Hair thinning happens when more follicles stop producing new strands than your body can replace, and the causes range from genetics and hormones to stress, nutrition, and styling habits. Losing between 50 and 150 hairs a day is normal. The problem starts when shedding outpaces regrowth, or when individual strands become finer and shorter over time. Understanding which category your thinning falls into is the first step toward addressing it.
How Hair Growth Works
Your hair follicles cycle through phases independently of each other. At any given moment, about 85% to 90% of the hairs on your head are in the growth phase, which lasts two to eight years. A small percentage (1% to 3%) are in a brief transition phase lasting about two weeks, and roughly 9% are in a resting phase that lasts two to three months before the hair sheds naturally. Thinning occurs when something disrupts this balance, either by shortening the growth phase, pushing too many follicles into the resting phase at once, or damaging follicles so they stop cycling altogether.
Pattern Thinning From Genetics
The most common cause of hair thinning in both men and women is hereditary pattern loss. A hormone called DHT (a byproduct of testosterone) binds to receptors on hair follicles and gradually shrinks them. Each cycle, the follicle produces a thinner, shorter strand until it eventually stops producing visible hair at all. This process is called miniaturization, and it’s why thinning from genetics tends to be gradual rather than sudden.
In men, this typically shows up as a receding hairline and thinning at the crown. In women, it more often appears as overall thinning across the top of the scalp, with the part line widening over time. The genetic component can come from either parent’s side of the family. If you notice a slow, progressive change in hair density rather than sudden shedding, genetics is the most likely explanation.
Stress-Related Shedding
A sudden increase in shedding, often described as clumps in the shower drain or hair falling out when you run your fingers through it, points to a condition called telogen effluvium. A physical or emotional stressor pushes a large number of follicles into the resting phase simultaneously. Two to three months later, all those hairs shed at once. The delay between the trigger and the visible thinning is what makes this so confusing: by the time your hair starts falling out, the stressful event may feel like old news.
Common triggers include major surgery, severe illness, job or financial stress, relationship problems, crash dieting, and high fevers. COVID infections have been a well-documented trigger in recent years. The good news is that telogen effluvium is temporary. The challenging part is that shedding continues for two to three months after the stressor is removed, so recovery feels slow. Most people see full regrowth within six to twelve months once the underlying cause resolves.
Hormonal Shifts
Your thyroid gland plays a direct role in hair growth. Both an underactive and an overactive thyroid can cause thinning, though the hair looks slightly different in each case. Hypothyroidism tends to produce dry, brittle, dull hair alongside increased shedding. Hyperthyroidism more often causes thinner individual strands with a greasy texture. In both cases, the growth phase of the hair cycle is cut short, pushing follicles into shedding prematurely. If your thinning comes with fatigue, weight changes, or temperature sensitivity, a thyroid issue is worth investigating with a simple blood test.
Postpartum thinning is another hormonal trigger that catches many new parents off guard. During pregnancy, elevated estrogen keeps follicles in the growth phase longer than usual, so hair often feels thicker. After delivery, those hormone levels drop and all the hair that “should have” been shed during pregnancy falls out at once, typically starting around three months postpartum. This resolves on its own within six to twelve months as hormones stabilize.
Polycystic ovary syndrome, menopause, and starting or stopping hormonal birth control can all shift the balance of hormones that influence hair follicle cycling. Women experiencing thinning alongside irregular periods, acne, or hot flashes may find that hormonal changes are the root cause.
Nutritional Gaps That Affect Hair
Hair follicles are among the most metabolically active cells in your body, and they’re sensitive to nutritional shortfalls. Iron is the nutrient most strongly linked to hair thinning. You don’t need to be anemic for low iron to affect your hair. Research suggests that iron stores need to reach a certain threshold, roughly 70 ng/mL of ferritin (the protein that stores iron in your blood), for optimal hair follicle function. Many people with thinning hair have ferritin levels technically in the “normal” range but below this threshold, a situation sometimes called nonanemic iron deficiency.
Vitamin D also plays a critical role. The vitamin D receptor on hair follicle stem cells is essential for initiating new growth cycles. Without adequate vitamin D signaling, stem cells in the follicle lose their ability to regenerate the lower portion of the hair strand, and new growth stalls. Since vitamin D deficiency is extremely common, especially in people who spend most of their time indoors or live in northern latitudes, it’s one of the easier causes to test for and correct.
Protein intake matters too. Hair is made almost entirely of a protein called keratin, and restrictive diets that cut protein significantly can lead to visible thinning within a few months. Crash diets and rapid weight loss are particularly notorious for triggering shedding.
Scalp Conditions
An unhealthy scalp can thin your hair even when your follicles are otherwise fine. Seborrheic dermatitis, the condition behind persistent dandruff and scalp flaking, is a common culprit. Excess oil production on the scalp creates an environment where a naturally occurring yeast called Malassezia overgrows, triggering inflammation and intense itching. The inflammation itself can disrupt follicle function, and the scratching that follows physically damages hair roots and obstructs new growth.
If your thinning is accompanied by a flaky, itchy, or red scalp, treating the scalp condition often allows hair to recover. Over-the-counter medicated shampoos that target yeast and inflammation are typically the first step.
Traction From Styling and Headgear
Thinning concentrated around your hairline or temples often signals traction alopecia, damage caused by repeated pulling on the hair. Tight ponytails, braids, cornrows, buns, and weaves are the most common causes. Hair extensions and heavy styles like long dreadlocks add weight that pulls on natural hair over time. Even non-cosmetic sources of tension, like tightly pinned nursing caps, helmets worn for long periods, or hair pulled taut under turbans or hijabs, can contribute.
Early traction alopecia is reversible if you reduce the tension. The warning signs are thinning or small bald spots at the hairline, tenderness at the roots, and tiny bumps where hair is being pulled. Left untreated for years, repeated tension causes scarring in the follicle, and scarred follicles cannot regrow hair. The key is catching it early and alternating to looser styles.
Medications That Cause Thinning
A surprisingly long list of medications can cause hair thinning as a side effect. Beyond the well-known example of chemotherapy drugs, which cause rapid and widespread hair loss, many everyday medications can trigger more gradual thinning. These include certain acne treatments (particularly those containing vitamin A derivatives), blood thinners, blood pressure medications like beta-blockers and ACE inhibitors, antidepressants, mood stabilizers, cholesterol-lowering drugs, epilepsy medications, NSAIDs, weight loss drugs, and some forms of hormone replacement therapy. Birth control pills can cause thinning both while taking them and after stopping.
Medication-related thinning usually follows the telogen effluvium pattern, appearing two to three months after starting the drug. If your thinning lines up with a new prescription, that connection is worth discussing. In many cases, switching to an alternative medication resolves the issue.
How to Tell What’s Causing Your Thinning
The pattern and timing of your thinning offer the best clues. Gradual thinning at the crown or a widening part that’s been progressing for years points toward genetic pattern loss. Sudden, diffuse shedding that started two to three months after a specific event suggests telogen effluvium. Thinning concentrated at the hairline or temples, especially if you wear tight hairstyles, suggests traction. Thinning paired with scalp flaking and itching points to a scalp condition.
A simple self-check: run your fingers through clean, dry hair and tug gently. One or two hairs coming out is normal. If noticeably more come out in a single pass, that’s a sign of excessive shedding. Other red flags include visible scalp where you couldn’t see it before, a ponytail that feels thinner, and more hair than usual on your pillow or in the drain.
Blood work can rule out or confirm several common causes at once. Testing ferritin, vitamin D, thyroid hormones, and a complete blood count covers the most frequent nutritional and hormonal triggers. For many people, thinning turns out to have more than one contributing factor, and addressing even one of them can make a noticeable difference.