The medical term for hair loss is alopecia. It refers to the loss of hair from any area of the body where hair normally grows, though most people use the term when talking about hair on the scalp. Alopecia isn’t a single condition, though. It’s an umbrella term that covers many different types of hair loss, each with its own cause, pattern, and outlook.
Losing some hair every day is completely normal. On average, you can expect to shed between 50 and 150 hairs daily. That sounds like a lot, but your scalp holds roughly 100,000 hairs at any given time, so this turnover is barely noticeable. When hair starts falling out faster than it regrows, or when follicles stop producing new hair altogether, that’s when thinning becomes visible.
How Hair Grows and Sheds Normally
Every hair on your head moves through a repeating cycle of three phases. The first is the growth phase, which lasts about 2 to 8 years for scalp hair. This is the longest stage and determines how long your hair can get. Next comes a short transition phase lasting about 2 weeks, where the follicle shrinks and the hair detaches from its blood supply. Finally, there’s a resting phase of 2 to 3 months, after which the old hair falls out and a new one begins growing in its place.
At any given time, most of your hair is in the growth phase while a small percentage is resting or shedding. Problems arise when something shifts this balance, pushing too many hairs into the resting or shedding phase at once, or when follicles shrink and eventually stop producing hair entirely.
Androgenetic Alopecia (Pattern Baldness)
The most common type of hair loss is androgenetic alopecia, often called male-pattern or female-pattern baldness. It affects both sexes, though it looks different in each. In men, it typically starts with a receding hairline and thinning at the crown. In women, hair thins more diffusely across the top of the scalp while the hairline usually stays intact.
The driving force behind this type is a hormone called DHT (dihydrotestosterone). Your body naturally produces DHT from testosterone, and in normal amounts it plays a role in regulating the hair growth cycle. But in people with a genetic predisposition, hair follicles on certain parts of the scalp are overly sensitive to DHT. This sensitivity causes the growth phase to shorten, producing thinner and shorter strands over time. Eventually, new hair growth slows or stops completely. This process, called follicle miniaturization, is gradual. It can take years before the thinning becomes obvious.
Genetics play a major role. Variations in the gene that controls androgen receptors can make those receptors respond more aggressively to DHT, accelerating the shrinking process. This is why pattern baldness tends to run in families.
Alopecia Areata (Autoimmune Hair Loss)
Alopecia areata looks very different from pattern baldness. Instead of gradual thinning, it causes smooth, round patches of hair loss that can appear suddenly on the scalp, beard, eyebrows, or elsewhere on the body. An estimated 700,000 Americans have this condition.
The cause is autoimmune: your immune system mistakenly attacks hair follicles, triggering inflammation that disrupts growth. Researchers believe both genetic and environmental factors contribute, but the exact trigger isn’t fully understood. In some cases, the patches regrow on their own. In others, the hair loss can spread to cover the entire scalp or even the whole body.
Three FDA-approved medications now exist specifically for severe alopecia areata. The most recent, approved in 2024, belongs to a class of drugs called JAK inhibitors, which work by calming the immune response that attacks the follicles.
Telogen Effluvium (Stress-Related Shedding)
If you’ve noticed handfuls of hair coming out in the shower or on your pillow a few weeks after a stressful event, you’re likely experiencing telogen effluvium. This type of hair loss happens when a physical or emotional shock pushes a large number of hairs into the resting phase all at once. About 4 to 6 weeks after the triggering event, those hairs start falling out together.
The list of known triggers is long: major surgery, serious illness, pregnancy and childbirth, extreme emotional stress like grief or divorce, crash dieting, hormonal changes, sleep deprivation, and certain medications. Nutritional deficiencies in iron, vitamin D, vitamin B, zinc, protein, or essential fatty acids can also increase your susceptibility and make episodes last longer.
The good news is that telogen effluvium is almost always temporary. The enhanced shedding typically continues for 2 to 3 months after the stressor is removed, and about 95% of people fully recover within 2 to 3 months after that. Acute episodes usually resolve in under 6 months total. In rare cases, the shedding becomes chronic, lasting beyond that window.
Anagen Effluvium (Rapid Hair Loss During Growth)
While telogen effluvium involves hairs that were already heading into a resting phase, anagen effluvium is the sudden loss of hairs that are still actively growing. It happens when something damages the rapidly dividing cells at the base of the hair follicle. These cells are among the fastest-growing in your body, which makes them vulnerable to anything that disrupts cell division.
Chemotherapy is the most well-known cause. The drugs target fast-dividing cells throughout the body, and hair matrix cells get caught in the crossfire. When these cells stop working, the hair shaft becomes thin and brittle, then breaks off near the scalp and falls out quickly. Unlike the gradual shedding of telogen effluvium, anagen effluvium can cause widespread, noticeable hair loss within days or weeks. Hair typically regrows once treatment ends, though the texture or color may change temporarily.
Traction Alopecia (Damage From Styling)
Traction alopecia is hair loss caused by repeated physical pulling on the hair. It’s entirely preventable but can become permanent if the styling habits continue long enough. Hairstyles most commonly linked to traction alopecia include:
- Cornrows
- Locs
- Tightly braided hair
- Tight buns, ponytails, and updos
- Hair extensions or weaves, especially on chemically relaxed hair
- Rollers worn to bed regularly
Even the constant friction of a hat, headscarf, or other covering can contribute, particularly if hair is pulled back tightly underneath. Early on, the damage is reversible. You might notice soreness at the hairline, small bumps, or thinning where the tension is greatest. But when the pulling continues over months or years, the follicles scar over. Where hair once grew, you’ll see smooth, shiny skin. At that point, regrowth is no longer possible.
Signs That Hair Loss Needs Medical Attention
Some patterns of hair loss point to underlying health issues worth investigating. A dermatologist will examine the scalp for inflammation, redness, sores, or scarring, any of which suggest a process that could cause permanent damage if untreated. Sudden, dramatic shedding without an obvious trigger is another reason to get evaluated.
Hair loss paired with unexplained weight changes, fatigue, or muscle weakness can signal a thyroid problem. In women, hair thinning combined with new facial or body hair growth, acne, or irregular periods may indicate excess androgens. In these situations, blood tests can check hormone levels, thyroid function, and nutritional deficiencies like iron or vitamin D to identify a treatable cause. Addressing the underlying condition often slows or reverses the hair loss itself.