What Causes Hair Loss? 12 Science-Backed Causes Explained

Hair loss is rarely caused by a single factor. In most cases, it's a combination of genetics, hormones, nutrition, and lifestyle factors that collectively tip the follicular balance toward shedding over growth. This guide systematically explains the 12 most common causes — and what you can do about each one.

1. DHT Sensitivity (Androgenetic Alopecia)

Dihydrotestosterone (DHT) is the primary driver of pattern baldness in both men and women. When the enzyme 5-alpha reductase converts testosterone to DHT, genetically susceptible follicles gradually miniaturize and eventually cease producing terminal hair. This accounts for approximately 95% of male hair loss and a significant proportion of female hair loss.

2. Iron Deficiency

Iron deficiency — particularly without frank anemia — is among the most under-diagnosed causes of hair loss. Studies show that ferritin (stored iron) levels below 40 ng/mL are associated with significantly increased telogen shedding, even when hemoglobin levels remain normal. Premenopausal women, vegetarians, and athletes are at highest risk.

3. Thyroid Disorders

Both hypothyroidism and hyperthyroidism disrupt the hair growth cycle. Hair follicles have thyroid hormone receptors, and inadequate or excessive thyroid hormone pushes follicles prematurely into the resting (telogen) phase. The good news: normalizing thyroid function with appropriate treatment usually reverses this type of hair loss within 6-12 months.

4. Chronic Stress and Elevated Cortisol

Prolonged psychological or physical stress elevates cortisol, which directly disrupts the Wnt/β-catenin signaling pathway that maintains follicle stem cell activity. A landmark 2021 Harvard study published in Nature identified the precise mechanism by which stress hormones suppress hair follicle regeneration — validating what clinicians had observed for decades.

5. Nutritional Deficiencies

Beyond iron, deficiencies in zinc, vitamin D, vitamin B12, and protein all contribute to hair loss. Zinc is required for DNA synthesis in rapidly dividing hair matrix cells. Vitamin D activates hair follicle vitamin D receptors to promote the anagen (growth) phase. Inadequate dietary protein limits keratin synthesis — the structural protein that hair is made of.

6. Autoimmune Conditions (Alopecia Areata)

In alopecia areata, the immune system attacks anagen hair follicles, causing sudden, often patchy hair loss. The condition affects approximately 6.8 million Americans. While stressful, alopecia areata is often self-resolving — about 80% of people with limited involvement regrow hair within 12 months. Severe or total cases may require immunosuppressive treatment.

7. Medications and Medical Treatments

Over 300 medications list hair loss as a potential side effect. The most commonly implicated include chemotherapy agents (most severe), anticoagulants, retinoids, beta-blockers, antidepressants, and hormonal contraceptives. If hair loss began after starting a new medication, discuss alternatives with your prescribing physician.

8. Scalp Conditions

Seborrheic dermatitis, psoriasis, folliculitis, and tinea capitis (ringworm) can all damage follicles and inhibit normal hair growth. Treating the underlying scalp condition typically resolves associated hair loss.

9. Hormonal Changes

Pregnancy-related hair loss (telogen gravidarum), postpartum shedding, PCOS, and menopause all involve hormonal shifts that affect the hair cycle. Postpartum shedding — which peaks 3-4 months after delivery — is temporary and resolves spontaneously, though it can be alarming in its severity.

10. Physical Trauma and Surgery

Major surgery, serious illness, extreme weight loss, or significant physical trauma can trigger diffuse shedding (telogen effluvium) 2-4 months later as follicles respond to the metabolic stress by entering the resting phase simultaneously.

11. Traction and Mechanical Damage

Chronic tension from tight hairstyles (braids, extensions, weaves, ponytails) damages the follicle over time. Early traction alopecia is reversible; advanced cases with follicular scarring are not. Prevention — avoiding chronically tight styles — is far more effective than treatment.

12. Aging

As we age, follicle stem cell activity declines, the anagen phase shortens, and the proportion of follicles in telogen increases. Hair shafts also become finer and more fragile. These changes are partly mediated by oxidative stress accumulation, which is why antioxidant-rich diets and targeted supplementation show some benefit in age-related thinning.