Hair Loss Causes: Complete Guide to Why You're Losing Hair
Over 30 distinct causes of hair loss exist, and treatment only works when you've correctly identified the cause. This guide covers them all, with guidance on how to identify your type.
Hair Loss by the Numbers
Hair loss is one of the most universal human experiences, but its causes are diverse and often misunderstood.
of men experience noticeable hair thinning by age 50 (androgenetic alopecia)
, American Hair Loss Association
of women experience noticeable hair loss at some point in their lifetime
distinct medical causes of hair loss have been identified, diagnosis matters
Critical point: Applying the wrong treatment for the wrong cause wastes time and money, and may delay addressing an underlying health issue. Diagnosis first, treatment second.
The Major Causes of Hair Loss
From the most common to the frequently overlooked, a comprehensive breakdown of each cause and what you can do about it.
Androgenetic Alopecia (Pattern Baldness)
Most CommonThe most prevalent form of hair loss, affecting both men and women. Driven by genetic sensitivity of follicles to DHT (dihydrotestosterone), which miniaturizes hair over time. In men it follows the Norwood scale (receding hairline + crown thinning). In women, it typically presents as diffuse thinning over the crown without complete hairline recession (Ludwig scale). Treatments: finasteride (men), minoxidil (both), dutasteride.
Telogen Effluvium
Very CommonDiffuse, sudden shedding triggered 2–3 months after a significant stressor: illness, surgery, childbirth, severe emotional stress, rapid weight loss, or nutritional deficiency. Follicles prematurely shift to the resting phase simultaneously, then shed en masse. Usually reversible within 6–12 months once the trigger is resolved. Chronic telogen effluvium (lasting >6 months) requires identification and treatment of an ongoing trigger.
Alopecia Areata
AutoimmuneAn autoimmune condition where the immune system attacks hair follicles, causing patchy hair loss on the scalp, beard, eyebrows, or anywhere on the body. Affects approximately 2% of the population. In most cases, hair regrows within a year without treatment, but recurrence is common. Severe forms (alopecia totalis = full scalp loss; alopecia universalis = full body loss) are rarer. Treatments include corticosteroids, JAK inhibitors (baricitinib, ritlecitinib. FDA approved 2022–2023).
Thyroid Disorders
Medical ConditionBoth hypothyroidism (underactive) and hyperthyroidism (overactive) can cause diffuse hair loss. Thyroid hormones regulate the hair growth cycle, imbalances disrupt this, pushing hairs into telogen prematurely. Hair loss from thyroid disorders often presents as generalized thinning rather than patterned loss. Blood test (TSH, T3, T4) confirms diagnosis. Hair typically recovers once thyroid levels are normalized with medication.
Nutritional Deficiencies
ReversibleIron deficiency anemia is the most common nutritional cause of hair loss, particularly in women with heavy periods or restrictive diets. Ferritin (stored iron) levels below 30 ng/mL are associated with increased hair shedding. Zinc, vitamin B12, vitamin D, and biotin deficiencies can also contribute. Crash dieting or very low calorie intake causes acute telogen effluvium. Blood work (ferritin, B12, zinc, vitamin D) can identify deficiencies.
Medication-Induced Hair Loss
Drug Side EffectNumerous medications can cause hair loss as a side effect, typically through telogen effluvium. Common culprits include: blood thinners (heparin, warfarin), beta-blockers, ACE inhibitors, antidepressants (SSRIs, especially sertraline), mood stabilizers (lithium), retinoids (isotretinoin/Accutane), hormonal medications (particularly progestins with androgenic activity), and chemotherapy agents. Hair loss is usually reversible after discontinuing the medication with physician supervision.
Hormonal Changes
HormonalHormonal shifts are a major trigger for hair loss. Postpartum shedding (3–6 months after childbirth) is one of the most common forms of telogen effluvium. Menopause reduces estrogen, which had been protective against DHT, many women experience accelerated androgenetic alopecia post-menopause. Polycystic ovary syndrome (PCOS) elevates androgens, causing both facial hair growth and scalp hair loss in women. Hormonal contraceptives with high androgenic activity can also trigger or worsen thinning.
Traction Alopecia
MechanicalHair loss caused by repeated tension on follicles from tight hairstyles, tight buns, ponytails, braids, weaves, and dreadlocks. Most commonly affects the hairline and temples. Early traction alopecia is reversible if the style is changed. Prolonged tension causes permanent follicle damage and scarring. Particularly prevalent among women of African descent due to specific hair styling practices. Prevention: looser styles, avoiding constant tension in the same areas.
Scalp Conditions
DermatologicalSeborrheic dermatitis (dandruff-related inflammation), psoriasis, ringworm (tinea capitis), and folliculitis can all cause or worsen hair loss. Seborrheic dermatitis is associated with Malassezia yeast overgrowth, the chronic inflammation it causes can disrupt the hair cycle over time. Scarring alopecias (lichen planopilaris, frontal fibrosing alopecia, central centrifugal cicatricial alopecia) permanently destroy follicles and require early aggressive treatment to halt progression.
Warning Signs: When to Act Fast vs. Monitor
Not all hair loss requires urgent medical attention, but some patterns signal conditions that need prompt evaluation.
See a Doctor Promptly
- Hair loss accompanied by fatigue, weight changes, or feeling cold/hot all the time (thyroid)
- Scalp pain, burning, or itching alongside hair loss (scarring alopecia)
- Hair loss with joint pain, rash, or other systemic symptoms (autoimmune)
- Sudden patchy loss in circles rather than diffuse thinning (alopecia areata)
- Hair loss with irregular periods, acne, or unexplained weight gain (PCOS)
Track & Monitor First
- Gradual recession at the hairline or temples (common androgenetic alopecia)
- Increased shedding (>150 hairs/day) after a major stressor 2–3 months prior
- Thinning at the crown only (Norwood vertex pattern)
- Widening part line in women (female pattern hair loss)
For these patterns, establishing a photo baseline and tracking changes over 3–6 months helps you and your dermatologist understand the rate of progression.
How to Identify Your Hair Loss Type
Pattern: Is the loss patterned (hairline, crown, temples) or diffuse (all-over thinning)? Patterned loss strongly suggests androgenetic alopecia. Diffuse loss suggests telogen effluvium, thyroid, or nutritional deficiency.
Timeline: Did it come on suddenly or gradually? Sudden, diffuse shedding 2–3 months after a major event is classic telogen effluvium. Slow, progressive recession over years points to androgenetic alopecia.
Distribution: Patchy circles = alopecia areata. Band-like hairline recession only = frontal fibrosing alopecia (women). Temples + crown + diffuse = androgenetic alopecia. Parting line only (women) = female pattern hair loss.
Associated symptoms: Itching, burning, or scalp pain = scarring alopecia or seborrheic dermatitis. Fatigue + hair loss = thyroid or anemia. Hair loss after childbirth = postpartum telogen effluvium.
Blood work: Ask your doctor to check: TSH (thyroid), CBC (anemia), ferritin (iron stores), B12, vitamin D, zinc, and in women, free/total testosterone and DHEA-S (androgens). This panel can identify or rule out most non-genetic causes.
Know What You're Dealing With. Start Tracking
Understanding your hair loss starts with objective data. Use AI-powered photo analysis to track density changes over time, document your pattern, and monitor whether interventions are working.
Analyze Your Hair LossFrequently Asked Questions
What is the most common cause of hair loss?▾
Androgenetic alopecia (male and female pattern hair loss) is by far the most common cause, accounting for approximately 95% of hair loss in men and the majority in women. It is driven by genetic sensitivity of hair follicles to dihydrotestosterone (DHT), a hormone derived from testosterone. By age 50, approximately 85% of men will experience significant thinning. The second most common cause is telogen effluvium, a temporary, diffuse shedding triggered by stress, illness, surgery, or nutritional deficiency.
Can stress alone cause hair loss?▾
Yes, stress can cause clinically significant hair loss through a condition called telogen effluvium. Major physical or emotional stressors (illness, surgery, bereavement, severe anxiety) shock hair follicles into the resting (telogen) phase simultaneously. The resulting shedding typically begins 2–3 months after the triggering event and can last 6–12 months. The good news: telogen effluvium is almost always reversible once the stressor is resolved. Chronic stress may also worsen androgenetic alopecia by elevating cortisol, which can increase DHT production.
Should I see a dermatologist about hair loss?▾
Yes, if you're experiencing noticeable hair loss, a dermatologist (ideally one specializing in hair disorders, called a trichologist) is the appropriate specialist to consult. They can distinguish between the 30+ potential causes through physical examination, a pull test, and blood work (checking thyroid, ferritin, B12, zinc, hormones). This is important because some causes, like thyroid disease, iron deficiency anemia, or autoimmune conditions, require specific treatment unrelated to typical hair loss medications. Self-diagnosing and using finasteride or minoxidil without ruling out other causes is not ideal.
Can hair loss be a sign of something serious?▾
Yes, in some cases. Sudden or diffuse hair loss can be a symptom of thyroid disorders (both hypo- and hyperthyroidism), lupus and other autoimmune conditions, iron deficiency anemia, significant nutritional deficiencies (B12, zinc), HIV/AIDS, syphilis, or as a side effect of chemotherapy, blood thinners, or certain psychiatric medications. Hair loss in conjunction with fatigue, weight changes, joint pain, rashes, or other systemic symptoms warrants prompt medical evaluation. For the majority of people, hair loss is not life-threatening, but it can be an early signal of an underlying condition that should be addressed.