In this article (10 sections)
Hair loss is the visible end-point of many different biological processes, and the most consequential first step in addressing it is figuring out which one is actually happening. The folk model that treats "hair loss" as a single condition with a single answer often steers people toward interventions that do nothing for the cause they actually have. Pattern hair loss has different drivers from postpartum shedding, from telogen effluvium triggered by stress or illness, from medication-induced hair loss, from thyroid- or iron-related shedding, and from inflammatory scalp diseases. The categories overlap and sometimes coexist, but each has a recognizable signature and a different evidence-based treatment path.
This pillar walks through the main causes, what their distinguishing features are, and which conditions push the diagnosis toward each. Use the children below for deeper coverage of specific categories.
Genetic and hormonal causes (the most common)
For the majority of adults who notice gradual, progressive thinning, the cause is androgenetic alopecia, also called pattern hair loss. It is genetic and hormonally mediated, driven by sensitivity of scalp follicles to dihydrotestosterone (DHT), a more potent androgen produced from testosterone by the enzyme 5-alpha-reductase. Susceptible follicles, concentrated on the top, front, and crown of the scalp, progressively shrink with repeated DHT exposure. The follicles on the back and sides are genetically resistant and are spared, which is why pattern loss produces the recognizable distribution.
What distinguishes pattern loss diagnostically:
- A characteristic distribution (temples and crown in men, widening part with relative sparing of the hairline in women)
- A gradual progression over years rather than weeks
- Family history of similar loss on either parent's side
- Hair becoming finer and shorter in affected areas before density visibly drops
Treatment options that address the underlying mechanism (5-alpha-reductase inhibitors and minoxidil) have decades of randomized trial evidence and are most effective when started early. The deeper biology lives on the androgenetic alopecia page and the DHT page.
The genetic causes and hormonal causes child pages cover these in more depth, including specifically how hormonal shifts during pregnancy, postpartum, perimenopause, and conditions like PCOS interact with pattern hair loss.
Telogen effluvium (stress-triggered diffuse shedding)
Telogen effluvium is a temporary, diffuse shedding pattern in which a larger-than-usual fraction of follicles enter the resting (telogen) phase of the hair cycle in synchrony, then shed three to four months later. The visible result is an alarming-looking burst of hair loss across the whole scalp, usually two to four months after a triggering event.
Common triggers include:
- A serious illness with high fever (including COVID-19 and severe flu)
- Major surgery, especially with general anesthesia
- Childbirth (postpartum telogen effluvium)
- Rapid weight loss or restrictive dieting
- Acute psychological stress (death of a loved one, divorce, job loss)
- A new medication started two to three months before the shedding began
- Severe iron deficiency or recent thyroid dysfunction
Telogen effluvium is diffuse rather than patterned, has a clear onset, and resolves on its own within three to six months once the trigger is removed. The follicles are not damaged; they have been pushed off schedule and re-enter anagen on their own. Our guide to stress and telogen effluvium covers the triggers, the timeline, and the underlying biology in more detail.
The most important diagnostic note is that telogen effluvium and pattern hair loss can coexist. A diffuse shed on top of slowly progressing pattern loss can reveal the underlying density loss for the first time. The shed itself recovers; the pattern loss does not without treatment.
Medical conditions
Several systemic conditions cause hair loss as part of their clinical picture. Diagnosing them is straightforward with a focused history and a few targeted blood tests, but they are commonly missed when hair loss is treated as a stand-alone cosmetic concern.
Thyroid disease. Both hypothyroidism and hyperthyroidism can produce diffuse hair shedding. The shedding tracks the underlying thyroid state and improves when thyroid function is corrected. TSH, free T4, and sometimes free T3 are the relevant labs.
Iron deficiency. Low ferritin (the storage form of iron) is associated with chronic telogen effluvium and is common in menstruating women, vegetarians, athletes, and frequent blood donors. Ferritin below 30 ng/mL is often used as a clinical threshold in hair-loss evaluation, although it is debated.
Autoimmune conditions. Alopecia areata, an autoimmune condition that targets hair follicles, produces sharply demarcated circular bald patches rather than diffuse or patterned thinning. It is a different process from pattern hair loss and is treated with immunomodulatory therapies, often topical or intralesional corticosteroids or, in severe cases, JAK inhibitors.
Polycystic ovary syndrome (PCOS). PCOS produces elevated androgen levels, which can drive female-pattern hair loss alongside other features like irregular menses, acne, and hirsutism. Our PCOS page covers the hair-loss dimension.
Inflammatory or scarring scalp disease. Lichen planopilaris, frontal fibrosing alopecia, central centrifugal cicatricial alopecia, and other scarring alopecias destroy follicles permanently through inflammation. They tend to present with symptoms (itching, burning, redness, scaling) that pattern loss does not produce, and they require dermatologist evaluation.
Each of these conditions has its own evaluation and treatment path. A focused medical history plus a small panel of labs (TSH, free T4, ferritin, CBC) covers the most common contributors. When inflammatory or scarring scalp disease is suspected, in-person dermatology assessment is the right next step.
Medications
Hair loss is a recognized side effect of dozens of common medications, sometimes as labeled adverse events and sometimes as well-documented off-label observations. The mechanism is usually telogen effluvium triggered two to three months after starting the medication, which then resolves if the medication is stopped or continued unchanged.
Common categories that can contribute to hair loss include:
- GLP-1 weight-loss medications (semaglutide, tirzepatide), where the trigger is more often the rapid weight loss than the drug itself
- Metformin, mediated through B12 effects
- Some antidepressants, particularly SSRIs and bupropion
- Some birth control formulations, depending on progestin androgenicity
- Beta blockers, ACE inhibitors, and some calcium channel blockers
- Anticoagulants (warfarin, heparin)
- Statins, in some users
- Anticonvulsants like valproate and gabapentin
- Chemotherapeutic agents (a distinct mechanism, anagen effluvium, in which actively growing hair shafts are damaged directly)
- Retinoids (oral isotretinoin and high-dose vitamin A)
The full coverage lives on the drug-induced hair loss cluster, with a child page per medication class. The general framework for distinguishing medication-related shedding from other causes is timing (onset two to three months after starting the drug), pattern (diffuse, not patterned), and the absence of other obvious triggers. Stopping the medication, when clinically appropriate, usually resolves the shedding within months.
Nutritional causes
Dietary deficiencies are a real but often overstated cause of hair loss. The genuine offenders, when they occur, can be identified by labs and corrected with reasonable success. The supplement-industry framing that hair loss reflects a general "nutrient deficiency" curable by their product is largely not supported by evidence.
The deficiencies with the strongest evidence for hair shedding are:
- Iron deficiency (ferritin)
- Vitamin D deficiency
- B12 deficiency (particularly in vegetarians and metformin users)
- Severe protein deficiency (rare in developed countries, common in restrictive dieting)
- Zinc deficiency (uncommon outside of specific medical conditions)
The deficiencies with weak or no evidence for hair shedding in well-nourished adults are biotin (deficiency is rare and supplementation does not help in non-deficient people), selenium, and various trace minerals marketed in hair vitamins. Our nutritional deficiencies guide covers the evidence in more depth, and the nutritional causes (covered above) child page covers the medical framework.
Stress
Stress is a real cause of hair loss, but the mechanism is more specific than the cultural shorthand suggests. Acute, severe stress (a major life event, serious illness, surgery, or trauma) can trigger telogen effluvium, with shedding appearing two to four months after the stressor and resolving within months once the stress resolves.
Chronic, low-level stress is not as well established as a hair-loss driver. The biology of chronic stress probably contributes to inflammatory and hormonal effects that can interact with pattern loss, but the evidence for chronic stress alone as a cause of progressive hair loss is weaker than the popular framing.
Stress does not, by itself, cause pattern hair loss. What it can do is unmask or accelerate the appearance of pattern loss in someone already developing it, because a wave of telogen shedding on top of slowly progressive pattern loss can reveal density changes that were not previously obvious. The stress and telogen effluvium guide covers the mechanism in detail.
Inflammatory and scalp conditions
Several scalp conditions can cause shedding through inflammation rather than through the hair cycle or hormones. Seborrheic dermatitis (the same condition that causes dandruff) produces a flaky, itchy scalp and can drive a mild inflammatory shedding. Folliculitis (inflammation of the follicles themselves) can produce focal shedding around inflamed areas. Psoriasis affecting the scalp can be associated with telogen effluvium in some cases.
These are usually distinguishable from pattern hair loss by symptoms: pattern loss is asymptomatic, while inflammatory scalp conditions itch, burn, flake, or hurt. Our companion guides cover itchy scalp and hair loss and ketoconazole shampoo (the standard adjunctive treatment for seborrheic dermatitis-related shedding).
Mechanical and styling causes
Sustained mechanical pulling on the hair shaft produces traction alopecia, a form of hair loss caused by the tension itself rather than a biological process at the follicle. It is most commonly associated with tight braids, ponytails, weaves, and extensions worn for long periods. Early traction alopecia is reversible if the tension is removed; advanced traction alopecia can produce permanent loss as follicles scar.
Ordinary hat-wearing does not cause hair loss, despite the persistent folk theory. Our does wearing a hat cause hair loss guide covers the specific cases where headwear is actually implicated.
When the cause is unclear
A reasonable threshold for medical evaluation is when self-assessment cannot confidently distinguish between the categories above, or when shedding is severe, prolonged, or accompanied by other symptoms. A focused history, a scalp exam, and a small panel of blood tests can usually resolve the differential. A telehealth assessment with a U.S.-licensed physician is one efficient way to start. Curekey's hair assessment walks through that process, and the how it works page covers what the consultation looks like.
The actionable point across all of these categories is that pattern hair loss responds best to treatment started early, before substantial follicle miniaturization, and the other categories respond best to identifying and addressing their specific trigger. The single biggest mistake in hair loss is not the choice of treatment; it is treating the wrong cause.
