What Causes Hair Loss: Types, Hormones, Genetics, and Treatment Options

This article is for informational purposes only. Consult a qualified healthcare professional for medical advice, diagnosis, or treatment.

The Biology of Hair Growth

Each of the roughly 100,000 hair follicles on the human scalp cycles independently through three phases. The anagen (growth) phase lasts 2–6 years for scalp hair, during which the hair shaft actively elongates from the base of the follicle. This is followed by catagen, a brief transitional phase of 2–3 weeks in which growth stops and the follicle shrinks. Finally, telogen (resting) lasts 3–4 months before the old hair sheds and a new anagen phase begins. At any one time, approximately 85–90% of scalp follicles are in anagen and 10–15% are in telogen. Shedding 50–100 hairs per day is entirely normal.

Common Types of Hair Loss

Androgenetic Alopecia: The DHT Pathway

Androgenetic alopecia (AGA), commonly called male or female pattern baldness, is the most prevalent form of hair loss. It results from the progressive miniaturization of genetically susceptible hair follicles under the influence of dihydrotestosterone (DHT). DHT is produced from testosterone by the enzyme 5-alpha reductase (5-AR), which is highly expressed in scalp follicles. DHT binds to androgen receptors in follicle dermal papilla cells, signaling them to shorten the anagen phase and progressively reduce the follicle's diameter over successive cycles until it produces only fine vellus hairs or ceases production entirely.

The genetics of AGA are polygenic and complex; over 350 genetic loci have been associated with the condition. The most important is a variant on the X chromosome encoding the androgen receptor (AR)—explaining why maternal grandfather baldness is a stronger predictor than paternal—but variants on dozens of autosomes also contribute. Women with AGA typically have intact hairlines but diffuse crown thinning because their lower androgen levels produce a less aggressive pattern.

Telogen Effluvium

Telogen effluvium (TE) is a reactive, diffuse shedding triggered when a physiological shock causes a large proportion of follicles to prematurely enter the telogen phase. The shedding becomes visible approximately 2–4 months after the trigger event. Common triggers include:

• Childbirth (postpartum effluvium affects up to 50% of new mothers)
• Major surgery or serious illness, including COVID-19 (hair loss is frequently reported 2–3 months after acute infection)
• Rapid weight loss, crash dieting, or caloric restriction
• Iron deficiency or ferritin levels below 30–40 ng/mL
• Hypothyroidism or hyperthyroidism
• Severe psychological stress
• Starting or stopping certain medications (retinoids, beta-blockers, anticoagulants)

Acute TE is self-limiting; hair regrowth typically resumes once the trigger is resolved. Chronic TE (lasting over 6 months) often has an unidentified or persistent underlying cause and requires investigation.

Alopecia Areata

Alopecia areata (AA) is an organ-specific autoimmune disease in which CD8+ T-cells attack the hair follicle immune privilege, disrupting the normal immunological protection that prevents follicle self-antigens from being recognized. It presents as one or more smooth, round bald patches, typically on the scalp but can affect any hair-bearing area. Alopecia totalis (entire scalp) and alopecia universalis (entire body) are severe variants. The condition is associated with other autoimmune disorders—particularly thyroid disease, vitiligo, and type 1 diabetes. JAK inhibitor medications (baricitinib, ritlecitinib) represent a significant treatment advance, with the FDA approving ritlecitinib for AA in 2023.

Evidence-Based Treatment Options

When to Seek Medical Evaluation

Dermatologist evaluation is recommended when shedding is severe, prolonged beyond 6 months, accompanied by scalp symptoms (redness, scaling, tenderness, pustules), or presents as well-defined bald patches. Sudden widespread shedding warrants blood work including thyroid function, complete blood count, ferritin, vitamin B12, and metabolic panel. Scarring alopecia requires urgent biopsy and treatment to prevent permanent follicle destruction.