Noticing extra hair on your pillow, in the shower drain, or in your brush is distressing — and it sends millions of Americans searching for answers. Hair loss (alopecia) affects approximately 80 million Americans at any given time. While most causes are benign and treatable, hair loss can be an early sign of underlying medical conditions that deserve attention.
How Much Hair Loss Is Normal?
Losing 50–100 hairs per day is entirely normal as part of the hair growth cycle. The scalp contains approximately 100,000 follicles, each cycling independently through growth (anagen, 2–6 years), transition (catagen), and resting (telogen, 2–3 months) phases. Concern is warranted when shedding noticeably exceeds this baseline, when the hairline recedes, when bald patches appear, or when density clearly decreases over months.
Common Types and Causes
Androgenetic Alopecia (Male and Female Pattern Hair Loss)
The most common type, affecting 50% of men by age 50 and up to 40% of women by age 70. It is driven by genetic sensitivity of hair follicles to dihydrotestosterone (DHT), a testosterone metabolite that progressively miniaturises susceptible follicles. In men, it follows the classic Hamilton-Norwood pattern (receding temples and crown). In women, it typically presents as diffuse thinning at the crown with preservation of the frontal hairline. FDA-approved treatments include topical minoxidil (men and women), oral finasteride (men), and low-level laser therapy.
Telogen Effluvium
A stress response in which a large proportion of follicles simultaneously shift from the growth to the resting phase, causing diffuse shedding 2–3 months after a triggering event. Common triggers include childbirth (postpartum hair loss), major surgery, severe illness, significant weight loss, crash dieting, major psychological stress, and COVID-19 infection. Shedding typically peaks at 3 months and resolves spontaneously within 6–12 months as the normal cycle resumes. Treatment addresses the underlying cause.
Nutritional Deficiencies
Iron deficiency is the most common nutritional cause of hair loss, particularly in women of reproductive age. Ferritin (iron storage) levels below 30 ng/mL are associated with hair shedding even without anaemia. Zinc, biotin, and protein deficiencies also impair hair growth. Crash diets and eating disorders are common hidden causes of sudden hair loss in young women.
Thyroid Disease
Both hypothyroidism and hyperthyroidism cause diffuse hair thinning. Thyroid hormones regulate the hair cycle; when levels are abnormal, follicles shift prematurely into the resting phase. Hair loss from thyroid disease is reversible with appropriate treatment — though regrowth may take 6–12 months after normalisation of thyroid function.
Alopecia Areata
An autoimmune condition in which the immune system attacks hair follicles, causing smooth, round patches of sudden hair loss on the scalp, beard, eyebrows, or body. It affects approximately 2% of Americans across a lifetime. In most cases, hair regrows spontaneously within a year; in others it progresses to total scalp (alopecia totalis) or body (alopecia universalis) hair loss. JAK inhibitors — baricitinib and ritlecitinib — are recently FDA-approved treatments showing significant regrowth in clinical trials.
Treatments That Work
- Minoxidil (Rogaine): Topical 2% or 5% solution or foam applied daily to the scalp stimulates follicle growth. Requires continuous use; stops working if discontinued.
- Finasteride: Oral 5-alpha reductase inhibitor that blocks DHT conversion. Effective in 90% of men for halting progression; ~65% see regrowth. Not approved for women (teratogenic in pregnancy).
- Platelet-Rich Plasma (PRP): Injections of concentrated growth factors from the patient's own blood show promising evidence for both androgenetic alopecia and alopecia areata.
- Address nutritional gaps: Test and supplement iron, vitamin D, zinc if deficient.
Sources
- American Academy of Dermatology. Hair loss: Who gets and causes. 2023.
- Blumeyer A, et al. Evidence-based guideline for the treatment of androgenetic alopecia. JDDG. 2011.
- King B, et al. Two Phase 3 Trials of Baricitinib for Alopecia Areata. NEJM. 2022.
- Mayo Clinic. Hair loss — Causes. 2023.