Dozens of diseases can cause hair loss, ranging from autoimmune conditions and hormonal disorders to infections and nutritional deficiencies. Some cause temporary shedding that reverses once the underlying condition is treated, while others destroy hair follicles permanently. Understanding which category a disease falls into matters because it determines whether regrowth is possible.
Autoimmune Diseases
Autoimmune conditions are among the most common disease-related causes of hair loss. In these conditions, the immune system mistakenly targets the body’s own tissues, and hair follicles are frequent casualties.
Alopecia Areata
In alopecia areata, the immune system attacks hair follicles directly, causing inflammation that shuts down hair production. Hair typically falls out in small, round patches about the size of a quarter, usually on the scalp. Around the edges of these patches, short broken hairs called “exclamation point” hairs are often visible, narrower at their base than their tip. The bare patches usually show no rash, redness, or scarring, though some people feel tingling or burning on the skin just before hair falls out. Nail changes like tiny pits or dents can also appear, especially in more severe cases.
People with other autoimmune or allergic conditions, including thyroid disease, psoriasis, vitiligo, hay fever, and eczema, are more likely to develop alopecia areata. Both genetics and environmental triggers play a role. In its mildest form, only a few coin-sized patches appear. In more extensive cases, it can progress to alopecia totalis (loss of all scalp hair) or the rare alopecia universalis (loss of nearly all body hair). Because the follicles are not destroyed, regrowth is possible, though the course is unpredictable.
Lupus
Lupus causes hair loss in two distinctly different ways. Systemic lupus erythematosus (SLE) can produce what’s known as “lupus hair,” dry, fragile short hairs along the frontal hairline that are so characteristic they can suggest the diagnosis from across the room. This type of hair loss signals active disease or a flare, and it often improves once the flare subsides. SLE can also trigger telogen effluvium, a form of diffuse shedding.
Discoid lupus erythematosus (DLE) is a different story. DLE attacks the scalp skin directly, causing scarring that permanently destroys hair follicles. As much as 60% of patients with scalp DLE progress toward permanent, irreversible hair loss. This makes early treatment critical, because once a follicle is scarred over, no amount of therapy can bring it back.
Other Autoimmune Conditions
Rheumatoid arthritis can cause diffuse, non-scarring hair shedding through telogen effluvium. Sjogren’s syndrome, which primarily affects moisture-producing glands, also produces various hair and skin changes. Scleroderma and dermatomyositis have their own hair-related manifestations as well.
Thyroid Disease
Both an underactive thyroid (hypothyroidism) and an overactive thyroid (hyperthyroidism) can cause widespread hair shedding. In hypothyroidism, the slowed metabolism impedes the division of skin and hair cells. Hair follicles get stuck in their resting phase and are slow to restart growth, leading to prolonged shedding along with dry, brittle, lackluster hair.
Hyperthyroidism triggers hair loss through a different pathway. An overactive thyroid increases the production of damaging molecules called reactive oxygen species, which cause oxidative stress that weakens hair shafts and reduces their tensile strength. The hair may not look as visibly damaged as it does with hypothyroidism, but shedding increases and strands break more easily. In both cases, hair loss is generally reversible once thyroid levels are brought back to a normal range, though full regrowth can take months.
Polycystic Ovary Syndrome (PCOS)
PCOS causes hair loss through excess androgens, particularly testosterone. The body converts testosterone into a more potent hormone called DHT, which binds to receptors on scalp hair follicles and causes them to shrink. Even when hair doesn’t fall out completely, it becomes visibly thinner and shorter.
The pattern of thinning in PCOS differs from male pattern baldness. Rather than a receding hairline, women with PCOS typically experience one of two patterns: thinning that starts at the mid-scalp and spreads outward in a circular manner, or thinning along the center part that widens toward the front in a triangular “Christmas tree” shape. Complete baldness is unusual, but the thinning can be significant enough to reveal large areas of scalp.
Iron Deficiency
Low iron stores are one of the most common and most overlooked causes of hair shedding, particularly in women. Research shows that optimal hair growth occurs when serum ferritin (a measure of stored iron) is around 70 ng/ml. In one study, 63% of women with hair loss had ferritin levels below 20 ng/ml. Many standard lab ranges consider ferritin “normal” at levels as low as 12 or 15 ng/ml, which may explain why iron-related hair loss is frequently missed. Studies suggest that treatment for hair loss works better when ferritin levels are above 40 ng/ml, and some researchers use a cutoff of 40 to 70 ng/ml as the minimum for healthy hair.
Iron deficiency doesn’t need to reach the level of full-blown anemia to affect your hair. If you’re experiencing diffuse shedding and your ferritin is in the low-normal range, it may still be a contributing factor worth addressing through diet or supplementation.
Inflammatory Scalp Conditions
Chronic scalp conditions like seborrheic dermatitis (severe dandruff) and psoriasis can lead to hair thinning through persistent inflammation and oxidative stress. The inflammation disrupts the hair growth cycle, pushing more follicles into their resting and shedding phases prematurely. It also weakens the anchoring force that holds hair in the follicle, so strands fall out before they’ve completed their full growth cycle.
In most cases, hair loss from these conditions is temporary and improves when the scalp inflammation is controlled. The exception is severe scalp psoriasis, which in rare cases can cause scarring and permanent loss. Psoriasis triggered by certain biologic medications can also carry a risk of permanent scarring alopecia.
Infectious Diseases
Several infections can cause hair loss, but secondary syphilis produces one of the most distinctive patterns. Known as “moth-eaten” alopecia, it creates multiple small, irregular patches of hair loss scattered across the scalp, giving it a ragged, eaten-away appearance. This pattern occurs in roughly 3% to 7% of people with secondary syphilis and can sometimes be the only visible symptom. The hair loss is non-scarring and can affect other hair-bearing areas of the body as well. Because it closely mimics alopecia areata, syphilis should be considered when patchy hair loss appears suddenly, particularly in adults with risk factors.
Fungal infections of the scalp (tinea capitis) are another infectious cause, especially in children. Bacterial infections like folliculitis decalvans can destroy follicles and cause permanent scarring hair loss if not treated early.
Telogen Effluvium: The Delayed Reaction
Many of the diseases above cause hair loss through a mechanism called telogen effluvium, where a physical or metabolic stressor pushes large numbers of hair follicles into their resting phase at once. The distinctive feature of telogen effluvium is its timing: hair shedding typically begins about three months after the triggering event. This delay often makes it hard to connect the shedding to its cause, since the illness, surgery, high fever, or hormonal shift may have resolved by the time hair starts falling out.
Common disease-related triggers include thyroid dysfunction, severe infections, lupus flares, significant weight loss, and iron deficiency. The shedding can be alarming, with handfuls of hair coming out in the shower or on a pillowcase. The good news is that telogen effluvium is almost always temporary. Once the underlying condition is identified and treated, hair typically begins regrowing within six to nine months, though it can take longer to return to its original fullness.
Scarring vs. Non-Scarring Hair Loss
The most important distinction in disease-related hair loss is whether the condition destroys the follicle. Non-scarring hair loss leaves the follicle intact, meaning regrowth is possible once the cause is addressed. This category includes alopecia areata, telogen effluvium, thyroid-related shedding, iron deficiency, and androgenetic alopecia from PCOS.
Scarring (cicatricial) alopecia permanently destroys the follicle, replacing it with scar tissue. Hair cannot regrow from a scarred follicle. Diseases in this category include discoid lupus, lichen planopilaris (the most common primary scarring alopecia, mostly affecting women over 50), central centrifugal cicatricial alopecia (the most common type of hair loss in Black women), and folliculitis decalvans. Burns, radiation, and tumors can also cause secondary scarring hair loss.
If you notice hair loss with visible scalp changes like redness, scaling, or smooth shiny patches where follicle openings have disappeared, that may indicate a scarring process. Early identification matters because treatment can stop further follicle destruction, even though it cannot reverse damage already done.