Medical Conditions That Cause Hair Loss

Most adult hair loss is androgenetic, the pattern-shaped thinning driven by genetics and hormones. But a meaningful share of new shedding has a medical driver underneath, and that distinction matters. Treating pattern hair loss when the real cause is an untreated thyroid disorder or low iron will produce frustrating results. Identifying the medical condition often does more for the hair than any topical or oral hair medication.

This page walks through the conditions most often implicated, what the workup typically looks like, and where the line sits between something that needs a primary care or dermatology visit and something that is just normal cycle variation.

Thyroid disease

Both an underactive thyroid (hypothyroidism) and an overactive one (hyperthyroidism) can cause diffuse shedding across the whole scalp. The mechanism is a disruption of the hair growth cycle. Thyroid hormone helps regulate how long follicles stay in the active growth (anagen) phase, and when levels swing too low or too high, more follicles drop into the resting (telogen) phase at once. Two to four months later, that wave of follicles sheds.

A basic thyroid panel covers most of what is worth knowing for hair purposes: TSH, free T4, and often free T3. A patient with shedding plus fatigue, cold intolerance, weight changes, or menstrual irregularity should consider testing. Hair usually recovers as thyroid levels normalize on treatment, but the timeline can run six to twelve months after labs stabilize.

For more on how diffuse cycle disruption looks day to day, see how the hair growth cycle works.

Iron deficiency

Iron deficiency is one of the better-studied nutritional contributors to hair shedding, particularly in women of reproductive age. The relevant lab is ferritin, the storage form of iron, not just hemoglobin. A patient can have a normal hemoglobin while ferritin is depleted, and the hair often responds to the ferritin level before the red blood cells do.

The threshold that matters for hair is debated. Some dermatologists treat ferritin below 40 to 70 ng/mL as worth correcting in patients with active shedding, even though standard "anemia" thresholds are much lower (Trost et al., J Am Acad Dermatol, 2006). The evidence is not airtight, but the cost of correcting low iron is low and the upside for hair when iron is the missing piece can be substantial.

Heavy menstrual bleeding, vegetarian or vegan diets, frequent blood donation, gastrointestinal issues, and pregnancy or postpartum recovery all raise the chance that low iron is a factor. See postpartum hair loss timeline for the related shedding pattern.

Autoimmune conditions

Alopecia areata is the most recognizable autoimmune cause of hair loss. The immune system attacks hair follicles, producing well-defined round or oval patches of complete bald scalp, sometimes the size of a coin, sometimes much larger. It can progress to alopecia totalis (all scalp hair) or alopecia universalis (all body hair) in a minority of cases. The hair loss is patchy and abrupt, not the gradual thinning of pattern hair loss, which makes it relatively easy to distinguish on exam.

Other autoimmune conditions, including lupus and some thyroid autoimmunity (Hashimoto's, Graves'), can affect hair through more than one mechanism. The pattern of loss, plus any other symptoms (joint pain, rashes, fatigue), guides the workup.

Alopecia areata treatment is its own area. It does not respond to standard pattern hair loss medications in the same way. A dermatologist evaluation is the appropriate step, not a hair loss telehealth visit alone.

PCOS and hormonal drivers in women

Polycystic ovary syndrome can drive hair changes in two directions at once: more body and facial hair (hirsutism) and thinner scalp hair. The mechanism involves elevated androgens, which act on scalp follicles in genetically susceptible women in the same way they do in pattern hair loss in men. PCOS is also associated with irregular periods, insulin resistance, and acne, which helps point the workup in the right direction.

For a detailed look at this pattern, see PCOS and hair loss. Hormonal hair loss in women is also covered more broadly in the women's hair loss pillar and in female pattern hair loss.

Scarring alopecias

This is the category that most needs a dermatologist rather than a telehealth visit for hair loss. Scarring alopecias are inflammatory conditions where the follicle is destroyed and replaced with scar tissue. Once that happens, the hair does not grow back from those follicles, regardless of treatment. Catching them early is what determines outcome.

The most common patterns:

• Lichen planopilaris. Inflammation around the follicles, often itchy or tender, with patches of scarring loss. Can affect any part of the scalp.
• Frontal fibrosing alopecia. A progressive recession of the frontal hairline that often takes the eyebrows with it. Predominantly affects women, especially postmenopausal, but men can be affected too. The hairline recession looks like a uniform band of pale, shiny skin rather than the angled temple recession of pattern loss.
• Central centrifugal cicatricial alopecia (CCCA). Starts at the crown and spreads outward, with scarring. Most common in Black women, and the cause is not fully understood.

Symptoms that should prompt a dermatology referral instead of trying topical or oral hair medications: itching, burning, tenderness, pustules, redness around hairs, or scalp that looks shiny and smooth where hair used to be. Pattern hair loss does not usually cause symptoms beyond the visual thinning itself.

Telogen effluvium triggers

Telogen effluvium is diffuse shedding triggered by a stressor: serious illness, surgery, childbirth, severe weight loss, certain medications, or major psychological stress. It is not a chronic condition in most cases. It shows up two to four months after the trigger and resolves over six to nine months once the trigger is gone. The pattern is generalized thinning across the whole scalp rather than the temple, crown, or part-line emphasis of pattern hair loss.

Stress-related shedding covers this in more depth, and the broader stress causes page walks through the mechanism.

Which labs are worth running

A reasonable hair-loss workup for a patient with new diffuse shedding usually includes:

• TSH, free T4 (thyroid)
• Ferritin (iron stores)
• Complete blood count
• Vitamin D, vitamin B12 in higher-risk patients

In women with additional symptoms (irregular periods, acne, hirsutism), adding free testosterone, DHEA-S, and prolactin makes sense. In men with hair loss plus low libido or fatigue, total testosterone is sometimes added, though pattern hair loss itself is not caused by low testosterone (it is driven by DHT activity at the follicle, not by overall testosterone levels). For more on that distinction, see what DHT is and why it causes pattern hair loss.

Labs do not need to be exhaustive. If the shedding pattern looks textbook pattern hair loss and there are no other symptoms, a smaller workup is reasonable. If the pattern is diffuse, sudden, or accompanied by scalp symptoms or other systemic signs, broaden the workup or see a dermatologist.

When pattern hair loss and a medical condition coexist

Common things happen commonly. A patient can have pattern hair loss and an iron deficiency at the same time. Treating one without the other leaves results on the table. The order generally goes: address the medical contributor first, then reassess hair after several months. If pattern loss is still progressing, that is when pattern-specific treatments enter the picture.

A medical assessment for hair loss should screen for these contributors before defaulting to medications. A licensed clinician through Curekey reviews symptoms and history with that broader lens, and can flag when a primary care or dermatology visit makes more sense than starting treatment. Start with a free hair assessment or read more about how the process works.