drier, coarser, and more prone to snapping. Excessive shedding during washing, brushing, or styling is another hallmark. If your hair loss is accompanied by fatigue, unexplained weight changes, feeling unusually cold or warm, or changes in your skin, a simple blood test can check your thyroid levels.
Low Iron and Nutritional Gaps
Iron deficiency is one of the more underrecognized causes of hair shedding, particularly in women. Your hair follicles need a steady supply of iron to fuel growth, and when your body’s iron stores drop, hair is one of the first things to suffer because it isn’t essential for survival. One study found that women experiencing diffuse hair shedding had an average ferritin level (the blood marker for stored iron) of about 16 ng/mL, compared to 60 ng/mL in women without hair loss. Women with ferritin at or below 30 ng/mL had 21 times the odds of experiencing this type of shedding.
Low vitamin D levels have also been linked to hair loss. Restrictive diets, heavy menstrual periods, and conditions that impair nutrient absorption can all contribute. If your shedding is diffuse and you can’t identify an obvious stressor, checking iron and vitamin D levels through a blood test is a reasonable step.
Autoimmune Hair Loss
Alopecia areata looks distinctly different from other forms of hair loss. Instead of generalized thinning, it causes smooth, round patches of baldness, often about the size of a quarter, though they can be larger or smaller. The immune system mistakenly attacks the hair follicles, and the affected skin may turn red, purple, brown, or gray. A few characteristic signs help identify it: tiny black dots visible in the follicle openings (broken-off hair shafts), short hairs that are wider at the top and taper toward the scalp (sometimes called exclamation point hairs), and new growth that comes in white.
Alopecia areata can affect anyone at any age, and it’s unpredictable. Some people have a single episode that resolves on its own, while others experience recurring patches. Three medications are now approved specifically to treat severe alopecia areata, all of which work by calming the immune response that targets the follicles.
Hormonal and Genetic Thinning
The most common long-term form of hair loss is pattern hair loss, driven by genetics and hormonal sensitivity. 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 on the top of the scalp while the hairline stays intact. This type of loss is gradual, progressing over years or decades, and it doesn’t involve the sudden shedding you see with stress-related or nutritional hair loss. The follicles progressively miniaturize, producing thinner, shorter, less visible hairs over time.
How to Tell If Your Shedding Is Abnormal
Because everyone sheds differently, it can be hard to know when shedding crosses the line. One approach dermatologists use is a pull test: grasping about 40 strands from a section of your scalp and tugging gently. If six or more strands come out, that’s considered active hair loss. You can try this yourself in different areas of your scalp to see if the shedding is concentrated in one spot or happening everywhere.
Pay attention to patterns. Hair collecting more than usual on your pillow, in the shower drain, or on your brush is worth noting, especially if it persists for more than a few weeks. Widening parts, visible scalp where you couldn’t see it before, and a ponytail that feels noticeably thinner are all signs that the loss has become significant enough to investigate. Photos taken in the same lighting over a few months can also help you track changes that are hard to notice day to day.
What You Can Do About It
The right response depends entirely on the cause. Stress-related shedding and postpartum hair loss typically resolve without intervention once the trigger passes. Being patient during those months is genuinely the most effective strategy, though gentle hair care (avoiding tight hairstyles, minimizing heat styling, and using a wide-tooth comb) can reduce breakage while your hair recovers.
If a nutritional deficiency is involved, correcting it through diet or supplementation can stop the shedding and promote regrowth, though it often takes three to six months to see a visible difference because of the time it takes new hairs to grow in. For thyroid-related hair loss, treating the underlying thyroid condition usually restores normal hair growth over time.
Pattern hair loss responds to different strategies since the underlying cause is ongoing. Topical treatments that stimulate blood flow to the follicles and oral medications that block the hormone responsible for follicle miniaturization are the most established options, both requiring consistent long-term use to maintain results. For alopecia areata, newer targeted therapies have shown strong regrowth results in clinical trials, giving people options that didn’t exist even a few years ago.
The most useful first step, regardless of what you suspect, is identifying the pattern of your hair loss: when it started, whether it’s patchy or diffuse, whether it came on suddenly or gradually, and what was happening in your life two to three months before you noticed it. That timeline often points directly to the cause.