Dozens of medical conditions can cause hair loss, ranging from hormonal imbalances and autoimmune diseases to nutritional deficiencies and infections. The type of hair loss, where it appears on your scalp, and whether it’s reversible all depend on the underlying cause. Understanding the difference matters because some conditions destroy hair follicles permanently, while others resolve once the root problem is treated.
Scarring vs. Non-Scarring Hair Loss
The single most important distinction in medical hair loss is whether the condition causes scarring. Non-scarring hair loss leaves the follicle intact, meaning hair can regrow once the trigger is addressed. Scarring (cicatricial) alopecia destroys the follicle itself, replacing it with scar tissue. Once that happens, regrowth in the affected area is no longer possible.
A dermatologist can usually tell the difference by examining your scalp closely. In scarring conditions, the tiny openings where hair normally exits the skin disappear entirely. In non-scarring conditions, those openings remain visible even where hair has thinned. This is why early diagnosis matters so much for scarring types: the goal shifts from regrowth to stopping further damage before more follicles are lost.
Autoimmune Conditions
Alopecia Areata
Alopecia areata is one of the most recognizable forms of medical hair loss. It typically appears as smooth, round patches on the scalp, though it can affect the eyebrows, beard, and body hair as well. It affects roughly 2% of the population in the U.S. and U.K., with global rates ranging from about 0.6% to 3.8%.
The condition is driven by immune cells that mistakenly attack hair follicles during their active growth phase. Certain white blood cells cluster around the base of the follicle and release inflammatory signals that disrupt the growth cycle, force the hair out prematurely, and prevent new growth from starting. Hair follicles aren’t destroyed, so regrowth is possible. In fact, three oral medications have now been approved for severe alopecia areata: the first in June 2022, the second in June 2023, and a third in 2024. In clinical trials of the newest option, more than 30% of patients had at least 80% scalp hair coverage after 24 weeks, and 25% saw nearly full regrowth.
Children with a family history of the condition are at higher risk, with some pediatric studies reporting prevalence rates between 10% and 50% in that group, pointing to a strong genetic component.
Lupus
Systemic lupus erythematosus (lupus) causes hair loss in two distinct ways. The first, sometimes called “lupus hair,” produces dry, fragile short hairs along the frontal hairline that break easily and give the appearance of fine, wispy fuzz on the anterior scalp. This type is non-scarring and often improves when the disease is better controlled.
The second type is more serious. Discoid lupus creates inflamed, discolored plaques on the scalp that destroy follicles and leave permanent scars. These patches often show a characteristic pattern of plugged follicles and color changes in the surrounding skin. Because discoid lupus causes irreversible loss, catching it early and starting treatment is critical.
Thyroid Disorders
Both an overactive thyroid (hyperthyroidism) and an underactive thyroid (hypothyroidism) can cause hair loss. Thyroid hormones play a direct role in regulating the hair growth cycle, and when production swings too high or too low, hair can stop growing altogether or shift prematurely into its resting phase.
This typically shows up as telogen effluvium, a pattern of diffuse shedding across the entire scalp rather than in patches. You might notice more hair than usual coming out during washing, brushing, or styling. The good news is that thyroid-related hair loss is usually reversible once hormone levels are stabilized with treatment, though regrowth can take several months.
Hormonal Imbalances and PCOS
Polycystic ovary syndrome (PCOS) is one of the most common hormonal causes of hair loss in women. The condition involves elevated levels of androgens (often called “male hormones,” though all women produce them in smaller amounts). These androgens, particularly a potent form called DHT, act on scalp follicles and cause a process called miniaturization. Over successive growth cycles, thick terminal hairs are gradually replaced by finer, shorter ones less than 0.03 mm in diameter. The result is a widening part and visible thinning across the top of the scalp, rather than the receding hairline pattern more common in men.
What makes this tricky is that androgen levels in your blood can sometimes appear normal while the scalp itself overproduces DHT locally. Some women with hair loss show increased activity of the enzymes that convert regular testosterone into DHT right at the follicle, even when standard blood tests look fine. This is why a dermatologist’s clinical exam, not just lab work, plays an important role in diagnosis.
Nutritional Deficiencies
Iron deficiency is the most studied nutritional cause of hair loss. Your body needs iron to produce the proteins that build hair fibers, and when iron stores drop too low, it deprioritizes hair growth. Doctors screen for this using ferritin, a blood marker that reflects your body’s iron reserves. Supplementation is often considered when ferritin falls below 70 ng/mL, though this isn’t a hard cutoff. One complication is that ferritin levels rise during inflammation or illness, so a “normal” reading can sometimes mask a true deficiency.
Other nutritional gaps linked to hair loss include vitamin D, zinc, and protein. Crash diets and restrictive eating patterns that cut protein intake sharply are a well-known trigger for diffuse shedding. The hair loss typically appears two to three months after the nutritional stress begins, which can make it hard to connect cause and effect.
Telogen Effluvium: The Catch-All Pattern
Many medical conditions don’t damage follicles directly. Instead, they shock the body enough to push a large percentage of hairs from their active growth phase into the resting phase all at once. This is telogen effluvium, and it’s the most common form of diffuse hair loss. The hallmark is a two-to-three-month delay between the triggering event and the shedding itself, which means you’re often pulling out clumps of hair long after you’ve recovered from whatever caused it.
Common triggers include high fever, severe infections, major surgery, childbirth, significant psychological stress, and thyroid disorders. Certain medications can also cause it, including some blood pressure drugs, antidepressants, anti-inflammatory drugs, and retinoids. Stopping birth control pills is another well-known trigger. Acute telogen effluvium typically resolves within six months as the growth cycle resets, but chronic cases can persist if the underlying cause isn’t addressed.
Scalp Infections
Fungal infections of the scalp (tinea capitis, commonly known as scalp ringworm) cause patchy hair loss that looks quite different from autoimmune or hormonal patterns. Two characteristic appearances help identify it. “Black dot” ringworm breaks hair shafts right at the scalp surface, leaving behind dark stubs that look like dots. “Gray patch” ringworm breaks hairs just above the surface, leaving short, dull stubs surrounded by flaky skin. The affected area is often scaly, itchy, or inflamed. Unlike autoimmune hair loss, fungal infections are contagious and require antifungal treatment. Once the infection clears, hair typically regrows.
Scarring Conditions Beyond Lupus
Several other conditions cause permanent, scarring hair loss. Frontal fibrosing alopecia produces a slowly receding band of hair loss along the front and sides of the hairline. Key signs include the absence of the tiny fine hairs that normally populate the hairline and subtle redness or scaling around remaining follicles. It primarily affects postmenopausal women, though cases in younger women and men are increasingly recognized.
Central centrifugal cicatricial alopecia (CCCA) begins at the crown and spreads outward. It’s most common in Black women and is associated with a grayish-white halo around affected follicles, a finding present in about 94% of cases. In late stages, both the follicle and its oil gland are replaced by dense scar tissue. Traction alopecia, caused by prolonged tension from tight hairstyles, starts as non-scarring but can become permanent if the pulling continues over years.
How Doctors Identify the Cause
Diagnosing medical hair loss usually starts with a close visual examination of the scalp and hair pattern, followed by targeted blood work when an underlying condition is suspected. The most commonly ordered tests include ferritin (to check iron stores), thyroid-stimulating hormone or TSH (to screen for thyroid disease), and androgen levels (to detect hormonal excess). A complete blood count may also be drawn to rule out anemia or signs of infection.
No single test covers every possible cause. Your doctor will use the pattern of loss, the timeline, your medical history, and your symptoms to narrow down which tests make sense. If a scarring condition is suspected, a small scalp biopsy may be needed to confirm the diagnosis and determine how much inflammation is still active, since that guides how aggressively to treat.